Prof. Dr. Christof Schulz

Institute for Combustion and Gas Dynamics
University of Duisburg-Essen

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  • Atmospheric-pressure particle mass spectrometer for investigating particle growth in spray flames
    Suleiman, S. and Nanjaiah, M. and Skenderovic, I. and Rosenberger, T. and Kunze, F. and Wlokas, I. and Kruis, F.E. and Wiggers, H. and Schulz, C.
    Journal of Aerosol Science 158 (2021)
    In this work, we introduce a new particle mass spectrometer (AP-PMS) that is able to detect particle-size distributions at ambient pressure using a three-stage pumping design. This device is demonstrated for direct sampling from the particle formation in spray-flame synthesis of iron oxide nanoparticles. Aerosol sampling is performed by a probe with integrated dilution that has been characterized and configured by computational fluid dynamics simulations and the chamber-skimmer system has been investigated by schlieren imaging. The system was validated by detailed characterization of a standardized sooting flame and by iron oxide nanoparticles generated in the SpraySyn burner from iron nitrate dissolved in a mixture of ethanol and 2-ethylhexanoic acid. The PMS results are compared to additional inline measurements with SMPS and ELPI + as well as with TEM measurements of thermophoretically sampled materials from the same location in the spray flame. © 2021 The Authors
    view abstract10.1016/j.jaerosci.2021.105827
  • Characterization of tracers for two-color laser-induced fluorescence thermometry of liquid-phase temperature in ethanol, 2–ethylhexanoic-acid/ethanol mixtures, 1-butanol, and o-xylene
    Prenting, M.M. and Shilikhin, M. and Dreier, T. and Schulz, C. and Endres, T.
    Applied Optics 60 (2021)
    The fluorescence spectra of dye solutions change their spectral signature with temperature. This effect is frequently used for temperature imaging in liquids and sprays based on two-color laser-induced fluorescence (2cLIF) measurements by simultaneously detecting the fluorescence intensity in two separate wavelength channels resulting in a temperature-sensitive ratio. In this work, we recorded temperature-dependent absorption and fluorescence spectra of solutions of five laser dyes (coumarin 152, coumarin 153, rhodamine B, pyrromethene 597, and DCM) dissolved in ethanol, a 35/65 vol.% mixture of ethanol/2-ethylhexanoic acid, ethanol/hexamethylsiloxane, o-xylene, and 1-butanol to investigate their potential as temperature tracers in evaporating and burning sprays. The dissolved tracers were excited at either 266, 355, and 532 nm (depending on the tracer) for temperatures between 296 and 393 K (depending on the solvent) and for concentrations ranging between 0.1 and 10 mg/l. Absorption and fluorescence spectra of the tracers were investigated for their temperature dependence, the magnitude of signal re-absorption, the impact of different solvents, and varying two-component solvent compositions. Based on the measured fluorescence spectra, the tracers were analyzed for their 2cLIF temperature sensitivity in the respective solvents. Coumarin 152 showed for single-component solvents the overall best spectroscopic properties for our specific measurement situation related to temperature imaging measurements in spray-flame synthesis of nanoparticles as demonstrated previously in ethanol spray flames [Exp. Fluids 61, 77 (2020)]. © 2021 Optical Society of America
    view abstract10.1364/AO.419684
  • Crumpled few-layer graphene: Connection between morphology and optical properties
    Musikhin, S. and Talebi-Moghaddam, S. and Corbin, J.C. and Smallwood, G.J. and Schulz, C. and Daun, K.J.
    Carbon 182 (2021)
    Comprehensive characterization of crumpled graphene materials is required to understand their morphologically dependent properties. This study connects the optical properties of crumpled few-layer graphene (FLG) aerosols and colloidal suspensions with particle morphology. The mass absorption cross-section (MAC) of crumpled FLG was measured in the aerosol phase and it is shown that such aerosols absorb visible light more efficiently than soot aerosols. The UV–Vis spectra of FLG suspensions in ethanol were correlated with the mean nanoparticle layer number and the lateral size. The measured optical properties were also simulated using discrete dipole approximation (DDA) applied to volume-reconstructed particles with refractive indices of graphene and graphite from literature. The measurements were more closely reproduced using the refractive index of graphite pellets rather than flat graphene on a substrate. Moreover, we show that a simpler Rayleigh–Debye–Gans (RDG) model can predict the wavelength dependence of the crumpled FLG absorption cross-section. © 2021
    view abstract10.1016/j.carbon.2021.06.052
  • Interrogating Gas-Borne Nanoparticles Using Laser-Based Diagnostics and Bayesian Data Fusion
    Menser, J. and Daun, K. and Schulz, C.
    Journal of Physical Chemistry C 125 (2021)
    We demonstrate how the evaporation properties of gas-borne nanoscale materials, here liquid silicon and germanium nanoparticles, can be obtained through a novel combination of in situ time-resolved laser-induced incandescence (TiRe-LII) and phase-selective laser-induced breakdown spectroscopy (PS-LIBS) based on Bayesian data fusion. This approach reduces the uncertainty in the parameters describing evaporation and condensation by more than a factor of 2 compared to the conventional path and has the capability to provide much needed particle-size-dependent information on nanomaterial phase transitions at high temperature. The inferred parameters are generally consistent with those repeated in the literature but with reduced uncertainty and an extended temperature range. © 2021 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c10026
  • Investigation of the combustion of iron pentacarbonyl and the formation of key intermediates in iron oxide synthesis flames
    Karakaya, Y. and Kluge, S. and Wiggers, H. and Schulz, C. and Kasper, T.
    Chemical Engineering Science 230 (2021)
    The information of the gas phase kinetics are relevant for the development of detailed reaction mechanisms as well as for process design and control in flame synthesis. In this study, the decomposition of iron pentacarbonyl and the reaction pathways towards iron oxide clusters and particles in laminar H2/O2/Ar low-pressure synthesis flames are investigated. Gas-phase species are analyzed by photoionization and electron ionization mass spectrometry. The extraction of a representative sample from the particle-laden flow of a synthesis flame by an intrusive sampling technique for the analysis is challenging, because iron-intermediate species can condense easily. Cations can be extracted from the flame with a high efficient ion sampling technique that results in high sensitivity. Iron-containing cations provide evidence of the presence of key intermediates, e.g., Fe(OH)2, Fe(OH)3, Fe2O3, and larger Fe-O-clusters which are the dominant intermediates with respect to particle formation and need to be considered in future gas-phase reaction mechanisms. © 2020 Elsevier Ltd
    view abstract10.1016/j.ces.2020.116169
  • Kinetics of the Thermal Decomposition of Ethylsilane: Shock-Tube and Modeling Study
    Sela, P. and Peukert, S. and Somnitz, H. and Janbazi, H. and Wlokas, I. and Herzler, J. and Fikri, M. and Schulz, C.
    Energy and Fuels (2021)
    The thermal decomposition of ethylsilane (H3SiC2H5, EtSiH3) is investigated behind reflected shock waves and the gas composition is analyzed by gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) in a temperature range of 990-1330 K and pressure range of 1-2.5 bar. The unimolecular decomposition of EtSiH3 is considered to be initiated via a molecular elimination of H2 (H3SiC2H5 → H2 + HSiC2H5) followed by reactions of cyclic silicon-containing species. The main observed stable products were ethylene (C2H4) and silane (SiH4). Measurements are performed with a large excess of a silylene scavenger (C2H2) to suppress bimolecular reactions caused by silylene (SiH2) and to extract unimolecular rate constants. A kinetics mechanism accounting for the gas-phase chemistry of EtSiH3 is developed, which consists of 24 Si-containing species, 31 reactions of Si-containing species, and a set of new thermochemical data. The derived unimolecular rate constant is represented by the Arrhenius expression kuni(T) = 1.96 × 1012 s-1 exp(-205 kJ mol-1/RT). The experimental data is reproduced very well by simulations based on the mechanism of this work and is in very good agreement with literature values. It is shown that EtSiH3 is a promising precursor for the synthesis of SiC nanoparticles. © 2021 American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03425
  • Large-scale synthesis of iron oxide/graphene hybrid materials as highly efficient photo-Fenton catalyst for water remediation
    Hammad, M. and Fortugno, P. and Hardt, S. and Kim, C. and Salamon, S. and Schmidt, T.C. and Wende, H. and Schulz, C. and Wiggers, H.
    Environmental Technology and Innovation 21 (2021)
    The Photo-Fenton reaction is an advanced oxidation process to break down organic pollutants in aqueous systems. Moreover, the scalable synthesis and engineering of stable catalysts with a high specific surface area is extremely important for the practical application of the Photo-Fenton process. In the current study, we developed a low-cost method for large-scale production of iron oxide/graphene nanostructures with a controllable graphene loading for the photo-Fenton reaction. Under optimal condition, high efficiencies of degradation (>99%) of methylene blue, rhodamine B, acid orange 7, and phenol at a concentration (60 mg/mL) were reached in 60 min under UV-A irradiation (1.6 mW/cm2) with mineralization of 72, 77, 82, and 48%, respectively. More importantly, the iron oxide/graphene nanocomposites exhibited good stability over a wide range of pH (from 3 to 9) and can be magnetically separated from the solution and repeatedly used with consistent photocatalytic performance. This enhanced removal efficiency of the iron oxide/graphene nanostructure compared to iron oxide nanoparticles is attributed to the accelerated transfer of photo-generated electrons between iron oxide and graphene and its relatively large surface area. The results demonstrate that the iron oxide/graphene system could be potentially utilized for many environmental treatment processes. © 2020 Elsevier B.V.
    view abstract10.1016/j.eti.2020.101239
  • Low-temperature and low-pressure effective fluorescence lifetimes and spectra of gaseous anisole and toluene
    Beuting, M. and Dreier, T. and Schulz, C. and Endres, T.
    Applied Physics B: Lasers and Optics 127 (2021)
    Fluorescence spectra and lifetimes of anisole and toluene vapor in nitrogen have been measured at conditions below ambient (257–293 K and 100–2000 mbar) upon excitation with 266-nm laser light to expand the applicable range of anisole and toluene laser-induced fluorescence (LIF) for conditions below room temperature that occur in expanding flows and cases with strong evaporative cooling. Anisole fluorescence spectra broaden with decreasing pressure while fluorescence lifetimes decrease simultaneously. This is consistent with a more pronounced effect of internal vibrational redistribution on the overall fluorescence signal and can be explained by significantly reduced collision rates. In the case of toluene, the transition from photo-induced heating to photo-induced cooling was observed for the first time for 266 nm. The data confirm predictions of earlier work and is particularly important for the advancement of the available photo-physical (step-ladder) models: since those transitions mark points where the molecules are already thermalized after excitation (i.e., no vibrational relaxation occurs during deactivation), they are important support points for fitting empirical parameters and allow analytical determination of the ground state energy transferred to the excited state. The data enable temperature and/or pressure sensing, e.g., in accelerating cold flows using laser-induced fluorescence of both tracers. © 2021, The Author(s).
    view abstract10.1007/s00340-021-07605-w
  • Multi-line SiO fluorescence imaging in the flame synthesis of silica nanoparticles from SiCl4
    Moussawi, A.E. and Endres, T. and Peukert, S. and Zabeti, S. and Dreier, T. and Fikri, M. and Schulz, C.
    Combustion and Flame 224 (2021)
    Flame synthesis is a powerful and scalable method for generating nanoparticles for a wide range of applications. The chemical interaction of the flame and the precursor combined with the spatial and temporal temperature distribution determine the product properties. For controlled nanoparticle synthesis that can also be scaled to industrial production rates, detailed knowledge of the underlying chemical kinetics and their interaction with the reactive flow is essential. Laser diagnostics has the capability to analyze the process by probing the concentration of important intermediates in shock tubes and reactive flows. The gas-phase synthesis of silica nanoparticles from SiCl4 in a premixed H2/O2 low-pressure flame reactor is studied by laser-induced fluorescence imaging of SiO mole fractions and temperature. The literature value-based spectroscopy model of SiO used for fitting the LIF spectra are validated based on absorption cross-sections measurements in a shock tube, where SiO is formed under precisely defined conditions (temperature, pressure, mole fraction) using a well-known kinetics mechanism for SiH4/CO2/Ar decomposition. Based on literature sources, a reaction mechanism is assembled to describe the oxidation of SiCl4 in the flame, which is then compared to the measured SiO mole fractions distribution to shed light on the current state of the understanding of SiCl4 combustion chemistry and to direct further refinements. © 2020 The Author(s)
    view abstract10.1016/j.combustflame.2020.12.020
  • Near-threshold soot formation in premixed flames at elevated pressure
    Mi, X. and Saylam, A. and Endres, T. and Schulz, C. and Dreier, T.
    Carbon 181 (2021)
    Soot formation at lean-threshold conditions referred to as “near-threshold sooting conditions” (i.e., with stoichiometry, φ, around 1.90 for ethene as a fuel) are studied in laminar premixed ethylene/air flames at pressure from 1 to 10 bar. Laser extinction is used to measure the soot volume fraction. Time-resolved laser-induced incandescence (TiRe-LII) is used to determine particle diameters from the LII signal temporal decay after pulsed laser heating. Thermophoretic sampling is applied to extract particle samples from the flame and ex situ transmission electron microscopy (TEM) is used to measure particle sizes and morphology. The soot volume fraction scales with pressure in a power-law function with the parameter n as 1.4 to 1.9 for flames at the equivalence ratio (φ = 2.1) even at the onset of soot formation. The elevated dependence of soot volume fraction on height above burner is detected with increasing pressure in the near-threshold sooting conditions. The measured soot diameter increases with pressure and equivalence ratio and its sensitivity to the equivalence ratio increases with increasing pressure. The TiRe-LII signal decay varies only little with height above burner and laser fluence in the near-threshold sooting flame (φ = 1.90–1.95), which indicates that the soot particle surface growth and oxidation are balanced. For a slightly sooting flame, TEM measurements from thermophoretically-sampled soot agree well with the LIIsim-evaluated particle size, indicating the reliability of TiRe-LII particle diameter determination under near-threshold conditions. © 2021 Elsevier Ltd
    view abstract10.1016/j.carbon.2021.05.014
  • Numerical Investigation of Remote Ignition in Shock Tubes
    Lipkowicz, J.T. and Nativel, D. and Cooper, S. and Wlokas, I. and Fikri, M. and Petersen, E. and Schulz, C. and Kempf, A.M.
    Flow, Turbulence and Combustion 106 (2021)
    Highly resolved two- and three-dimensional computational fluid dynamics (CFD) simulations are presented for shock-tube experiments containing hydrogen/oxygen (H2/O2) mixtures, to investigate mechanisms leading to remote ignition. The results of the reactive cases are compared against experimental results from Meyer and Oppenheim (Proc Combust Inst 13(1): 1153–1164, 1971. https://doi.org/10.1016/s0082-0784(71)80112-1) and Hanson et al. (Combust Flame 160(9): 1550–1558, 2013. https://doi.org/10.1016/j.combustflame.2013.03.026). The results of the non-reactive case are compared against shock tube experiments, recently carried out in Duisburg and Texas. The computational domain covers the end-wall region of the shock tube and applies high order numerics featuring an all-speed approximate Riemann scheme, combined with a 5th order interpolation scheme. Direct chemistry is employed using detailed reaction mechanisms with 11 species and up to 40 reactions, on a grid with up to 2.2 billion cells. Additional two-dimensional simulations are performed for non-reactive conditions to validate the treatment of boundary-layer effects at the inlet of the computational domain. The computational domain covers a region at the end part of the shock tube. The ignition process is analyzed by fields of localized, expected ignition times. Instantaneous fields of temperature, pressure, entropy, and dissipation rate are presented to explain the flow dynamics, specifically in the case of a bifurcated reflected shock. In all cases regions with locally increased temperatures were observed, reducing the local ignition-delay time in areas away from the end wall significantly, thus compensating for the late compression by the reflected shock and therefore leading for first ignition at a remote location, i.e., away from the end wall where the ignition would occur under ideal conditions. In cases without a bifurcated reflected shock, the temperature increase results from shock attenuation. In cases with a bifurcated reflected shock, the formation of a second normal shock and shear near the slip line is found to be crucial for the remote ignition to take place. Overall, the two- and three-dimensional simulations were found to qualitatively explain the occurrence of remote ignition and to be quantitatively correct, implying that they include the correct physics. © 2020, The Author(s).
    view abstract10.1007/s10494-020-00219-w
  • Phase-sensitive detection of gas-borne Si nanoparticles via line-of-sight UV/VIS attenuation
    Asif, M. and Menser, J. and Endres, T. and Dreier, T. and Daun, K. and Schulz, C.
    Optics Express 29 (2021)
    The distinct optical properties of solid and liquid silicon nanoparticles are exploited to determine the distribution of gas-borne solid and liquid particles in situ using line-of-sight attenuation measurements carried out across a microwave plasma reactor operated at 100 mbar. The ratio between liquid and solid particles detected downstream of the plasma varied with measurement location, microwave power, and flow rate. Temperatures of the liquid particles were pyrometrically-inferred using a spectroscopic model based on Drude theory. The phase-sensitive measurement supports the understanding of nanoparticle formation and interaction and thus the overall gas-phase synthesis process. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstract10.1364/OE.426528
  • Plug-flow reactor and shock-tube study of the oxidation of very fuel-rich natural gas/DME/O2 mixtures
    Kaczmarek, D. and Herzler, J. and Porras, S. and Shaqiri, S. and Fikri, M. and Schulz, C. and Atakan, B. and Maas, U. and Kasper, T.
    Combustion and Flame 225 (2021)
    A polygeneration process with the ability to provide work, heat, and useful chemicals according to the specific demand is a promising alternative to traditional energy conversion systems. By implementing such a process in an internal combustion engine, products like synthesis gas or unsaturated hydrocarbons and very high exergetic efficiencies can be obtained through partial oxidation of natural gas, in addition to the already high flexibility with respect to the required type of energy. To enable compression ignition with natural gas as input, additives such as dimethyl ether are needed to increase the reactivity at low temperatures. In this study, the reaction of fuel-rich natural gas/dimethyl ether (DME) mixtures is investigated to support the further development of reaction mechanisms for these little studied reaction conditions. Temperature-resolved species concentration profiles are obtained by mass spectrometry in a plug-flow reactor at equivalence ratios ϕ = 2, 10, and 20, at temperatures between 473 and 973 K and at a pressure of 6 bar. Ignition delay times and product-gas analyses are obtained from shock-tube experiments, for ϕ = 2 and 10, at 710 – 1639 K and 30 bar. The experimental results are compared to kinetic simulations using two literature reaction mechanisms. Good agreement is found for most species. Reaction pathways are analyzed to investigate the interaction of alkanes and DME. It is found that DME forms radicals at comparatively low temperatures and initiates the conversion of the alkanes. Additionally, according to the reaction pathways, the interaction of the alkanes and DME promotes the formation of useful products such as synthesis gas, unsaturated hydrocarbons and oxygenated species. © 2020
    view abstract10.1016/j.combustflame.2020.10.004
  • Pyrolysis of diethyl carbonate: Shock-tube and flow-reactor measurements and modeling
    Sela, P. and Zhang, Y. and Herzler, J. and Fikri, M. and Schulz, C. and Peukert, S.
    Proceedings of the Combustion Institute 38 (2021)
    Shock-tube and flow-reactor experiments were applied to investigate the thermal decomposition of diethyl carbonate (DEC). The formation of CO2, C2 H2 , and C2H5H was measured with GC/MS and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) behind reflected shock waves. The same products were also detected by GC/MS in flow reactor experiments. All experiments combined span a temperature range of 663 K–1203 K at pressures between 1.0 and 2.0 bar. Time-resolved species concentration profiles from HRR-TOF-MS and product compositions from GC/MS measurements were simulated applying a detailed reaction mechanism for DEC combustion. A master-equation analysis was conducted based on computed energies from G4 calculations. Quantum chemical calculations confirm that DEC primarily decomposed by six-center elimination followed by rapid decomposition of the alkoxy acid. Measured DEC decomposition rate constants k (T) at p ≈1.5 bar could be represented by the Arrhenius equation. The theoretical analysis also included dipropyl carbonate decomposition and the reactivities of DEC and DPC were compared and considered in the context of reactivity of dialkyl carbonates under pyrolytic conditions.
    view abstract10.1016/j.proci.2020.07.052
  • Room-temperature Fe:ZnSe laser tunable in the spectral range of 3.7–5.3 µm applied for intracavity absorption spectroscopy of CO2 isotopes, CO and N2O
    Fjodorow, P. and Frolov, M.P. and Korostelin, Y.V. and Kozlovsky, V.I. and Schulz, C. and Leonov, S.O. and Skasyrsky, Y.K.
    Optics Express 29 (2021)
    We demonstrate an intracavity absorption spectroscopy system based on a broadband single-crystal pulsed Fe:ZnSe laser. The laser operates at room-temperature and is continuously tunable in the spectral range of 3.76–5.29 µm. The long-wavelength emission up to 5.29 µm is a record achievement for Fe:ZnSe lasers, to the best of our knowledge. The developed laser system is applied for measurements of gaseous absorption inside the laser resonator. We demonstrate sensitive detection of (i) CO2 isotopes in the atmosphere and in human breath, (ii) CO in breath (after cigarette smoking) and in the smoke of a smoldering paper, and (iii) N2O in a gas flow. The achieved detection limits are: 0.1 ppm for 12CO2 and 13CO2, 3 ppm for CO, and 1 ppm for N2O. The sensitivity of the current system is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 107 are discussed. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstract10.1364/OE.422926
  • Spatial distribution of gas-phase synthesized germanium nanoparticle volume-fraction and temperature using combined in situ line-of-sight emission and extinction spectroscopy
    Liu, G. and Asif, M. and Menser, J. and Dreier, T. and Mohri, K. and Schulz, C. and Endres, T.
    Optics Express 29 (2021)
    In this study, emission and extinction spectroscopy were combined to in situ measure temperature and volume fraction distributions of liquid germanium nanoparticle gas-phase synthesized in an argon/hydrogen/germane flow through a microwave plasma. Emission of the hot particles and extinction against a continuous background were recorded by a spectrometer in the 380-703 nm and 230-556 nm ranges, respectively, selected based on the specific optical properties of the material. Absorption coefficients were deconvoluted from line-of-sight attenuation (LOSA) measurements by a least-square algorithm and then used to determine the local volume fraction distribution. The temperature field was derived from the line-of-sight emission (LOSE) spectra with the prior knowledge of absorption coefficients. A multi-wavelength reconstruction model was developed for the determination of the spatially-resolved distribution of the measured quantities assuming a stationary axisymmetric flow. Advantages of the method include experimental simplicity, low cost, and adaptability to up-scaled reactor sizes. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstract10.1364/OE.418922
  • Spray-flame synthesis of LaMO3(M = Mn, Fe, Co) perovskite nanomaterials: Effect of spray droplet size and esterification on particle size distribution
    Angel, S. and Schneider, F. and Apazeller, S. and Kaziur-Cegla, W. and Schmidt, T.C. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute 38 (2021)
    Perovskite nanomaterials such as LaMnO3, LaFeO3, and LaCoO3were synthesized in a spray flame from metal nitrates dissolved in combustible liquids. The addition of low-boiling solvents such as 2-ethylhexanoic acid (2-EHA) to the ethanol-based solutions supports the formation of phase-pure particles with unimodal particle-size distribution in the 10-nm range attributed to enhanced evaporation through micro-explosions. Nevertheless, in many cases, a second particle mode with sizes of a few hundred nanometers is formed. In this paper, we investigate two possible reasons for the appearance of large particles. Firstly, we analyze the effect of the oxygen dispersion gas flow applied in the two-fluid nozzle on the droplet size distributions of burning sprays using phase Doppler anemometry. We identified that an increase of the dispersion gas flow significantly decreases the number concentration of large droplets (>30 μm), which causes a significant increase of the BET surface area of as-synthesized LaMnO3and LaCoO3with increasing dispersion gas flow from 60 m2/g (5 slm dispersion gas) to 100 m2/g (8 slm). Secondly, the esterification in the mixture of solvents towards ethyl-2-ethylhexanoate, which is associated with the release of water as a byproduct, was analyzed by GC/MS. The ester concentration in the iron-containing solution was found to be up to nine times higher than in cobalt or manganese precursor solutions. Simultaneously, the produced LaFeO3materials show lower BET surface areas and the increasing dispersion gas flow has a minor effect on this material than on the cobalt and manganese perovskite cases. We attribute this to the fact that water formed during esterification forces the hydrolysis of iron nitrate and the formation of large particles within the droplets. © 2021 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.proci.2020.07.116
  • Survivability of the thermographic phosphors YAG:Pr and SMP:Sn in a premixed flame
    Kopf, A. and Bardi, M. and Kohler, E. and Endres, T. and Bruneaux, G. and Schulz, C.
    Measurement Science and Technology 32 (2021)
    The survivability of two thermographic phosphors, YAG:Pr3+ and SMP:Sn2+ ((Sr,Mg)3(PO4)2:Sn2+), in a combustion environment is studied in the post-flame regime of a premixed propane/air flame. While the laser-induced luminescence of YAG:Pr3+ can be exploited for in situ temperature measurements in the exhaust gas above the flame, there is no in situ detectable luminescence for SMP:Sn2+ for any reaction conditions within the stability limits of the flame. The phosphor particles are recaptured above the flame and ex situ analyzed for chemical or structural changes using microscopic analysis (SEM/EDX) and x-ray diffraction. The microscopic analysis of post-flame YAG:Pr3+ does not show any alteration, whereas morphology and chemical composition of SMP:Sn2+ have been modified upon passing through the reaction zone, which is responsible for the loss of its luminescence properties. © 2021 IOP Publishing Ltd.
    view abstract10.1088/1361-6501/abf57b
  • Virtual Special Issue of Recent Advances in Gas-Phase Synthesis of Functional Materials for Energy
    Schulz, C. and Li, S.
    Energy and Fuels 35 (2021)
    view abstract10.1021/acs.energyfuels.1c00796
  • A group additivity methodology for predicting the thermochemistry of oxygen-containing organosilanes
    Janbazi, H. and Schulz, C. and Wlokas, I. and Wang, H. and Peukert, S.
    International Journal of Chemical Kinetics 52 (2020)
    A combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group-additivity values (GAVs) for this class of molecules. The reference species include 62 stable single-bonded, 19 cyclic, and nine double-bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature. © 2020 The Authors. International Journal of Chemical Kinetics published by Wiley Periodicals LLC
    view abstract10.1002/kin.21410
  • A six-compound, high performance gasoline surrogate for internal combustion engines: Experimental and numerical study of autoignition using high-pressure shock tubes
    Cancino, L.R. and da Silva, A., Jr. and De Toni, A.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C. and Curran, H.J.
    Fuel 261 (2020)
    This paper presents experimental and modeling data for the autoignition of a novel, six-component, high performance gasoline surrogate fuel comprising ethanol, n-heptane, i-octane, 1-hexene, methylcyclohexane, and toluene (AL-P-I-O-N-A). Experimental tests are conducted in two high-pressure shock tubes to determine the ignition delay time as a function of pressure, temperature and equivalence ratio. Ignition delay times were measured at 10 and 30 bar in the temperature range from 749 to 1204 K and equivalence ratios ranging from 0.35 to 1.30. A modified Arrhenius equation is defined to mathematically describe the ignition delay time of the proposed surrogate. For experimental data with temperatures higher than 900 K, a multiple linear regression identified the pressure dependence exponent of 0.72 and stoichiometry dependence exponent of 0.62, as well as a global activation energy of ≈109 kJ/mol. A simplistic approach to mechanism reduction based on the elimination of reactions with no relevant rate of progress was used in order to reduce an extensive detailed kinetics model (hierarchically constructed with more than 17800 reactions). The reduced detailed kinetics model with 4885 elementary reactions among 326 chemical species was used for numerical simulations. Comparisons between the experimental and numerical data are favorable, with the predictions using the reduced kinetics model differing by less than 0.056% when compared to the complete mechanism. It was observed that for low temperatures the proposed reduced kinetics model agrees only qualitatively with the measurements. In order to understand the likely cause of this discrepancy a brute force sensitivity analysis on IDT was performed, elucidating the more influencing reactions on the ignition delay times. The experimental data obtained in this research was compared to available data in the literature in terms of anti-knock index (AKI) and for a scaled pressure of 30 bar (τ30) at a stoichiometric composition. A modified Arrhenius equation was then fitted and an AKI dependence exponent of -1.11 was obtained, inferring that the higher the AKI the higher the IDT, independent of fuel composition at temperatures lower than the NTC region. This trend should be confirmed by further studies. © 2019 Elsevier Ltd
    view abstract10.1016/j.fuel.2019.116439
  • An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
    Porras, S. and Kaczmarek, D. and Herzler, J. and Drost, S. and Werler, M. and Kasper, T. and Fikri, M. and Schießl, R. and Atakan, B. and Schulz, C. and Maas, U.
    Combustion and Flame 212 (2020)
    Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions. © 2019 The Authors
    view abstract10.1016/j.combustflame.2019.09.036
  • Characterization of few-layer graphene aerosols by laser-induced incandescence
    Musikhin, S. and Fortugno, P. and Corbin, J.C. and Smallwood, G.J. and Dreier, T. and Daun, K.J. and Schulz, C.
    Carbon 167 (2020)
    Gas-phase synthesis is a promising route for producing large amounts of high quality few-layer graphene (FLG) nanoparticles economically, but optimizing these processes requires a detailed understanding of the formation kinetics, which in turn demands diagnostics for characterizing this material in situ. This work reports the first laser-induced incandescence measurements on FLG aerosols. Temporally- and spectrally-resolved incandescence signals from FLG particles are measured and used to calculate pyrometric temperatures. Differences between incandescence signals and pyrometric temperatures obtained from FLG and aerosolized soot nanoaggregates are attributed to the larger absorption cross-section and specific surface area of FLG compared to soot. LII signal intensity is found to vary linearly with particle number concentration measured independently by a condensation particle counter. Overall, these results demonstrate the potential for laser-induced incandescence to measure FLG nanoparticle mass (volume) fraction and active surface area in situ, as well as to differentiate graphene from other types of carbonaceous nanomaterials online. © 2020 Elsevier Ltd
    view abstract10.1016/j.carbon.2020.05.052
  • Characterization of tracers for two-color laser-induced fluorescence liquid-phase temperature imaging in sprays
    Prenting, M.M. and Bin Dzulfida, M.I. and Dreier, T. and Schulz, C.
    Experiments in Fluids 61 (2020)
    Abstract: The variation of the fluorescence spectral signature of tracer solutions with temperature enables temperature imaging measurements in liquids and sprays by simultaneously recording and rationing the fluorescence intensity detected in two separate wavelength channels. In this work, we recorded fluorescence spectra of ethanol-based solutions of nine laser dyes used as tracers (PTP, stilbene 1, coumarin 152, coumarin 153, rhodamine B, rhodamine 101, pyrromethene 597, DCM, and pyridine 1) after excitation at either 266, 355, or 532 nm (depending on the dye) for temperatures between 298 and 348 K (close to the boiling point of the solvent), and for concentrations (depending on dye) around 10 mg/l (i.e., ~ 10–5 mol/l). The influence of signal self-absorption was investigated for the tracers best suited for thermometry, rhodamine B and coumarin 152, where the latter is almost unaffected due to its large Stokes shift. In thin-film (100 µm) cells, possible concentration effects on the fluorescence spectrum were investigated in the absence of signal self-absorption in the 0.1–10 and 0.5–50 mg/l range for rhodamine B and coumarin 152, respectively. Sensitivities of the two-color intensity ratios were determined for two selected color detection channels for each tracer characterized by their center wavelength and spectral half width and conditioned on averaged intensities of larger than 10% of the spectral peak of their respective fluorescence spectrum. The use of coumarin 152 that showed the overall best spectroscopic properties was demonstrated for temperature imaging in a burning ethanol spray. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00348-020-2909-9
  • CO-concentration and temperature measurements in reacting CH4/O2 mixtures doped with diethyl ether behind reflected shock waves
    He, D. and Shi, L. and Nativel, D. and Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 (2020)
    The oxidation of CH4/diethyl ether mixtures was studied with laser absorption-based time-resolved temperature and CO concentration measurements behind reflected shock waves. Fuel-rich (equivalence ratio ϕ = 2.0) mixtures were studied because of their relevance for mechanism development for partial oxidation reactions in the context of polygeneration processes and measurements at ϕ = 0.5 and 1.0 were used to verify the mechanism performance in an extended range of equivalence ratios. Temperature and CO concentration were measured via absorption using two fundamental vibrations of CO (ν" = 0, P20 and ν" = 1, R21) with two mid-IR quantum-cascade lasers near 4.8546 and 4.5631 µm. Interference from broadband absorption of CO2 in the region near 4.56 µm was quantified based on measured temperature-dependent CO2 absorption cross-sections and mechanism-based prediction of CO2 concentrations. The measured temporal CO-concentration and temperature profiles were compared with simulations based on two mechanisms (Fikri et al., 2017; Yasunaga et al., 2010). For mixtures with ϕ = 0.5, the two mechanisms show similar results, and well reproduce the experimental data. At ϕ = 1.0 and 2.0, the Fikri et al. mechanism shows very good agreement with the experiments whereas the Yasunaga et al. mechanism predicts a too fast CO-concentration and temperature rise. © 2020 The Combustion Institute
    view abstract10.1016/j.combustflame.2020.02.024
  • Determination of gas-phase absorption cross-sections of FeO in a shock tube using intracavity absorption spectroscopy near 611 nm
    Fjodorow, P. and Lalanne, M.R. and He, D. and Nanjaiah, M. and Pilipodi-Best, A. and Baev, V.M. and Cheskis, S. and Herzler, J. and Fikri, M. and Wlokas, I. and Schulz, C. and Rahinov, I.
    Proceedings of the Combustion Institute (2020)
    We report state-resolved absorption cross-section measurement and oscillator-strength evaluation of the gas-phase iron oxide (FeO) orange system near 611 nm. Intracavity absorption spectroscopy (ICAS) with a homemade broadband dye laser was applied for time-resolved measurements of absorption spectra of shock-activated mixtures of iron pentacarbonyl and carbon dioxide (diluted in argon), generating gas-phase FeO. The measurements were performed with a time resolution of 170 μs in the spectral range of 16,316-16,353 cm-1 that includes a large number of FeO absorption lines. Across the 8-cm diameter of the shock tube, ICAS leads to an effective absorption path length of 260 m. Absorption cross-section values of 0.5 × 10-18-4 × 10-18 cm2 were determined for temperatures around 2200 K and pressures of ~1.3 bar. Pressure- and temperature-independent oscillator strengths for individual ro-vibronic transitions within the 611-nm band of FeO orange system are reported for the first time. These data are generally applicable for quantitative absorption measurements of flame studies of iron chemistry, where FeO plays a key role as intermediate species. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.06.251
  • Ethanol ignition in a high-pressure shock tube: Ignition delay time and high-repetition-rate imaging measurements
    Nativel, D. and Niegemann, P. and Herzler, J. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute (2020)
    Ethanol is known to be prone to pre-ignition in internal combustion engines under high-load conditions and its ignition shows large deviations from ideal, spatially, and temporally-homogeneous ignition in shock tubes at moderate temperatures (800-950 K). In this context, the ignition of stoichiometric ethanol/O 2 mixtures with various levels of inert gas dilution was investigated in a high-pressure shock tube at =20 bar between 800 and 1250 K. Ignition delay times were determined from spatially integral detection of chemiluminescence emission. Additionally, high-repetition-rate color imaging enabled the differentiation of the luminescence in time, space, and spectral range between various ignition modes. In the low-temperature range (800-860 K), different inhomogeneous ignition modes were identified. The addition of small amounts of helium into the undiluted fuel/air mixture was found to be efficient to mitigate pre-ignition, attributed to a variation in heat transfer and thus suppression of the build-up of local temperature inhomogeneities. The experiments in case of spatially homogeneous ignition show very good agreement with the predictions based on three detailed kinetics mechanisms (Zhang et al., CNF 190 (2018) 74, Frassoldati et al., CNF 159 (2012) 2295, and Zhou et al. CNF 197 (2018) 423), inhomogeneities, however, resulted in a shortening of the ignition delay times up to a factor of 2.6. © 2020 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2020.07.021
  • Experimental and numerical investigation of iron-doped flames: FeO formation and impact on flame temperature
    Nanjaiah, M. and Pilipodi-Best, A. and Lallane, M. and Fjodorow, P. and Schulz, C. and Cheskis, S. and Kempf, A. and Wlokas, I. and Rahinov, I.
    Proceedings of the Combustion Institute (2020)
    Gas-phase iron compounds strongly affect the flame structure already at very low concentrations, which implies the control of combustion efficiency, pollution formation, and materials synthesis in flames. The impact of iron pentacarbonyl on low-pressure premixed flames was investigated experimentally and numerically for a broad range of equivalence ratios. The burner was operated in top-to-bottom orientation, causing a strong effect of buoyancy on the flow field, a configuration, also known as buoyancy-opposed flame. The application of ultra-sensitive broadband intracavity laser absorption diagnostics enabled path-integrated measurements of gas-phase FeO in the particle-laden flow. Spatially-resolved temperature distributions were measured via OH laser-induced fluorescence. The measurements were complemented by detailed simulations of the down-firing flame to determine the (one-dimensional) flow field on the centerline of the burner. The experimental findings were the basis for extension of existing reaction schemes for iron-doped flames and a new skeletal scheme was proposed. Measured temperatures and normalized FeO concentrations were used to validate both the detailed and the skeletal scheme. The results of the optimization and reduction procedure helped to improve the understanding of the structure of the iron-doped flame and the role of iron-cluster formation in the interaction mechanisms which cause the flame inhibition or promotion by iron-compounds. © 2020 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2020.07.006
  • Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor
    Atakan, B. and Kaiser, S.A. and Herzler, J. and Porras, S. and Banke, K. and Deutschmann, O. and Kasper, T. and Fikri, M. and Schießl, R. and Schröder, D. and Rudolph, C. and Kaczmarek, D. and Gossler, H. and Drost, S. and Bykov, V. and Maas, U. and Schulz, C.
    Renewable and Sustainable Energy Reviews 133 (2020)
    Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. © 2020 The Authors
    view abstract10.1016/j.rser.2020.110264
  • Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance
    Hammad, M. and Hardt, S. and Mues, B. and Salamon, S. and Landers, J. and Slabu, I. and Wende, H. and Schulz, C. and Wiggers, H.
    Journal of Alloys and Compounds 824 (2020)
    Magnetic nanoparticle-mediated hyperthermia has shown great potential in cancer therapy. However, upscaling of the synthesis of iron oxide nanoparticle with the required narrow size distribution remains challenging. This paper describes the reproducible and scalable synthesis of citric acid-functionalized iron oxide nanoparticles optimized for hyperthermia treatment. Iron oxide nanoparticles were synthesized by a spray flame method, which is eco-friendly and cost-effective. To the best of our knowledge, this is the first study reporting spray-flame synthesis of small iron oxide nanoparticles (approx. 7 nm) with narrow size distribution (polydispersity index ≪ 0.1). The citric acid-coated iron oxide nanoparticles revealed a hydrodynamic size of approx. 37 nm and a high magnetic saturation of 69 Am2/kg at room temperature. The magnetic hyperthermia study showed a significantly enhanced value of the intrinsic loss power (4.8 nHm2/kg), which is 1.5-fold higher than the best commercially available equivalents. The improved heating efficiency and small hydrodynamic size of citric acid-coated iron oxide nanoparticles demonstrate that the system could potentially be used as a nanoplatform for hyperthermia treatment. © 2020 Elsevier B.V.
    view abstract10.1016/j.jallcom.2020.153814
  • High-pressure shock-tube study of the ignition and product formation of fuel-rich dimethoxymethane (DMM)/air and CH4/DMM/air mixtures
    Herzler, J. and Fikri, M. and Schulz, C.
    Combustion and Flame 216 (2020)
    Ignition delay times (IDTs) of fuel-rich CH4/dimethoxymethane (DMM)/air mixtures (ϕ = 2 and 10) were measured in a high-pressure shock tube at a pressure of 30 bar and compared to simulations based on reaction mechanisms for DMM from literature. Additionally, IDT of DMM/air mixtures were measured at similar conditions in a wide range of equivalence ratios (ϕ = 0.5, 1, and 2). Those mechanisms that predict the IDTs of DMM within the experimental uncertainties also predict the IDTs of the fuel-rich CH4/DMM/air mixtures very well although they were not designed for these conditions. For the measurements at ϕ = 10, product gas samples were extracted from the shock tube test section at around 14–24 ms after arrival of the reflected shock wave by a fast-opening valve and analyzed with gas chromatography. Besides CO, H2, and H2O, ethane, ethylene, acetylene, benzene, propene and toluene were observed as main reaction products. © 2020 The Combustion Institute
    view abstract10.1016/j.combustflame.2020.03.008
  • Impact of shock-tube facility-dependent effects on incident- and reflected-shock conditions over a wide range of pressures and Mach numbers
    Nativel, D. and Cooper, S.P. and Lipkowicz, T. and Fikri, M. and Petersen, E.L. and Schulz, C.
    Combustion and Flame 217 (2020)
    In real shock tubes, deviations from the ideal gas-dynamic behavior can affect experiments and complicate data analysis and interpretation. These non-ideal effects depend on the shock-tube geometry and therefore, results (e.g., ignition delay times) may vary between different experimental facilities. To clarify the influence of geometry and operating procedures, these effects were investigated in four geometrically different shock tubes located in two laboratories, Texas A&M University and the University of Duisburg-Essen. Incident shock-wave attenuation and pressure rise (dp*/dt) were measured behind reflected shock waves over a 2.1–4.1 Mach number and a 0.1–3.0 MPa post-reflected-shock pressure range. A strong influence of the Mach number on dp*/dt was observed for all facilities and conditions, whereas only a slight influence was found for shock-wave attenuation. Both dp*/dt and attenuation were higher by about a factor of two for the shock tubes with approximately half the inner diameter (8.0 vs. 16.2 cm). These findings are analyzed through correlations with initial pressure, inner diameter, Mach number, and specific heat ratio. The implication of non-ideal effects on experiments with reactive mixtures and related combustion experiments is discussed. Extreme conditions of dp*/dt were derived from the correlations and used to understand the effects of an equivalent dT*/dt on simulated ignition delay times of two reactive systems (CH4/air and C7H16/air). It was found that smaller shock-tube diameters with respectively larger dp*/dt show shorter ignition delay times (especially at temperatures below 1000 K for the C7H16/air case). Therefore, the geometry constraints must be considered in simulations through dp*/dt inputs in the chemical kinetics simulation for the extreme cases to account for non-ideal effects. © 2020
    view abstract10.1016/j.combustflame.2020.03.023
  • Laser-based CO concentration and temperature measurements in high-pressure shock-tube studies of n-heptane partial oxidation
    He, D. and Nativel, D. and Herzler, J. and Jeffries, J.B. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 126 (2020)
    This paper presents a laser-based absorption technique for measuring temperature and CO concentration in high-pressure shock tubes. Two fundamental vibrations of CO (v" = 0, P8, 4.73 µm and v" = 1, R21, 4.56 µm) were selected for high-temperature sensitivity with a reduced influence from pressure broadening compared to previous work. Single-pass absorption (80 mm path length) was measured with two quantum-cascade lasers. The technique was demonstrated by measuring time-resolved temperature for non-reactive mixtures at 1100–1960 K and 1.2–9.7 bar. During partial oxidation of n-heptane, temperature and CO concentrations were measured with 4 µs time resolution at 1360–1670 K and 5.8–8.2 bar. Interference from broadband CO2 absorption was quantified and subtracted. Measured data in the burnout state are in excellent agreement with predictions from kinetics mechanisms (Mehl et al. Proc Combust Inst 33:193, 2011; Zhang et al. Combust Flame 172:116, 2016) over the entire range of operating conditions, which validates the performance of the current laser-absorption technique in reactive-mixture measurements. Additionally, time-resolved temperature and CO-concentration measurements agree well with predictions based on the Mehl et al. mechanism. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-020-07492-7
  • Monitoring formaldehyde in a shock tube with a fast dual-comb spectrometer operating in the spectral range of 1740–1790 cm–1
    Fjodorow, P. and Allmendinger, P. and Horvath, R. and Herzler, J. and Eigenmann, F. and Geiser, M. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 126 (2020)
    A dual-frequency-comb spectrometer based on two quantum-cascade lasers is applied to kinetics studies of formaldehyde (HCHO) in a shock tube. Multispectral absorption measurements are carried out in a broad spectral range of 1740–1790 cm–1 at temperatures of 800–1500 K and pressures of 2–3 bar. The formation of HCHO from thermal decomposition of 1,3,5-trioxane (C3H6O3, 0.9% diluted in argon) and the subsequent oxidation of formaldehyde is monitored with a time resolution of 4 µs. The rate coefficient of the decomposition of C3H6O3 (i.e., HCHO formation) is found to be k1 = 6.0 × 1015 exp(− 205.58 kJ mol−1/RT) s–1. For the oxidation studies, mixtures of 0.36% C3H6O3 and 1% O2 in argon are used. The information of all laser lines, along with the consideration of individual signal variance of each line, is utilized for kinetic and spectral analysis. The experimental kinetic profiles of HCHO are compared with simulations based on the mechanisms of Zhou et al. (Combust Flame, 197:423–438, 2018) and Cai and Pitsch (Combust Flame, 162:1623–1637, 2015). © 2020, The Author(s).
    view abstract10.1007/s00340-020-07545-x
  • Selective cyclohexene oxidation with O2, H2O2and: Tert -butyl hydroperoxide over spray-flame synthesized LaCo1- xFexO3nanoparticles
    Büker, J. and Alkan, B. and Fu, Q. and Xia, W. and Schulwitz, J. and Waffel, D. and Falk, T. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 10 (2020)
    The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with environmentally friendly oxidizing agents such as O2, H2O2 and tert-butyl hydroperoxide (TBHP) has great potential to replace existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo1-xFexO3 catalysts was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity and allylic selectivity, but O2 resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe substitution was studied, which did not show any beneficial synergistic effects. LaCoO3 was found to be the optimum catalyst for cyclohexene oxidation with O2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol-1. The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure. 7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with 2-cyclohexene-1-hydroperoxide. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0cy00906g
  • Self-assembled nano-silicon/graphite hybrid embedded in a conductive polyaniline matrix for the performance enhancement of industrial applicable lithium-ion battery anodes
    Wiggers, H. and Sehlleier, Y.H. and Kunze, F. and Xiao, L. and Schnurre, S.M. and Schulz, C.
    Solid State Ionics 344 (2020)
    Nano-structured silicon-based composite materials have generated significant excitement for use as anode materials in high-performance Li-ion batteries. For making these materials commercially applicable, a high Coulombic efficiency at the first cycle must be achieved. Additionally, scalable synthesis routes need to be developed to provide access to practically-relevant material quantities. In this work, we propose a strategy for the production of Si/graphite/polyaniline (Si/graphite/PANI) composites that addresses both above mentioned challenges. Si nanoparticles were produced in a pilot-plant-scale microwave-plasma reactor using monosilane (SiH4) as precursor. This process enables the formation of high-purity Si nanoparticles with controllable crystal sizes at a production rate of 45 g/h. Si/graphite hybrids are fabricated through self-assembly by electrostatic attraction. The Si/graphite/PANI nanocomposite is then prepared by in situ polymerization of aniline monomer in the presence of the Si/graphite hybrid. With this approach, ~40 g of Si/graphite/PANI composite per batch can be produced at lab scale. The scalability of the underlying processes enables the use for commercial products. The nanocomposite shows favorable characteristics inherited from its three components: Si nanoparticles provide high capacity, graphite acts as an electrical conductor and gives a high Coulombic efficiency, and the polyaniline coating further enhances the electrical conductivity and protects the entire structure. A very good Coulombic efficiency of 86.2% at the initial cycle is recorded for this nanocomposite material. Galvanostatic charge/discharge tests demonstrate that this material can deliver a discharge capacity of 2000 mAh/g with a very good capacity retention of 76% after 500 cycles at a discharge rate of 0.5C (1.25 A/g). The capacity is 870 mAh/g measured at 5C (12.5 A/g). © 2019 Elsevier B.V.
    view abstract10.1016/j.ssi.2019.115117
  • Shock-tube study of the decomposition of octamethylcyclotetrasiloxane and hexamethylcyclotrisiloxane
    Sela, P. and Peukert, S. and Herzler, J. and Schulz, C. and Fikri, M.
    Zeitschrift fur Physikalische Chemie 234 (2020)
    Shock-tube experiments have been performed to investigate the thermal decomposition of octamethylcyclotetrasiloxane (D4, Si4O4C8H24) and hexamethylcyclotrisiloxane (D3, Si3O3C6H18) behind reflected shock waves by gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) in a temperature range of 1160-1600 K and a pressure range of 1.3-2.6 bar. The main observed stable products were methane (CH4), ethylene (C2H4), ethane (C2H6), acetylene (C2H2) and in the case of D4 pyrolysis, also D3 was measured as a product in high concentration. A kinetics sub-mechanism accounting for the D4 and D3 gas-phase chemistry was devised, which consists of 19 reactions and 15 Si-containing species. The D4/D3 submechanism was combined with the AramcoMech 2.0 (Li et al., Proc. Combust. Inst. 2017, 36, 403-411) to describe hydrocarbon chemistry. The unimolecular rate coefficients for D4 and D3 decomposition are represented by the Arrhenius expressions k total/D4(T) = 2.87 × 1013 exp(-273.2 kJ mol-1/RT) s-1 and k total/D3(T) = 9.19 × 1014 exp(-332.0 kJ mol-1/RT) s-1, respectively. © 2020 Walter de Gruyter GmbH, Berlin/Boston 2020.
    view abstract10.1515/zpch-2020-0005
  • Spray-flame synthesis of La(Fe, Co)O3 nano-perovskites from metal nitrates
    Angel, S. and Neises, J. and Dreyer, M. and Friedel Ortega, K. and Behrens, M. and Wang, Y. and Arandiyan, H. and Schulz, C. and Wiggers, H.
    AIChE Journal 66 (2020)
    Nano-sized perovskites were synthesized in a spray flame from nitrate precursors dissolved in ethanol and in ethanol/2-ethylhexanoic acid (2-EHA) mixtures. Experiments with ethanol led to a broad particle-size distribution and to the formation of undesired phases such as La2CoO4, La2O3, and Co3O4. The addition of 2-EHA can initiate micro explosions of the burning droplets and has been systematically investigated toward the formation of single-phase, high-surface-area LaCoO3 and LaFeO3 with a narrow size distribution. To investigate the effect of 2-EHA, temperature-dependent changes of the chemical composition of the precursor solutions were analyzed with ATR-FTIR between 23 and 70°C. In all cases, the formation of esters was identified while in the solutions containing iron, additional formation of carboxylates was observed. The synthesized materials were characterized by BET SSA, XRD, SAED and EDX-TEM and their catalytic activity was analyzed, reaching 50% CO conversion at temperatures below 160 and 300°C for LaCoO3 and LaFeO3, respectively. © 2019 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstract10.1002/aic.16748
  • Spray-flame synthesis of LaMO3 (M = Mn, Fe, Co) perovskite nanomaterials: Effect of spray droplet size and esterification on particle size distribution
    Angel, S. and Schneider, F. and Apazeller, S. and Kaziur-Cegla, W. and Schmidt, T.C. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute (2020)
    Perovskite nanomaterials such as LaMnO3, LaFeO3, and LaCoO3 were synthesized in a spray flame from metal nitrates dissolved in combustible liquids. The addition of low-boiling solvents such as 2-ethylhexanoic acid (2-EHA) to the ethanol-based solutions supports the formation of phase-pure particles with unimodal particle-size distribution in the 10-nm range attributed to enhanced evaporation through micro-explosions. Nevertheless, in many cases, a second particle mode with sizes of a few hundred nanometers is formed. In this paper, we investigate two possible reasons for the appearance of large particles. Firstly, we analyze the effect of the oxygen dispersion gas flow applied in the two-fluid nozzle on the droplet size distributions of burning sprays using phase Doppler anemometry. We identified that an increase of the dispersion gas flow significantly decreases the number concentration of large droplets (>30 μm), which causes a significant increase of the BET surface area of as-synthesized LaMnO3 and LaCoO3 with increasing dispersion gas flow from 60 m2/g (5 slm dispersion gas) to 100 m2/g (8 slm). Secondly, the esterification in the mixture of solvents towards ethyl-2-ethylhexanoate, which is associated with the release of water as a byproduct, was analyzed by GC/MS. The ester concentration in the iron-containing solution was found to be up to nine times higher than in cobalt or manganese precursor solutions. Simultaneously, the produced LaFeO3 materials show lower BET surface areas and the increasing dispersion gas flow has a minor effect on this material than on the cobalt and manganese perovskite cases. We attribute this to the fact that water formed during esterification forces the hydrolysis of iron nitrate and the formation of large particles within the droplets. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.07.116
  • Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902051
  • Studying the influence of single droplets on fuel/air ignition in a high-pressure shock tube
    Niegemann, P. and Herzler, J. and Fikri, M. and Schulz, C.
    Review of Scientific Instruments 91 (2020)
    The interaction of fuel and lubricant droplets with gaseous fuel/air mixtures close to autoignition is relevant in the context of unwanted early autoignition in spark-ignition internal combustion (IC) engines. To study the influence of droplets on the ignition of fuel/air mixtures independent from the in-cylinder pressure/temperature history, the shock-tube technique in combination with an injection system was established, which enables the generation and injection of single droplets or droplet clusters of n-dodecane and lubricant base oil behind reflected shock waves at pressures and temperatures representative for the compression phase of IC engines. Injected droplets were imaged by high-repetition-rate laser-induced fluorescence. The ignition process was observed by imaging in the visible and UV simultaneously through the shock-tube end wall with a combination of color- and UV-sensitive high-repetition-rate cameras. It was found that the amount and composition of the injected liquid are important factors determining the extent of the interference with the ongoing autoignition of the premixed fuel/air bath gas. For a stoichiometric mixture of primary reference fuels (PRF95) in air, the droplets significantly accelerate ignition especially in the negative temperature coefficient regime at around 760 K. The comparison of the timing of local ignition and the occurrence of volumetric ignition indicates that only in cases where the surrounding gas is close to autoignition, the droplets can trigger early autoignition. This required temporal and spatial coincidence might explain the high level of randomness of early autoignition in engines. © 2020 Author(s).
    view abstract10.1063/5.0024614
  • Thermochemistry of organosilane compounds and organosilyl radicals
    Janbazi, H. and Schulz, C. and Wlokas, I. and Wang, H. and Peukert, S.
    Proceedings of the Combustion Institute (2020)
    Si-C-H-containing radicals and stable species are present in the oxidation of silicon-organic compounds such as methyl and ethyl silanes, which are frequently-used precursors for the synthesis of silicon-based nanoparticles and coatings via combustion processes. Silicon-containing intermediates interact with flame radicals and thus play a major role in flame chemistry and influence flame propagation. Mechanistic understanding of these effects is hampered by very limited thermochemical properties available for relevant organosilane species. This paper presents quantum-chemical calculations and isodesmic reaction schemes for the determination of temperature-dependent heat of formation, entropy, and heat capacity of Si-C-H radicals and molecules, from which group additivity values (GAVs) were obtained from combinatorial considerations. The data for 22 stable Si-C-H species are revised using isodesmic reactions and the related 24 GAVs were refined by considering 19 additional stable Si-C-H species. In addition, quantum chemical calculations are made to calculate the thermochemistry of 61 radicals and used to derive 56 GAVs for Si-C-H containing radicals for the first time. © 2020 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2020.06.114
  • A Cr 4+ :forsterite laser for intracavity absorption spectroscopy in the spectral range of 1.2–1.4 µm
    Fjodorow, P. and Löhden, S. and Hellmig, O. and Schulz, C. and Baev, V.M.
    Optics Express 27 (2019)
    We present a broadband cw Cr 4+ :forsterite laser operating at room temperature with a lasing threshold of 0.8 W that is tunable in the spectral range from 7246 to 8361 cm –1 (1196–1380 nm). This laser is applied for highly sensitive measurements of gaseous absorption inside the cavity. The maximum sensitivity demonstrated in the experiment corresponds to an effective absorption path length of L eff = 2500 km. The spectral bandwidth of laser emission varies from 3 to 150 cm –1 depending on the laser pulse duration, enabling broadband multi-component absorption measurements. We demonstrate sensitive detection of various species (with estimated detection limits), such as H 2 O (25 ppt), O 2 (3 ppm), CO 2 (150 ppb), CH 4 (2 ppb), HCl (6 ppb) and HF (2 ppt) using lab-scale absorption lengths of about one meter. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstract10.1364/OE.27.011122
  • A New Methodology to Study the Mechanisms of Combustion-Chamber Deposit Formation and the Effects of Engine Parameters on the Quantity and Morphology of Combustion-Chamber Deposits
    Ganeau, L. and Alves Fortunato, M. and Pilla, G. and Bruneaux, G. and Schulz, C.
    SAE Technical Papers (2019)
    In this work, a methodology is developed to study engine deposit formation mechanisms. It relies on analyzing the deposit with electron microscopy for morphology and infrared absorption spectroscopy for identifying typical chemical functions. Two lab-scale experiments are used to calibrate these measurement techniques by creating deposits through the two main phases: Liquid film and soot deposition. To test this methodology, an optical engine is used to create a library of deposits. Two main deposit morphologies are found: A homogeneous underlayer as well as soot-like agglomerates. The underlayer is attributed to a fuel-film mechanism whereas the latter is attributed to particles formed through the combustion process. The influence of engine parameters, such as injection phasing and cooling temperature, on the quantity and morphology of the deposits is studied. Various substrate materials, such as quartz, sapphire, aluminum, and steel were used on the piston surface to investigate the materials influence on the morphology, the composition, and the quantity of the deposits. © 2019 SAE Japan and SAE International.
    view abstract10.4271/2019-01-2355
  • Absolute SiO concentration imaging in low-pressure nanoparticle-synthesis flames via laser-induced fluorescence
    Chrystie, R.S.M. and Ebertz, F.L. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 125 (2019)
    In this paper, we present a strategy for imaging measurements of absolute concentration values of gas-phase SiO in the combustion synthesis of silica, generated from the reaction of hexamethyldisiloxane (HMDSO) precursor in a lean (ϕ = 0.6) hydrogen/oxygen/argon flame. The method is based on laser-induced fluorescence (LIF) exciting the Q(42) rotational transition within the A1Π − X1Σ (1, 0) electronic band system of SiO at 231 nm. Corrections for temperature-dependent population of the related ground state are based on multi-line SiO–LIF thermometry utilizing transitions within the A1Π − X1Σ (0, 0) electronic band around 234 nm. Corrections for local collisional quenching are based on measured effective fluorescence lifetimes from the temporal signal decay using a short camera gate stepped with respect to the laser pulse. This fluorescence lifetime measurement was confirmed with additional measurements using a fast photomultiplier. The resulting semi-quantitative LIF signal was photometrically calibrated using Rayleigh scattering from known gas samples at various pressures and laser energies as well as with nitric oxide LIF. The obtained absolute SiO concentration values in the HMDSO-doped flames will serve as a stringent test case for recently developed flame kinetic mechanisms for this class of gas-borne silicon dioxide nanoparticle synthesis. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-019-7137-8
  • Comparative study of flame-based SiO 2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? < 0.6), inert-gas diluted H 2 /O 2 /Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A 1 Π-X 1 Σ + (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO 2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.07.024
  • Comparative study of flame-based SiO2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? < 0.6), inert-gas diluted H2/O2/Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A1Π-X1Σ+ (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.07.024
  • Detailed simulation of iron oxide nanoparticle forming flames: Buoyancy and probe effects
    Sellmann, J. and Rahinov, I. and Kluge, S. and Jünger, H. and Fomin, A. and Cheskis, S. and Schulz, C. and Wiggers, H. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 37 (2019)
    Nanoparticle formation in flames is strongly influenced by the residence-time-temperature history inside the flame. We study how the temperature history can be intentionally modified by orienting flames either in an upward-firing or downward-firing configuration. We also investigate the influence of unintended residence-time modifications caused by sampling nozzles. These phenomena are investigated by experiments and simulations for the synthesis of iron oxide nanoparticles from premixed iron-pentacarbonyl-doped hydrogen/oxygen flat flames. The experiments apply molecular-beam sampling with a particle mass spectrometer to measure particle sizes and a quartz microbalance to detect the presence of condensed matter. The simulations rely on a finite-rate chemistry approach with species-specific diffusion, particle dynamics are described by a bi-modal population balance model. It is demonstrated that the downward-burning flame forms a detached stagnation point, causing longer residence times at elevated temperature than an upward- or horizontally firing flame, permitting the growth of larger particles. These iron oxide particles are eventually formed in the recombination zone of the flame, but no condensed matter was found in the reaction zone. The experiments also observed the formation of particles in the preheat zone, but their composition and all aspects of their disappearance remain uncertain. Current models do, however, suggest the formation of iron particles and their subsequent evaporation and combustion. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.06.041
  • Detector calibration and measurement issues in multi-color time-resolved laser-induced incandescence
    Mansmann, R. and Sipkens, T.A. and Menser, J. and Daun, K.J. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 125 (2019)
    Time-resolved laser-induced incandescence is used to infer the size distribution of gas-borne nanoparticles from time-resolved pyrometric measurements of the particle temperature after pulsed laser heating. The method is highly sensitive to aspects of the measurement strategy that are often not considered by practitioners, which often lead to discrepancies between measurements carried out under nominally identical conditions. This paper therefore presents a well-documented calibration procedure for LII systems and quantifies the uncertainty in pyrometric temperatures introduced by this procedure. Calibration steps include corrections for: (1) signal baseline, (2) variable transmission through optical components, and (3) detector characteristics (i.e., gain and spectral sensitivity). Candidate light sources are assessed for their suitability as a calibration reference and the uncertainty in calculated calibration factors is determined. The error analysis is demonstrated using LII measurements made on a sooting laminar diffusion flame. We present results for temperature traces of laser-heated particles determined using two- and multi-color detection techniques and discuss the temperature differences for various combinations of spectral detection channels. We also summarize measurement artifacts that could bias the LII signal processing and present strategies for error identification and prevention. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-019-7235-7
  • Development and evaluation of a chemical kinetics reaction mechanism for tetramethylsilane-doped flames
    Janbazi, H. and Karakaya, Y. and Kasper, T. and Schulz, C. and Wlokas, I. and Peukert, S.
    Chemical Engineering Science 209 (2019)
    Tetramethysilane (TMS) is a precursor for flame synthesis of silica (SiO2) nanoparticles. A chemical reaction mechanism was developed for the oxidation of TMS in a lean low-pressure (p ≈ 30 mbar) H2/O2/Ar flame using species mole fractions, obtained from molecular-beam mass spectrometry (MBMS) measurements in a matrix-supported flat flame doped with 600 ppm TMS. The thermodynamic data of Si-containing species were determined from quantum-chemical calculations at the G4 level of theory. The formation and subsequent consumption of Si(OH)4, one of the main products of TMS oxidation, and the formation of Si4O10H4 clusters are hypothesized to be the primary pathway in the synthesis of silica nanoparticles. The reaction rate coefficients are either estimated via an algorithmic optimization procedure or are assumed based on analogies to similar reactions in the literature. The mechanism was further validated based on MBMS measurements with the same base flame doped with 400 and 800 ppm TMS. © 2019 Elsevier Ltd
    view abstract10.1016/j.ces.2019.115209
  • Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions
    Bertin, E. and Münzer, A. and Reichenberger, S. and Streubel, R. and Vinnay, T. and Wiggers, H. and Schulz, C. and Barcikowski, S. and Marzun, G.
    Applied Surface Science 467-468 (2019)
    Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.10.061
  • Evalution of Drude parameters for liquid Germanium nanoparticles through aerosol-based line-of-sight attenuation measurements
    Daun, K.J. and Menser, J. and Asif, M. and Musikhin, S. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 226 (2019)
    The objective of this study is to infer Drude model parameters for liquid germanium nanoparticles from extinction measurements made across an aerosol within a microwave plasma reactor using a halogen lamp (410–700 nm) and a laser-driven light source (205–585 nm). The plasma frequency and relaxation time are inferred using Rayleigh theory, Mie theory, and a fourth-order Mie approximation. These parameters are compared with those found using the ellipsometry-derived complex dielectric function as well as the bulk density and electrical resistivity of liquid germanium. The analysis is carried out in a probabilistic context using Bayesian inference, which accounts for both the measurement noise and model error. While all the candidate models can reproduce the shape of the experimentally-derived extinction spectra, the Bayesian inference showed that extinction-derived parameters differed from those obtained from the density and electrical resistivity in a statistically-significant way. This highlights the limitations of the free-electron model that underpins Drude theory, and suggests potential opportunities for model refinement. © 2019 Elsevier Ltd
    view abstract10.1016/j.jqsrt.2019.01.021
  • Excitation wavelength dependence of the fluorescence lifetime of anisole
    Baranowski, T. and Dreier, T. and Schulz, C. and Endres, T.
    Physical Chemistry Chemical Physics 21 (2019)
    Photo-physical models that describe the pressure- and temperature-dependent fluorescence quantum yield of organic fluorescence tracers rely on an accurate prediction of the initial excited-state population, collision-dependent relaxation processes, and state-dependent relaxation processes. In case the initial excited-state population distribution reached after the laser excitation equals on average the thermal distribution, the fluorescence quantum yield becomes pressure independent. This initial distribution critically depends on the temperature-dependent ground-state population before excitation as well as the excitation wavelength. The ability to predict this behavior is a critical check for the validity of the existing photophysical models. The dependence of the effective fluorescence lifetime of anisole on the excitation wavelength (256-270 nm) was investigated at temperatures between 325 and 525 K for pressures between 1 and 4 bar. For each temperature, a unique excitation wavelength was found where the fluorescence lifetime is pressure-independent. The comparison of the experimental results with the predictions based on the established photophysical step-ladder models revealed a systematic underestimation of the required excitation photon energies for direct excitation into the thermalized level. An improved modeling approach based on quantum chemistry calculations for implementing simulated excitation spectra and state-dependent transition probabilities overcomes these limitations. Our results show for the example of anisole that the fluorescence step-ladder models that exist for aromatic fluorescence tracers must be modified to correctly predict the effect of the excitation wavelength. © 2019 the Owner Societies.
    view abstract10.1039/c9cp01472a
  • Fuel effects on NO formation in diesel-like jets in a vessel
    Ottenwälder, T. and Raffius, T. and Schulz, C. and Grünefeld, G. and Koß, H.-J. and Pischinger, S.
    Combustion and Flame 206 (2019)
    We demonstrated recently that NO concentration measurements are feasible even in the core of largely non-sooting diesel-like jets by combined laser-induced fluorescence (LIF) and spontaneous Raman scattering (SRS). However, the question arises whether previous findings hold also for other diesel-like jets. The current study focuses on fuel effects. In the previous NO measurements, n-heptane was used. It is replaced by pure di-n-butylether (DNBE) and a tailor-made blend of 50% DNBE and 50% n-octanol. These fuels are promising biofuel candidates and lead to an interesting variation of mixing during combustion (MDC). The determination of NO concentrations turns out to be generally feasible with the blend on the jet centerline in the quasi-steady phase of the injection event. The corresponding uncertainty is about ± 28 %. By contrast, some of the NO-LIF measurements in sooting DNBE jets are discarded, primarily due to increased light attenuation. For the remaining NO concentrations with DNBE the corresponding uncertainty is about ± 40 %. For the blend, results indicate that NO formation is very similar to the one in the n-heptane jets. Thus, the net effect of changed volatility and oxygenation is seemingly weak. By contrast, quasi-steady centerline NO concentrations are apparently significantly affected by MDC for pure DNBE. Relatively high NO concentrations are observed in this case, although products of highly fuel-rich fluid parcels are also present there. This study indicates the importance of MDC in such jets. © 2019 The Combustion Institute
    view abstract10.1016/j.combustflame.2019.04.053
  • High-temperature gas-phase kinetics of the thermal decomposition of tetramethoxysilane
    Sela, P. and Peukert, S. and Herzler, J. and Sakai, Y. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    The decomposition of tetramethoxysilane (Si(OCH3)4, TMOS) was studied in shock-tube experiments in the 1131-1610 K temperature range at pressures ranging from 1.9 to 2.3 bar behind reflected shock waves combining gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). The initial reaction is a four-center elimination to form methanol. At elevated temperatures, TMOS also decomposes via a O-C bond scission forming a methyl (CH3) and the corresponding OSi(OCH3)3 radical. The main observed products were methane (CH4), methanol (CH3OH), ethylene (C2H4), and ethane (C2H6). The yields of these products increase with temperature. A kinetics mechanism from literature (Chu et al. 1995), which quantitatively accounts for the observed products in the decomposition of TMOS, was adopted and updated. The mechanism contains 13 silicon species and 24 reactions with silicon-containing species. It was combined with the methanol mechanism of Burke et al. (2006). The measured global rate constant for TMOS decomposition was found to be koverall[TMOS→products] = 2.9 × 1011exp(- 225 kJ mol-1/RT)s-1. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.05.084
  • High-Temperature Unimolecular Decomposition of Diethyl Ether: Shock-Tube and Theory Studies
    Sela, P. and Sakai, Y. and Choi, H.S. and Herzler, J. and Fikri, M. and Schulz, C. and Peukert, S.
    Journal of Physical Chemistry A 123 (2019)
    The unimolecular decomposition of diethyl ether (DEE; C2H5OC2H5) is considered to be initiated via a molecular elimination and a C-O and a C-C bond fission step: C2H5OC2H5 → C2H4 + C2H5OH (1), C2H5OC2H5 → C2H5 + C2H5O (2), and C2H5OC2H5 → CH3 + C2H5OCH2 (3). In this work, two shock-tube facilities were used to investigate these reactions via (a) time-resolved H-atom concentration measurements by H-ARAS (atomic resonance absorption spectrometry), (b) time-resolved DEE-concentration measurements by high repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS), and (c) product-composition measurements via gas chromatography/MS (GC/MS) after quenching the test gas. The experiments were conducted at temperatures ranging from 1054 to 1505 K and at pressures between 1.2 and 2.5 bar. Initial DEE mole fractions between 0.4 and 9300 ppm were used to perform the kinetics experiments by H-ARAS (0.4 ppm), GC/MS (200-500 ppm), and HRR-TOF-MS (7850-9300 ppm). The rate constants, ktotal (ktotal = k1 + k2 + k3) derived from the GC/MS and HRR-TOF-MS experiments agree well with each other and can be described by the Arrhenius expression, ktotal(1054-1467 K; 1.3-2.5 bar) = 1012.81±0.22 exp(-240.27 ± 5.11 kJ mol-1/RT) s-1. From the H-ARAS experiments, overall rate constants for the bond fission channels, k2+3 = k2 + k3 have been extracted. The k2+3 data can be well described by the Arrhenius equation, k2+3(1299-1505 K; 1.3-2.5 bar) = 1014.43±0.33 exp(-283.27 ± 8.78 kJ mol-1/RT) s-1. A master-equation analysis was performed using CCSD(T)/aug-cc-pvtz//B3LYP/aug-cc-pvtz and CASPT2/aug-cc-pvtz//B3LYP/aug-cc-pvtz molecular properties and energies for the three primary thermal decomposition processes in DEE. The derived experimental data is very well reproduced by the simulations with the mechanism of this work. With regard to the branching ratios between bond fissions and elimination channels, uncertainties remain. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.jpca.9b04186
  • Investigating temporal variation in the apparent volume fraction measured by time-resolved laser-induced incandescence
    Sipkens, T.A. and Menser, J. and Mansmann, R. and Schulz, C. and Daun, K.J.
    Applied Physics B: Lasers and Optics 125 (2019)
    In many time-resolved laser-induced incandescence (TiRe-LII) experiments, it is common practice to relate the intensity emitted by laser-heated nanoparticles to the detected LII signal through a factor (here called the intensity scaling factor, ISF) that includes the particle volume fraction and other parameters that may not be the focus of the analysis. While, in the absence of evaporation or sublimation, the ISF should theoretically remain constant with respect to time, recent multi-wavelength measurements show that, in reality, it may vary with both time and fluence. We consider four candidate effects that contribute to this behavior: particle annealing; polydispersity in the nanoparticle-size distribution; background luminosity due to emission from nanoparticles in the line-of-sight before and behind the probe volume; and the temporal resolution of the detector. We demonstrate these effects by simulating TiRe-LII data for in-flame soot at atmospheric pressure, using new simplified heat transfer and annealing models. Analysis of experimental signals collected from flame-generated soot at atmospheric pressure reveals trends in the ISF similar to those predicted by simulations. These temporal variations provide important insights that can help to diagnose problems in TiRe-LII experiments and improve TiRe-LII models. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-019-7251-7
  • Laser spectroscopic investigation of diesel-like jet structure using C8 oxygenates as the fuel
    Raffius, T. and Ottenwälder, T. and Schulz, C. and Grünefeld, G. and Koß, H.-J. and Pischinger, S.
    Fuel 235 (2019)
    Di-n-butyl ether (DNBE) and n-octanol have very low sooting tendencies in diesel-like combustion, as demonstrated in previous engine studies. This finding is not fully understood for pure DNBE, because it has a very high cetane rating (∼100). In order to investigate the underlying mechanisms, the structure of diesel-type jets is analyzed by a number of optical diagnostics, such as spontaneous Raman scattering (SRS), laser-induced fluorescence (LIF), OH* luminescence imaging, Mie scattering, and shadowgraphy. Pure DNBE and a tailor-made blend of 50% DNBE and 50% n-octanol as well as neat n-heptane are used as the fuel in separate experiments. The jets are probed in a simulated engine-like environment in a high-pressure combustion vessel. In particular, the inner flame structure is analyzed by SRS and LIF. This yields information on the local temperature and the concentrations of O2, CO, and polycyclic aromatic hydrocarbons (PAH). For the first time, O2 is quantitatively detected in the core of a diesel-like flame by resonance-enhanced SRS. Thereby, air entrainment into the inner flame core is assessed. Results show that air entrainment is particularly strong for pure DNBE, explaining its high soot oxidation rate and overall low sooting tendency. High entrainment is primarily attributed to the low heat-release rate of DNBE, which is likely an effect of its high ignitability. Thus, it can be concluded that the high cetane rating of pure DNBE does not only lead to relatively poor pre-combustion mixture preparation and consequently considerable soot formation but seemingly also to particularly strong soot oxidation. Moreover, the jet structure turns out to be very similar for the DNBE/n-octanol blend and neat n-heptane, indicating that the net effect of volatility and fuel oxygenation is weak. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.07.124
  • Mixing processes in the transonic, accelerated wake of a central injector
    Richter, J. and Beuting, M. and Schulz, C. and Weigand, B.
    Physics of Fluids 31 (2019)
    The compressible accelerated mixing layer of a central injector was thoroughly investigated experimentally to provide a data set that can be used for validating numerical simulations. A drop-shaped central injector was mounted upstream of a rectangular convergent-divergent nozzle, through which air was accelerated to a Mach number of 1.7. The free-stream Reynolds number at the point of injection was 6.245 × 104. Four different measurement techniques - short-time illuminated schlieren imaging, laser schlieren, laser-induced thermal acoustics, and laser-induced fluorescence (LIF) - were applied to visualize the flow structures and to measure the predominant frequency of periodic flow features, the Mach number and temperature, and the injectant distribution. Instantaneous images show that the mixing layer was dominated by a series of alternating vortices. The mixing layer’s self-similarity could be proven by means of injectant mass fraction profiles, which were derived from LIF measurements. The growth rate of the mixing layer was shown to approximately follow the 1 2-power law. It was concluded from comparison to literature data that the growth rate is primarily determined by the free-stream Reynolds number, whereas the free-stream Mach number (compressibility effects) and the injectant amount play a minor role. These experimental data were used to validate three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes simulations using the shear-stress transport turbulence model. It was shown that the vortex shedding frequency and the mixing layer growth rate as well as the wake velocity deficit were underestimated by the simulations. This indicates that the flow physics of vortex formation were not entirely reproduced. © 2019 Author(s).
    view abstract10.1063/1.5055749
  • NIR sensor for aqueous urea solution film thickness and concentration measurement using a broadband light source
    Lubnow, M. and Dreier, T. and Schulz, C.
    Applied Optics 58 (2019)
    We demonstrate a multi-wavelength near-infrared (NIR) broadband absorption sensor for the simultaneous monitoring of layer thickness and urea concentration of aqueous urea solutions. Samples were prepared in thin-layer quartz transmission cells. Film thickness and urea mass fraction (at constant temperature) were determined from measured transmittance ratios in characteristic wavelength bands selected by narrowband filters in front of the detector and converted to absorbance ratios. Suitable emission bands were selected depending on the sensitivity of the NIR absorption spectrum of the solution with respect to temperature and solute concentration. For this purpose, Fourier transform IR spectra of aqueous urea solutions were recorded in the 1250–2500 nm wavelength range for urea concentrations between 0 and 40 wt.% and temperatures between 298 K and 338 K. A prototype sensor was designed using a continuous-wave fiber-coupled incoherent tungsten lamp, subsequent intensity modulation, and lock-in detection of the transmitted radiation. The sensor concept was validated with measurements using a calibration cell providing liquid layers of variable thicknesses (7–1000 μm). © 2019 Optical Society of America
    view abstract10.1364/AO.58.004546
  • Power and syngas production from partial oxidation of fuel-rich methane/DME mixtures in an HCCI engine
    Banke, K. and Hegner, R. and Schröder, D. and Schulz, C. and Atakan, B. and Kaiser, S.A.
    Fuel 243 (2019)
    Polygeneration is the coupling of energy conversion and conversion towards useful chemicals, providing a route towards more flexible and efficient energy systems. In this work, we explore a particular concept of polygeneration using an internal combustion engine as a reactor for partial oxidation to generate synthesis gas in variable combinations with mechanical work and heat. Experiments were performed in a single-cylinder engine operated in homogeneous-charge compression-ignition (HCCI) mode on a mixture of methane and air with dimethyl ether (DME) as a reactivity-enhancing additive. For intake temperature from 100 to 190 °C, the range of stable, non-sooting operation with acceptable pressure-rise rates was determined in terms of equivalence ratio and DME mole fraction in the fuel. At 150 °C intake temperature, 8.7–9.5% DME were needed to stabilize operation at equivalence ratios between about 1.3 and 2.7. Experimental results from fuel-rich conditions with equivalence ratios ranging from 1.65 to 2.34 were compared to simulations with a homogeneous, single-zone engine model. The concept of exergy was used to investigate the thermodynamic performance of the polygeneration engine. The effect of the equivalence ratio on work and heat output, thermal and exergetic efficiency, and selectivity towards useful product species was investigated. In the experiments a work output of up to 160 J (ϕ = 1.65) per cycle (IMEP = 4.82 bar) and exergetic efficiencies of up to 81.5% (ϕ = 2.34) were achieved. The simultaneous generation of synthesis gas had a selectivity of up to 72% for hydrogen and 79% for carbon monoxide (both at ϕ = 2.34). © 2019 The Authors
    view abstract10.1016/j.fuel.2019.01.076
  • Shock-tube study of methane pyrolysis in the context of energy-storage processes
    Nativel, D. and Shu, B. and Herzler, J. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    The thermal decomposition of methane (10? mol% in inert gases) was investigated behind reflected shock waves. Product spectra were measured via GC/MS after reactions with initial temperatures between 1285 and 2400?K at pressures of 1.5?bar in a single-pulse shock tube and of 30?bar in a high-pressure shock tube with rapid gas sampling via a fast opening valve. In the 1.5-bar experiments, additional time-resolved absorption measurements in the mid-IR were carried out. The temporal variation in CH4 concentration was measured at 836-2495?K and 1.8?bar with interband-cascade lasers near 3.39?μm. Time-resolved temperatures were determined via CO two-line thermometry with two quantum-cascade lasers near 4.56 and 4.85?μm for initial post-shock temperatures of 1715-2573?K at 2.2?bar. The results were compared to simulations based on three different literature mechanisms (Cai and Pitsch, 2015; Porras et al., 2017; Wang et al., 2007) as well as with the new rate constant of methane dissociation from Wang et al. (2016).
    view abstract10.1016/j.proci.2018.06.083
  • Shock-tube study of the ignition and product formation of fuel-rich CH 4 /air and CH 4 /additive/air mixtures at high pressure
    Herzler, J. and Sakai, Y. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Higher-value chemicals can be produced from methane with small exergy losses by partial oxidation if the chemical conversion proceeds in an internal combustion engine (ICE) as a polygeneration process (Gossler and Deutschmann, 2015). Kinetics models are not sufficiently validated for the very fuel-rich and high-pressure conditions relevant for this process. Therefore, ignition delay times of fuel-rich methane/(additive)/air mixtures were measured in a shock tube at about 30bar and temperatures between 600 and 1650K. n-heptane and diethylether were used as additives to increase the reactivity of the fuel so that the polygeneration process can be realized in an ICE at HCCI conditions at lower compression temperatures. At φ=2, measured ignition delay times agree well with simulations using different mechanisms from literature. Synthesis gas (CO, H 2 ) is the main product at these conditions (Sen et al., 2016). For the production of higher hydrocarbons, the equivalence ratio must be increased. Very fuel-rich mixtures (φ=10) were used because the temperature increase during the reaction of these mixtures is quite low (<450K), so that post-ignition temperatures stay below the lower limit of soot formation. Only for mixtures with n-heptane as additive, good agreement of measured and simulated ignition delay times is found. The other mixtures show strong deviations with all mechanisms. As a further parameter to improve and validate the mechanisms at φ=10, product distributions after ignition were determined by sampling in the cooling phase with a fast-opening valve and GC/MS analysis. Besides H 2 and H 2 O, CO and higher hydrocarbons like C 2 H 2 , C 2 H 4 , C 2 H 6 , and C 6 H 6 were detected as main products. About half of the carbon of the consumed methane is converted to CO, the other half to higher hydrocarbons. The product distributions are well predicted by simulations.
    view abstract10.1016/j.proci.2018.05.120
  • Shock-tube study of the ignition and product formation of fuel-rich CH4/air and CH4/additive/air mixtures at high pressure
    Herzler, J. and Sakai, Y. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Higher-value chemicals can be produced from methane with small exergy losses by partial oxidation if the chemical conversion proceeds in an internal combustion engine (ICE) as a polygeneration process (Gossler and Deutschmann, 2015). Kinetics models are not sufficiently validated for the very fuel-rich and high-pressure conditions relevant for this process. Therefore, ignition delay times of fuel-rich methane/(additive)/air mixtures were measured in a shock tube at about 30bar and temperatures between 600 and 1650K. n-heptane and diethylether were used as additives to increase the reactivity of the fuel so that the polygeneration process can be realized in an ICE at HCCI conditions at lower compression temperatures. At φ=2, measured ignition delay times agree well with simulations using different mechanisms from literature. Synthesis gas (CO, H2) is the main product at these conditions (Sen et al., 2016). For the production of higher hydrocarbons, the equivalence ratio must be increased. Very fuel-rich mixtures (φ=10) were used because the temperature increase during the reaction of these mixtures is quite low (<450K), so that post-ignition temperatures stay below the lower limit of soot formation. Only for mixtures with n-heptane as additive, good agreement of measured and simulated ignition delay times is found. The other mixtures show strong deviations with all mechanisms. As a further parameter to improve and validate the mechanisms at φ=10, product distributions after ignition were determined by sampling in the cooling phase with a fast-opening valve and GC/MS analysis. Besides H2 and H2O, CO and higher hydrocarbons like C2H2, C2H4, C2H6, and C6H6 were detected as main products. About half of the carbon of the consumed methane is converted to CO, the other half to higher hydrocarbons. The product distributions are well predicted by simulations.
    view abstract10.1016/j.proci.2018.05.120
  • Spontaneous-Raman-scattering measurements in diesel-like n-heptane jets: Spectroscopy and flame structure
    Raffius, T. and Schulz, C. and Ottenwälder, T. and Grünefeld, G. and Heufer, K.A. and Koß, H.-J. and Pischinger, S.
    Fuel 236 (2019)
    It was demonstrated recently that quantitative temperature and CO measurements can be conducted by spontaneous Raman scattering (SRS) in non-sooting diesel-like biofuel jets in a vessel. In contrast, n-heptane is used as the fuel in the present study. This leads to slightly increased sooting tendency. One-dimensional temperature and CO measurements are generally performed successfully. Fuel-injection pressure is varied and the flame structure is characterized, including the jet core. Effects of pre-combustion mixture inhomogeneity are particularly observed at lower injection pressure (700 bar). Strong deviations from adiabatic-equilibrium conditions in terms of temperature and CO mole fraction are observed in particularly fuel-rich fluid parcels. CO formation is apparently affected by turbulent mixing there. Relatively low temperatures are measured around the flame lift-off region, in particular for the highest injection pressure (1500bar), indicating turbulence-chemistry interaction. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.08.165
  • Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900168
  • SpraySyn-A standardized burner configuration for nanoparticle synthesis in spray flames
    Schneider, F. and Suleiman, S. and Menser, J. and Borukhovich, E. and Wlokas, I. and Kempf, A. and Wiggers, H. and Schulz, C.
    Review of Scientific Instruments 90 (2019)
    In many scientific communities, the definition of standardized experiments has enabled major progress in process understanding. The investigation of the spray-flame synthesis of nanoparticles at a well-defined standard burner by experiment and simulation makes it possible to produce a comprehensive data set with various established and novel measuring methods. In this work, we introduce the design of the SpraySyn burner as a new standard for a free-jet type burner that offers well-defined and simulation-friendly boundary conditions and geometries as well as accessibility for optical diagnostics. A combustible precursor solution is fed through a centrally located capillary and aerosolized with an oxygen dispersion gas flow. The spray flame is stabilized by a premixed flat methane/oxygen pilot flame fed via a porous bronze matrix surrounded by a stabilizing nitrogen coflow emanating through the same porous matrix, providing easy-to-calculate boundary conditions for simulations. This burner design enables the use of a wide choice of solvents, precursors, and precursor combinations. Best-practice operating instructions and parameters are given, and large-eddy simulations are performed demonstrating the suitability of the SpraySyn burner for computational fluid dynamics simulations. For ensuring reproducible operation across labs, we define a consumer-camera-based flame characterization scheme for the quantitative assessment of the flame geometry such as flame length, diameter, tilt angle, and photometric distribution of visible chemiluminescence along the center axis. These parameters can be used for benchmarking the pilot and spray flame by each user of the SpraySyn burner with the reference flames. © 2019 Author(s).
    view abstract10.1063/1.5090232
  • Structures of carbonaceous nanoparticles formed in various pyrolysis systems
    Jander, H. and Borchers, C. and Böhm, H. and Emelianov, A. and Schulz, C.
    Carbon 150 (2019)
    In the pyrolysis of different hydrocarbon/carbon suboxid fuels formation of carbon particles with the special view to their structures was examined. For this, the following three very different pyrolysis systems were investigated experimentally i)a pyrolysis reactor, ii)a shock tube and iii)a plasma reactor with respect to the influence of varying reaction parameters on the carbonaceous nanoparticles. The particles formed in these reaction systems were studied in view of their morphology and state of crystallization by use of electron microscopy (Philips CM30)at low- and high resolution combined with micro-diffraction measurements. As to be seen at low resolution of the transmission electron microscopy studies, the particle sizes in the pyrolysis reactor and shock tube do not differ significantly, but distinguished considerably from those particle sizes obtained in the plasma reactor. While the particles obtained in the pyrolysis reactor and shock-tube had particle diameters of about d≈ 30 nm, the particles in the plasma reactor consisted of fluffy-like units, and their sizes were about d≈ 4 nm. The various carbon layers consisted of different polyaromatic hydrocarbon units with variable sizes arranged to diverse states in the course of graphitization. © 2019 Elsevier Ltd
    view abstract10.1016/j.carbon.2019.02.034
  • Synthesis of silicon nanoparticles in a pilot-plant-scale microwave plasma reactor: Impact of flow rates and precursor concentration on the nanoparticle size and aggregation
    Kunze, F. and Kuns, S. and Spree, M. and Hülser, T. and Schulz, C. and Wiggers, H. and Schnurre, S.M.
    Powder Technology 342 (2019)
    This work is devoted to scale-up the microwave plasma synthesis of silicon nanoparticles from gaseous precursor monosilane (SiH4), previously investigated in lab-scale processes, to the pilot-plant-scale with production rates up to 200 g/h. The aim is to ensure reproducible, long-term operation of the reactor through gas-dynamic stabilization of the reacting flow and to control particle size and morphology via the gas flow velocity and the precursor concentration. Based on a newly designed nozzle, the lab-scale approach of stabilizing the plasma flow via a tangential sheath gas flow and an axial precursor gas flow was successfully transferred to the pilot-plant scale. At precursor concentrations up to 16 vol% of SiH4 diluted in argon and hydrogen, the as-synthesized particles have similar characteristics compared to those from lab-scale reactors. They are spherical, crystalline, mostly soft-agglomerated, and show a log-normal size distribution with a geometric standard deviation around 1.45 as expected for self-preserving aerosol size-distributions. In contrast to lab-scale experiments, an increase in SiH4 concentration up to 48 vol% does not lead to further growth of isolated primary particles but promotes aggregate formation from smaller primary particles. This is attributed to massive initial nucleation of very small particles due to strong supersaturation and their subsequent strong aggregation while suppressing complete coalescence due to the limited residence time at high temperature. © 2018 Elsevier B.V.
    view abstract10.1016/j.powtec.2018.10.042
  • The influence of selected aromatic fluorescence tracers on the combustion kinetics of iso-octane
    Sommerer, J. and Fikri, M. and Herzler, J. and Maas, U. and Schießl, R. and Schulz, C.
    Fuel 244 (2019)
    The influence of four single-ring aromatic compounds on the ignition process of iso-octane is investigated by shock-tube experiments and numerical simulations. Methylbenzene (toluene), dimethylbenzene (xylene), trimethylbenzene (without distinction of isomers), and methoxybenzene (anisole) are often used as tracers in laser-induced fluorescence (LIF) combustion diagnostics where the presence of the tracer should have a minimal influence on the engine performance. Ignition delay times for tracer-blended iso-octane/air mixtures were measured at 750–1500 K at a pressure of 40 bar, and for equivalence ratios ϕ of 0.5 and 1.0. Measured ignition delay times of the blends containing 5–10 vol.% of the respective tracer are very close to the ignition delay times of pure iso-octane/air mixtures at the same pressure, temperature and equivalence ratio. Simulations of the auto-ignition involving detailed chemical kinetics of the alkane and the aromatics predict ignition delay times that are in good agreement with the experimental results. The simulations are used for a more detailed analysis of the interaction between the alkane and the aromatics during ignition. It is shown that the tracers and the fuel are consumed at similar rates in the blended mixtures, although mixtures of pure tracer with air are significantly less reactive than the corresponding mixtures of fuel and air. Sensitivity analyses are used to investigate the coupling between the sub-mechanisms of the tracers and the fuel. The chain branching reactions of the fuel leading to the formation of OH radicals are found to be the most sensitive reactions in the pre-ignition phase. The interaction between tracer and fuel at low temperatures can be characterized as follows: The fuel initiates radical formation, and the aromatic compound accompanies the commencing reaction by using the radicals originating from the fuel for their own decomposition. In the early steps of reaction, chain branching is caused by the tracer at a slower pace compared to the fuel. Therefore, the tracer-based reaction channels merely follow the reaction progress by the pace defined by the fuel. © 2019 Elsevier Ltd
    view abstract10.1016/j.fuel.2019.01.190
  • Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1–xFexO3 Perovskites
    Waffel, D. and Alkan, B. and Fu, Q. and Chen, Y.-T. and Schmidt, S. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    ChemPlusChem 84 (2019)
    Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo1–xFexO3 perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo0.8Fe0.2O3 showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol−1. Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201900429
  • All gas-phase synthesis of graphene: Characterization and its utilization for silicon-based lithium-ion batteries
    Münzer, A. and Xiao, L. and Sehlleier, Y.H. and Schulz, C. and Wiggers, H.
    Electrochimica Acta 272 (2018)
    We report on a gas-phase synthesis method for the preparation of free-standing few-layer graphene in a microwave plasma reactor using pure ethanol as precursor. This scalable synthesis route produces gas-phase graphene (GPG) with lab-scale production rates up to a few hundred mg/h. The physico-chemical properties of the resulting GPG were characterized by XRD, FTIR-, and Raman spectroscopy, electrical conductivity measurements, XPS, and HRTEM in combination with EELS. The materials’ properties were compared with those of reduced graphene oxide (rGO) made by the established Hummers’ method. The results indicate that the gas-phase synthesis method provides highly-ordered few-layer graphene with extraordinary high purity, very low oxygen content of less than 1 at.%, and high specific conductivity. Both graphene materials were processed in combination with gas-phase synthesized silicon nanoparticles towards silicon-graphene nanocomposites for Li-ion battery anodes. Subsequent electrochemical testing revealed that the gas-phase graphene significantly enhances the long-term stability and Coulomb efficiency of the composite compared to pristine silicon and outperforms the composite fabricated from reduced graphene oxide. © 2018 Elsevier Ltd
    view abstract10.1016/j.electacta.2018.03.137
  • Application of toluene LIF to transonic nozzle flows to identify zones of incomplete molecular mixing
    Beuting, M. and Richter, J. and Weigand, B. and Dreier, T. and Schulz, C.
    Optics Express 26 (2018)
    Toluene laser-induced fluorescence (LIF) has been applied to image the mixing deficit on the molecular level in the transonic wake of two different blunt-body injectors in a compressible accelerated nozzle flow. A single-color excitation and two-color detection scheme is employed to measure the signal red-shift caused by the quenching effect of molecular oxygen on the fluorescence of toluene, which reduces and red-shifts the LIF signal if both substances interact on a molecular level. To this end, toluene is injected alternatingly with O2-contaning and O2-free carrier gas into the air main flow. Differences of both signals mark regions where mixing on molecular level is incomplete. A zone of molecular mixing deficit extending several millimeters in stream-wise direction is identified. The effect of local variations in temperature on the sensitivity of this technique is discussed using photo-physical data measured in a stationary low-temperature cell. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstract10.1364/OE.26.010266
  • Combined production of power and syngas in an internal combustion engine – Experiments and simulations in SI and HCCI mode
    Wiemann, S. and Hegner, R. and Atakan, B. and Schulz, C. and Kaiser, S.A.
    Fuel 215 (2018)
    In this work, an internal combustion engine is used as a reactor for partial oxidation to produce syngas together with mechanical work. Experiments were performed in a single-cylinder engine operated on methane/air. Spark-ignition (SI) and homogeneous-charge compression-ignition (HCCI) were investigated. For HCCI, 5 mol% n-heptane were added to the fuel to reduce auto-ignition temperatures. With spark ignition at ϕ = 1.56, the product gas contained up to 8.6 mol% CO and 7.7 mol% H2 at 71.5% exergetic efficiency, while at ϕ = 0.72 roughly the same mechanical work was generated, but with only 42.5% exergetic efficiency. Under the richer conditions achievable in HCCI combustion, syngas content increased to 15.8 mol% CO and 17.9 mol% H2, and the exergetic efficiency to 81.5%. A homogeneous single-zone model coupled with a detailed reaction mechanism was used to simulate the process. The experimental results and the simulation were in good agreement for operating points without frequent misfires. © 2017
    view abstract10.1016/j.fuel.2017.11.002
  • Direct Measurement of High-Temperature Rate Constants of the Thermal Decomposition of Dimethoxymethane, a Shock Tube and Modeling Study
    Peukert, S. and Sela, P. and Nativel, D. and Herzler, J. and Fikri, M. and Schulz, C.
    Journal of Physical Chemistry A 122 (2018)
    Shock-tube experiments have been performed to investigate the thermal decomposition of the oxygenated hydrocarbon dimethoxymethane (DMM; CH 3 OCH 2 OCH 3 ). The primary initial reaction channels of DMM decomposition are considered to be the two bond fissions: CH 3 OCH 2 OCH 3 → CH 3 O + CH 2 OCH 3 (1) and CH 3 OCH 2 OCH 3 → CH 3 + OCH 2 OCH 3 (2). In the present work, two shock-tube facilities and three different detection techniques have been combined: Behind reflected shock waves, we have carried out time-resolved measurements of (i) the formation of H atoms using the highly sensitive H-ARAS (Atomic Resonance Absorption Spectrometry) technique and (ii) the depletion of the DMM reactant by high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). In addition, (iii) the temperature-dependent composition of stable reaction products was measured in single-pulse shock-tube experiments via gas chromatography (GC/MS). The experiments span a temperature range of 1100-1430 K, a pressure range of 1.2-2.5 bar, and initial reactant mole fractions from 0.5 ppm (for H-ARAS experiments) up to 10 000 ppm (for HRR-TOF-MS experiments). Experimental rate constants k total , k total = k 1 + k 2 , obtained from these three completely different methods were in excellent agreement among each other, i.e., deviations are within ±30-40%, and they can be well represented by the Arrhenius expression k total (T) = 10 13.28±0.27 exp(-247.90 ± 6.36 kJ mol -1 /RT) s -1 (valid over the 1100-1400 K temperature and the 1.2-2.5 bar pressure range). By replacing the respective k total values used in a recently published DMM chemical kinetics combustion mechanism (Vermeire et al. Combust. Flame 2018, 190, 270-283), it was also possible to successfully reproduce measured product distributions. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpca.8b06558
  • Editorial
    Schulz, C. and Wang, H.
    Progress in Energy and Combustion Science 64 (2018)
    view abstract10.1016/j.pecs.2017.09.002
  • Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid
    Xu, G.-L. and Xiao, L. and Sheng, T. and Liu, J. and Hu, Y.-X. and Ma, T. and Amine, R. and Xie, Y. and Zhang, X. and Liu, Y. and Ren, Y. and Sun, C.-J. and Heald, S.M. and Kovacevic, J. and Sehlleier, Y.H. and Schulz, C. and Mattis, W.L. and Sun, S.-G. and Wiggers, H. and Chen, Z. and Amine, K.
    Nano Letters 18 (2018)
    Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na+ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li+, K+, Mg2+, and Al3+ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b04193
  • Experimental investigation of the influence of the pressure gradient on the transonic mixing behavior in blunt-body wakes using tracer LIF
    Beuting, M. and Schulz, C. and Richter, J. and Weigand, B.
    2018 Fluid Dynamics Conference (2018)
    Toluene laser induced-fluorescence (LIF) has been used to study the effect of a stream-wise pressure gradient on the mixing behavior in the transonic wake of two different types of central injectors feeding a low momentum flow into an accelerating co-flow. For this purpose, an optically-accessible, modular flow channel with a rectangular cross-section has been utilized: Following a nozzle module comprising a Ma = 1.7 convergent–divergent nozzle and the injector assembly, three interchangeable test modules imposing a pressure gradient by featuring different opening angles (0°, 0.25°, and 1.25°) can be attached. The two injectors investigated distinguish by the extent of their trailing edge into the sub-and supersonic region of the nozzle flow. It was found that the self-similarity of the mixing layer is preserved in all cases independent of the imposed pressure gradient. Calculated growth rates of the mixing layer agreed well with the half-power law. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.
    view abstract10.2514/6.2018-3543
  • Gas-phase synthesis of functional nanomaterials: Challenges to kinetics, diagnostics, and process development
    Schulz, C. and Dreier, T. and Fikri, M. and Wiggers, H.
    Proceedings of the Combustion Institute (2018)
    Gas-phase synthesis of nanoparticles enables production of high-purity materials with well-controlled properties in continuous flow processes. It is an established technology for a couple of (mostly inorganic) commodities with more or less specific materials characteristics. However, increasing process understanding and control provides a chance for scale-up of highly specialized lab-scale technologies used for the manufacturing of unique materials to industrial scale. Nanoparticles with adjustable composition and size distributions are of interest for a wide variety of applications from coatings to electronics to functional materials, e.g., for energy conversion and storage. For the synthesis of materials with desired properties, the reaction conditions must be well controlled. Understanding the decomposition kinetics and mechanisms of vaporized precursor compounds, cluster formation, and the potential interaction with bath gases and flame chemistry is a prerequisite for a targeted synthesis of materials. The scientific challenges concerning the precursor chemistry and particle formation and the experimental and theoretical approaches to overcome them have a large overlap with those in combustion science. Kinetics experiments are carried out in shock-tube reactors with optical and mass spectrometric detection of intermediate and product species, and in flow reactors with laser-based detection of temperature and species concentration as well as molecular-beam sampling techniques. Reaction conditions such as temperature, intermediate species concentration and particle size must be determined . in situ in lab-scale nanoparticle reactors and the definition of standardized experiments that allow to build-up large data bases for model development is important. The scale-up to pilot-plant-scale based on experimentally validated simulations finally helps to prove the viability of new technologies and their application on mass markets such as materials for batteries or catalysis. © 2018.
    view abstract10.1016/j.proci.2018.06.231
  • High-Temperature Rate Constants for H + Tetramethylsilane and H + Silane and Implications about Structure–Activity Relationships for Silanes
    Peukert, S. and Herzler, J. and Fikri, M. and Schulz, C.
    International Journal of Chemical Kinetics 50 (2018)
    The shock-tube technique has been used to investigate the reactions H + SiH4 → H2 + SiH3 (R1) and H + Si(CH3)4 → Si(CH3)3CH2 + H2 (R2) behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. For reaction (R1), the experiments covered a temperature range of 1170–1251 K and for (R2) 1227–1320 K. In both cases, the pressures were near 1.5 bar. In these experiments, H atoms were monitored with atomic resonance absorption spectrometry. Fits to the H-atom temporal concentration profiles applying postulated chemical kinetic reaction mechanisms were used for determining the rate constants k1 and k2. Experimental rate constants were well represented by the Arrhenius equations k1(T) = 2.75 × 10−9 exp(−37.78 kJ mol−1/RT) cm3 s−1 and k2(T) = 1.17 × 10−7 exp(−86.82 kJ mol−1/RT) cm3 s−1. Transition state theory (TST) calculations based on CBS-QB3 and G4 levels of theory show good agreement with experimentally obtained rate constants; the experimental values for k1 and k2 are ∼40% lower and ∼50% larger than theoretical predictions, respectively. For the development of a mechanism describing the thermal decomposition of tetramethylsilane (Si(CH3)4; TMS), also TST-based rate constants for reaction CH3 + Si(CH3)4 → Si(CH3)3CH2 + CH4 (R3) were calculated. A comparison between experimental and theoretical rate constants k2 and k3 with available rate constants from the literature indicates that Si(CH3)4 has very similar reactivity toward H abstractions like neopentane (C(CH3)4), which is the analog hydrocarbon to TMS. Based on these results, the possibility of drawing reactivity analogies between hydrocarbons and structurally similar silicon-organic compounds for H-atom abstractions is discussed. © 2017 Wiley Periodicals, Inc.
    view abstract10.1002/kin.21140
  • High-Temperature Rate Constants for the Reaction of Hydrogen Atoms with Tetramethoxysilane and Reactivity Analogies between Silanes and Oxygenated Hydrocarbons
    Peukert, S. and Yatsenko, P. and Fikri, M. and Schulz, C.
    Journal of Physical Chemistry A 122 (2018)
    The shock-tube technique has been used to investigate the H-abstraction reaction H + Si(OCH3)4 → H2 + Si(OCH2)(OCH3)3 behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. The experiments covered a temperature range of 1111-1238 K, and pressures of 1.3-1.4 bar. H atom concentrations were monitored with atomic resonance absorption spectrometry (ARAS). Fits to the temporal H atom concentration profiles based on a developed chemical kinetics reaction mechanism were used for determining bimolecular rate constants. Experimental total H-abstraction rate constants were well represented by the Arrhenius equation ktotal(T) = 10-9.16±0.24 exp(-25.5 ± 5.6 kJ mol-1/RT) cm3 s-1. Transition state theory (TST) calculations based on the G4 level of theory show excellent agreement with experimentally obtained rate constants, i.e., the theory values of k(T) deviate by less than 25% from the experimental results. Regarding H abstractions, we have compared the reactivity of C-H bonds in Si(OCH3)4 with the reactivity of C-H bonds in dimethyl ether (CH3OCH3). Present experimental and theoretical results indicate that at high temperatures, i.e., T > 500 K, CH3OCH3 is a good reactivity analog to Si(OCH3)4, i.e., kH+Si(OCH3)4(T) ∼ 1.5 × kH+CH3OCH3(T). On the basis of these results, we discuss the possibility of drawing reactivity analogies between oxygenated silanes and oxygenated hydrocarbons. Copyright © 2018 American Chemical Society.
    view abstract10.1021/acs.jpca.8b03160
  • LIISim: a modular signal processing toolbox for laser-induced incandescence measurements
    Mansmann, R. and Terheiden, T. and Schmidt, P. and Menser, J. and Dreier, T. and Endres, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 124 (2018)
    Evaluation of measurement data for laser-induced incandescence (LII) is a complex process, which involves many processing steps starting with import of data in various formats from the oscilloscope, signal processing for converting the raw signals to calibrated signals, application of models for spectroscopy/heat transfer and finally visualization, comparison, and extraction of data. We developed a software tool for the LII community that helps to evaluate, exchange, and compare measurement data among research groups and facilitate the application of this technique by providing powerful tools for signal processing, data analysis, and visualization of experimental results. A common file format for experimental data and settings simplifies inter-laboratory comparisons. It can be further used to establish a public measurement database for standardized flames or other soot/synthetic nanoparticle sources. The open-source concept and public access to the software development should encourage other scientists to validate and further improve the implemented algorithms and thus contribute to the project. In this paper, we present the structure of the LIISim software including the materials database concept, signal-processing algorithms, and the implemented models for spectroscopy and heat transfer. With two application cases, we show the operation of the software how data can be analyzed and evaluated. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-018-6934-9
  • Methodology for the investigation of ignition near hot surfaces in a high-pressure shock tube
    Niegemann, P. and Fikri, M. and Wlokas, I. and Röder, M. and Schulz, C.
    Review of Scientific Instruments 89 (2018)
    Autoignition of fuel/air mixtures is a determining process in internal combustion engines. Ignition can start either homogeneously in the gas phase after compression or in the vicinity of hot surfaces. While ignition properties of commercial fuels are conventionally described by a single quantity (octane number), it is known that some fuels have a varying propensity to the two processes. We present a new experimental concept that generates well-controlled temperature inhomogeneities in the shock-heated gases of a high-pressure shock tube. A shock-heated reactive mixture is brought into contact with a heated silicon nitride ceramic glow plug. The glow-plug temperature can be set up to 1200 K, higher than the post-reflected-shock gas temperatures (650-1050 K). High-repetition-rate chemiluminescence imaging is used to localize the onset of ignition in the vicinity of the hot surface. In experiments with ethanol, the results show that in most cases under shock-heated conditions, the ignition begins inhomogeneously in the vicinity of the glow plug and is favored because of the high wall temperature. Additionally, the interaction of geometry, external heating, and gas-dynamic effects was investigated by numerical simulations of the shock wave in a non-reactive flow. © 2018 Author(s).
    view abstract10.1063/1.5017275
  • Numerical investigation of transonic mixing behavior in the wake of a central injector at different reynolds numbers
    Richter, J. and Weigand, B. and Beuting, M. and Schulz, C.
    2018 Fluid Dynamics Conference (2018)
    This paper reports on the prediction of transonic mixing layers that develop downstream of two different types of central injectors. The injectors are placed in a convergent-divergent nozzle and differ only by the position of their trailing edges up-and downstream of the nozzle throat. Therefore, both sub-and supersonic wake flows are considered. The basic flow feature of these two wake flows is fundamentally different, however, both share that the development of the mixing layer strongly depends on the free-stream and injection flow condition. The position of the central injectors within the convergent-divergent nozzle is therefore of central importance. In this work, different streamwise injector positions corresponding to free-stream Reynolds numbers of 0.62 to 1.05 × 105 (initially subsonic wakes) and 1.38 to 1.63 × 105 (supersonic wakes) are numerically (2D URANS) investigated. In all subsonic mixing layers, regular vortex shedding was dominant. The vortex formation mechanism responded very sensitively to the flow conditions regarding the point of vortex formation and the shedding frequency. The resulting growth rate of the mixing layer could be notably enhanced at the injector position closest to the nozzle throat. In comparison, the mixing layers of all supersonic configurations were significantly narrower. In these cases, the mixing layer growth is influenced by the strength of two oblique shock waves that form downstream of the trailing edge. With increasing free-stream Mach number these shocks became stronger and the growth rate increased slightly. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.
    view abstract10.2514/6.2018-3544
  • Quantitative nitrogen oxide measurements by laser-induced fluorescence in diesel-like n-heptane jets with enhanced premixing
    Ottenwälder, T. and Schulz, C. and Raffius, T. and Koß, H.-J. and Grünefeld, G. and Heufer, K.A. and Pischinger, S.
    Combustion and Flame 188 (2018)
    Maintaining low NOx emissions over the operating range of diesel engines continues to be a major issue. However, quantitative measurements of nitric oxide (NO) are lacking especially in the core of diesel jets even if optical measurement techniques like laser-induced fluorescence (LIF) are used. NO may be present in the jet core due to enhanced fuel/air premixing. Close to the flame axis, severe light attenuation occurs. However, the diesel-like jets investigated in this study are largely non-sooting, so that light attenuation is reduced. n-heptane is used as diesel-surrogate fuel in the current investigations, which are performed in a combustion vessel. Quantitative NO measurements are conducted based on the knowledge of attenuation, temperature, the estimated equivalence ratio, and a calibration procedure. Attenuation turns out to be the most important factor in terms of measurement uncertainty. To avoid interfering LIF emissions by PAHs (polycyclic aromatic hydrocarbons) and O2, line imaging with a relatively high spectral resolution is conducted. Thereby, a higher accuracy can be achieved than by state-of-the-art planar imaging set-ups. Results show that NO initially forms throughout the jet cross-section. Quasi-steady NO concentrations measured in the jet core are compared to detailed kinetic simulations, taking the residence time of the fluid parcels in the main reaction zone into account. The residence-time “corrected” NO concentrations seemingly show a significant amount of prompt NO and are essentially consistent with simulation results for a corresponding equivalence ratio that was previously determined by Raman scattering. © 2017 The Combustion Institute
    view abstract10.1016/j.combustflame.2017.09.035
  • Response surface and group additivity methodology for estimation of thermodynamic properties of organosilanes
    Janbazi, H. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I. and Peukert, S.
    International Journal of Chemical Kinetics 50 (2018)
    Group additivity methods simplify the determination of thermodynamic properties of a wide range of chemically related species involved in detailed reaction schemes. In this paper, we expand Benson's group additivity method to organosilanes. Based on quantum-chemical calculations, the thermodynamic data of 22 stable silicon-organic species are calculated, presented in the form of NASA polynomials, and compared to the available experimental data. Based on this theoretical database, a complete set of 24 Si- and C-atom-centered, single-bonded and nonradical group additivity values for enthalpy of formation, standard entropy, and heat capacity at temperatures from 200 to 4000 K is derived through unweighted multivariate linear regression. © 2018 Wiley Periodicals, Inc.
    view abstract10.1002/kin.21192
  • Shock-tube study of the decomposition of tetramethylsilane using gas chromatography and high-repetition-rate time-of-flight mass spectrometry
    Sela, P. and Peukert, S. and Herzler, J. and Fikri, M. and Schulz, C.
    Physical Chemistry Chemical Physics 20 (2018)
    The decomposition of tetramethylsilane was studied in shock-tube experiments in a temperature range of 1270-1580 K and pressures ranging from 1.5 to 2.3 bar behind reflected shock waves combining gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). The main observed products were methane (CH4), ethylene (C2H4), ethane (C2H6), and acetylene (C2H2). In addition, the formation of a solid deposit was observed, which was identified to consist of silicon- and carbon-containing nanoparticles. A kinetics sub-mechanism with 13 silicon species and 20 silicon-containing reactions was developed. It was combined with the USC-MechII mechanism for hydrocarbons, which was able to simulate the experimental observations. The main decomposition channel of TMS is the Si-C bond scission forming methyl (CH3) and trimethylsilyl radicals (Si(CH3)3). The rate constant for TMS decomposition is represented by the Arrhenius expression ktotal[TMS → products] = 5.9 × 1012 exp(-267 kJ mol-1/RT) s-1. © the Owner Societies.
    view abstract10.1039/c7cp06827a
  • Soot formation in shock-wave-induced pyrolysis of acetylene and benzene with H 2 , O 2 , and CH 4 addition
    Drakon, A. and Eremin, A. and Mikheyeva, E. and Shu, B. and Fikri, M. and Schulz, C.
    Combustion and Flame 198 (2018)
    Experiments on the pyrolysis of C 2 H 2 /Ar and C 6 H 6 /Ar mixtures with addition of H 2 , O 2 , and CH 4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm -1 using quantum-cascade lasers. For this purpose, 0.5–0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O 2 , and CH 4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H 2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H 2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH 4 and O 2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot. © 2018 The Combustion Institute
    view abstract10.1016/j.combustflame.2018.09.014
  • Strategy for determining absolute concentration levels of SiO in low pressure gas phase synthesis flames for silica nanoparticles
    Chrystie, R. and Ebertz, F. and Dreier, T. and Schulz, C.
    Optics InfoBase Conference Papers Part F103-LACSEA 2018 (2018)
    Silica nanoparticles are conveniently synthesized in gas phase H<inf>2</inf>/O<inf>2</inf> premixed flames and a silicon-carrying precursor (e.g., hexamethyldisiloxane, HMDSO). For flame kinetics mechanism validation including particle growth a technique for absolute concentration measurements of the intermediate SiO based on laser-induced fluorescence and Rayleigh scattering is demonstrated. © 2018 The Author(s).
    view abstract10.1364/LACSEA.2018.LW3C.6
  • Temperature, pressure, and oxygen quenching behavior of fluorescence spectra and lifetimes of gas-phase o-xylene and 1,2,4-trimethylbenzene
    Benzler, T. and Endres, T. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 124 (2018)
    Ortho-xylene (1,2-dimethylbenzene, XL) and 1,2,4-trimethylbenzene (TMB) are promising aromatic fluorescence tracer species for gas-phase imaging measurements of concentration, temperature, and oxygen partial pressure. In the present work, temperature-dependent gas-phase ultraviolet absorption spectra of XL and TMB were measured. In the investigated temperature range (296–725 K), the absorption bands red-shift with increasing temperature for both species and their absorption cross-sections increase. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (XL 296–1025 K, TMB 296–775 K), pressure (1–10 bar), and O2 concentration using a streak camera coupled to a spectrometer. The fluorescence spectra of both species show a noticeable red-shift with increasing temperature and O2 concentration. In N2 as bath gas, the fluorescence lifetime of XL and TMB decreases by three orders of magnitude at the peak temperatures compared to room temperature. For both species, fluorescence quenching by N2 (up to 10 bar) is temperature-dependent and is strongest at about 500 K. Quenching by O2 shortens the fluorescence lifetime for both species significantly. This effect is much reduced at higher temperatures. The temperature dependence of the Stern–Volmer coefficients that describe the effect of O2 quenching can be approximated by an exponential decay. Semi-empirical exponential fits to all investigated data (for XL and TMB) as well as published data for toluene were used to provide signal prediction models that are capable of predicting the signal intensities over a wide range of environmental conditions. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00340-018-6937-6
  • The influence of hydrogen and methane on the growth of carbon particles during acetylene pyrolysis in a burnt-gas flow reactor
    Peukert, S. and Sallom, A. and Emelianov, A. and Endres, T. and Fikri, M. and Böhm, H. and Jander, H. and Eremin, A. and Schulz, C.
    Proceedings of the Combustion Institute (2018)
    The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N2/C2H2, and N2/C2H2 with addition of H2 or CH4 surrounded by a rich C2H4/air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H2 to the C2H2/N2 flow on both, particle sizes and soot volume fractions. The effect of CH4 addition to the C2H2/N2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N2:CH4:C2H2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH4, while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C2H4/air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH4 on the soot particle growth. © 2018.
    view abstract10.1016/j.proci.2018.05.049
  • Water film thickness imaging based on time-multiplexed near-infrared absorption
    Lubnow, M. and Jeffries, J.B. and Dreier, T. and Schulz, C.
    Optics Express 26 (2018)
    We demonstrate the imaging of the thickness of liquid water thin films in the 100–1500 µm range at a constant temperature by monitoring the pixel-by-pixel ratio of absorbance at two near-infrared (NIR) wavelengths near 1400 nm detected with a fast framing InGaAs focal-plane array camera. Experiments were performed in reflection mode with films of pure water and water/ethanol mixtures supported on opaque surfaces using two illumination–detection configurations. One scheme uses specular reflection of incident and reflected linearly polarized diode-laser light at Brewster’s angle, which enables detection of signal light that has twice traversed the liquid film with negligible interference from unwanted partial reflections of the incoming beams at the front surface interfaces (air/window and window/water for films constrained by a cover plate or air/water for free-standing films). The second configuration located the detection camera perpendicular above the surface where the detected light was transmitted through the sample and diffusely scattered from the support surface. Imaging measurements of film thickness using both configurations were successfully demonstrated. Time-resolved measurements capture the dynamics of flowing water films or waves generated by droplet impingement. © 2018 Optical Society of America
    view abstract10.1364/OE.26.020902
  • Water film thickness imaging based on time-multiplexed near-infrared absorption
    Lubnow, M. and Dreier, T. and Schulz, C.
    Optics InfoBase Conference Papers Part F103-LACSEA 2018 (2018)
    We demonstrate the imaging of water film thickness at constant temperature by exploiting absorbance ratios of near-infrared (NIR) radiation at two wavelengths in the water absorption spectrum around 1400 nm with light delivered by diode lasers and signal registered by a fast framing InGaAs focal plane array camera. Measurements are performed in reflection mode from opaque film support surfaces. © 2018 The Author(s).
    view abstract10.1364/LACSEA.2018.LW2C.3
  • A quantum chemical and kinetics modeling study on the autoignition mechanism of diethyl ether
    Sakai, Y. and Herzler, J. and Werler, M. and Schulz, C. and Fikri, M.
    Proceedings of the Combustion Institute 36 (2017)
    A detailed chemical kinetics model has been developed to elucidate the auto-ignition behavior of diethyl ether (DEE) under conditions relevant for internal combustion engines. The present model is composed of a C0-C4 base module from literature and a DEE module. For the low-temperature oxidation mechanism, the reactions of ROO and QOOH radicals were studied previously with a quantum-chemical and transition state theory approach by Sakai et al. (2015). In the present study, the potential energy surfaces for the unimolecular reactions of OOQOOH isomers and 1- and 2-ethoxyethyl radicals were determined with a CBSQB3 composite method. In the presence of an OOH group, the reaction barrier of the hydrogen shift from the β site (terminal carbon atom) decreases as it does in alkane oxidation but there is no effect on the hydrogen shift from the α site (next to the ether oxygen atom). Therefore, the reaction barriers of OOQOOH isomers have the same trend as the corresponding ROO radical and rate constants for the reactions of OOQOOH isomers were determined. The constructed model was validated against the recent data of ignition delay times provided in literature by Werler et al. (2015). The agreement is good over the temperature range 500-1300K and pressure range 1-40bar, although, open questions remain regarding the non-consensus at 900-1150K and 40bar. Reaction-path and sensitivity analyses attribute the importance of the reactivity at the α site to the decrease of the C H bond dissociation energy due to the ether oxygen atom. © 2016.
    view abstract10.1016/j.proci.2016.06.037
  • A Shock Tube and Modeling Study about Anisole Pyrolysis Using Time-Resolved CO Absorption Measurements
    Shu, B. and Herzler, J. and Peukert, S. and Fikri, M. and Schulz, C.
    International Journal of Chemical Kinetics 49 (2017)
    The pyrolysis of anisole (C6H5OCH3) was studied behind reflected shock waves via highly sensitive absorption measurements of CO concentration using a rotational transition in the fundamental vibrational band near 4.7 µm. Time-resolved CO mole fractions were monitored in shock-heated C6H5OCH3/Ar mixtures between 1000 and 1270 K at 1.3–1.6 bar. The decomposition of C6H5OCH3 proceeds exclusively via homolytic dissociation, with reaction rate k1, forming methyl (CH3) and phenoxy (C6H5O) radicals. The subsequent decomposition of C6H5O by ring rearrangement and bond dissociation yields CO. To determine the rate constant k2 of C6H5O decomposition avoiding secondary reactions, allyl phenyl ether (C6H5OC3H5) was used as an alternative source for C6H5O. Its decomposition was studied between 970 and 1170 K at ∼1.4 bar. The potential-energy surface of C6H5O dissociation has been reevaluated at the G4 level of theory. Rate constants determined from unimolecular rate theory are in good agreement with the present experiments. However, the obtained rates k2 = 9.1 × 1013 exp(−220.3 kJ mol−1/RT)s−1 are significantly higher than those reported before (factor 6, 2, and 1.5 faster than those data reported by Lin and Lin, J. Phys. Chem. 1986, 90, 425–431; Frank et al., 1994; Carstensen and Dean, 2012, respectively). Good agreement was found between the measured CO concentration profiles and simulations based on the mechanism of Nowakowska et al. after substituting k2 by the value obtained from experiments on C6H5OC3H5 in this work. The bimolecular reaction of C6H5O and CH3 toward cresol was identified as the most important reaction influencing the CO concentration at longer reaction time. © 2017 Wiley Periodicals, Inc.
    view abstract10.1002/kin.21105
  • Experimental and numerical study of a HMDSO-seeded premixed laminar low-pressure flame for SiO2 nanoparticle synthesis
    Feroughi, O.M. and Deng, L. and Kluge, S. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    Silicon dioxide nanoparticles are generated in a lean hydrogen/oxygen flat flame doped with small amounts of hexamethyldisiloxane (HMDSO) stabilized by a water-cooled sintered bronze matrix. The burner is housed in an optically-accessible low-pressure (3kPa) chamber. Temperature fields were determined via multi-line laser-induced fluorescence (LIF) using added NO as target species. Gas-phase silicon oxide (SiO) was detected via laser-induced fluorescence (LIF) by exciting the weakly temperature-dependent rovibrational Q11(32) transition in the A-X (0,0) vibronic band system at 235.087nm. Semi-quantitative concentration profiles as a function of height-above-burner (HAB) were obtained after exploiting the measured temperature fields and correcting measured LIF intensities for the temperature-dependence of the ground-state population and collisional quenching using measured effective fluorescence lifetimes. Particle sizes were determined as a function of HAB via molecular-beam sampling with subsequent particle mass spectrometry (PMS). The experimental data were used to develop a simple kinetics model of HMDSO combustion and SiO2 particle precursor formation with subsequent nucleation and particle growth in the H2/O2 flame. The model was incorporated in a CFD simulation to account for facility effects that arise from modified flow fields and heat transfer between the flame and the reactor chamber. © 2016 Elsevier Ltd.
    view abstract10.1016/j.proci.2016.07.131
  • Flame-temperature, light-attenuation, and CO measurements by spontaneous Raman scattering in non-sooting diesel-like jets
    Raffius, T. and Schulz, C. and Ottenwälder, T. and Grünefeld, G. and Koß, H.-J. and Brands, T. and Pischinger, S.
    Combustion and Flame 176 (2017)
    Quantitative spatially resolved measurements of temperature and species are lacking particularly in the core of combusting diesel jets. Major problems are light attenuation and interfering light emissions. However, these factors are reduced in non-sooting diesel-like jets, as demonstrated in the present work, because light is not attenuated by soot and interfering LIF (laser-induced fluorescence) from PAHs (polycyclic aromatic hydrocarbons) is substantially lower. The current results show that thermometry by SRS (spontaneous Raman scattering) excited by a UV (ultraviolet) laser is therefore feasible even in the core of a non-sooting diesel-like jet in a combustion vessel. Two diagnostic approaches are assessed. The first one is based on the spectral band shape of the Stokes (red-shifted) ro-vibrational SRS from N2, whereas the ratio of integrated ro-vibrational Stokes to anti-Stokes (blue-shifted) N2-SRS bands is exploited in the second one. It turns out that the first method is advantageous in terms of light attenuation by molecular species, the influence of interfering emissions, and resulting single-shot capability. However, these investigations also show that the anti-Stokes N2-SRS signal can be used for quantification of light attenuation. This is particularly attractive because this SRS band at ∼235 nm nearly coincides with a LIF emission from NO at ∼237 nm, leading to improved attenuation correction of NO-LIF. Furthermore, the recorded spectra indicate that additional quantitative species measurements by SRS are feasible in the non-sooting jet. For instance, the mole fraction of CO is quantified in this work for the first time in the jet core. © 2016 The Combustion Institute
    view abstract10.1016/j.combustflame.2016.09.027
  • Ignition delay times of Jet A-1 fuel: Measurements in a high-pressure shock tube and a rapid compression machine
    De Toni, A.R. and Werler, M. and Hartmann, R.M. and Cancino, L.R. and Schießl, R. and Fikri, M. and Schulz, C. and Oliveira, A.A.M. and Oliveira, E.J. and Rocha, M.I.
    Proceedings of the Combustion Institute 36 (2017)
    Ignition delay time (IDT) measurements for Jet A-1 fuel samples have been performed with a rapid compression machine (RCM) and a high-pressure shock tube (ST). The IDT measurements span a pressure range from 7 to 30. bar, a temperature range from 670. K to 1200. K, and fuel/air equivalence ratios ϕ(symbol). from 0.3 to 1.3. Expressions fitting the experimental data sets were obtained, with fitting parameters being provided. The combined RCM/ST data aimed at providing information on the two-stage ignition behavior and on the transition from NTC chemistry to high-temperature radical chain-branching, which are important and hard to meet targets in the development of chemical surrogates. © 2016 The Combustion Institute.
    view abstract10.1016/j.proci.2016.07.024
  • Inline coating of silicon nanoparticles in a plasma reactor: Reactor design, simulation and experiment
    Münzer, A. and Sellmann, J. and Fortugno, P. and Kempf, A. and Schulz, C. and Wiggers, H.
    Materials Today: Proceedings 4 (2017)
    Gas-phase synthesis of nanoparticles (NPs) in hot plasmas is a promising approach to produce pure, highly specific, and complex nanomaterials at large production rates. Post-processing of the material by particle coating, embedding, or surface functionalization is often required to adjust the materials' properties with respect to their utilization in functional structures. Due to the high surface-to-volume ratio, the nanoparticles' surface properties strongly influence the processing and thus their applicability. We report on a scalable and continuous gas-phase synthesis process of silicon nanoparticles by a high-temperature single-step plasma process with subsequent inline coating. Our process requires a two-stage supply of process gases: First, silicon nanoparticles (Si-NPs) are formed from the gaseous precursor monosilane (SiH4) after its decomposition in the plasma zone. Secondly, the coating agent ethylene (C2H4) is mixed with the hot, particle-laden gas flow downstream of the plasma zone via a specifically-designed coating nozzle. To facilitate a homogeneous intermixing of C2H4 and the nanoparticle-laden gas stream, fluid dynamics simulations were performed to design and optimize the geometry of the coating nozzle. The process conditions can be varied to tune the decomposition process of gaseous C2H4 in respect to coating the Si-NP surface. As a result, we are able to tune the composition of the nanoparticles. Product characterization by X-ray diffraction, Raman, FTIR and X-ray photoelectron spectroscopy revealed that either SiC, or silicon with a carbon-like or a polyethylene-like shell is produced respectively, with increasing distance of the coating nozzle from the plasma. For all process conditions, spherical, coated particles with a highly-crystalline silicon core were observed as indicated by TEM measurements.
    view abstract10.1016/j.matpr.2017.09.176
  • Instantaneous 3D imaging of highly turbulent flames using computed tomography of chemiluminescence
    Mohri, K. and Göers, S. and Schöler, J. and Rittler, A. and Dreier, T. and Schulz, C. and Kempf, A.
    Applied Optics 56 (2017)
    The computed tomography of chemiluminescence (CTC) technique was applied for the first time to a real highly turbulent swirl flame setup, using a large number of CCD cameras (N q = 24 views), to directly reconstruct the three-dimensional instantaneous and time-averaged chemiluminescence fields. The views were obtained from a 172.5° region (in one plane) around the flame, and the CTC algorithm [Floyd et al., Combust. Flame 158, 376 (2011)] was used to reconstruct the flame by discretizing the domain into voxels. We investigated how the reconstructions are affected by the views’ arrangement and the settings of the algorithm, and considered how the quality of reconstructions should be assessed to ensure a realistic description of the capabilities of the technique. Reconstructions using N q ≤ 12 were generally better when the cameras were distributed more equiangularly. When N q was severely low (e.g., 3), the reconstruction could be improved by using fewer voxels. The paper concludes with a summary of the strengths and weaknesses of the CTC technique for examining a real turbulent flame geometry and provides guidance on best practice. © 2017 Optical Society of America.
    view abstract10.1364/AO.56.007385
  • Laser-induced atomic emission of silicon nanoparticles during laser-induced heating
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Optics 56 (2017)
    The temporal luminescence behavior of silicon atoms during and after laser-heating of gas-borne silicon nano-particles was investigated. Silicon nanoparticles were formed in the exhaust stream of a microwave plasma reactor at 100 mbar. The observed prompt atomic line intensities correspond with thermal excitation of the evaporated species. A prompt signal at 251.61 and 288.15 nm originating from the 3s23p2 → 3s23p4s transitions showed a lifetime of 16 ns that matches the documented excited-state lifetime for the respective transitions. A secondary delayed signal contribution with similar peak intensities was observed commencing approximately 100-300 ns after the laser pulse and persisting for hundreds of nanoseconds. This signal contribution is attributed to electron impact excitation or recombination after electron impact ionization of the silicon evaporated as a consequence of the laser heating of the plasma leading to non-thermal population of electronically excited silicon. The observations support a nanoparticle evaporation model that can be used to recover nanoparticle sizes from time-resolved LII data. © 2017 Optical Society of America.
    view abstract10.1364/AO.56.000E50
  • Mass spectrometric analysis of clusters and nanoparticles during the gas-phase synthesis of tungsten oxide
    Kluge, S. and Wiggers, H. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    The combustion synthesis of nanoscale tungsten-oxide particles from tungsten hexafluoride is investigated in a low-pressure hydrogen/oxygen flat flame. The reactor is equipped with molecular-beam sampling of post-flame gases at variable height above burner (HAB). Main species of the flame, intermediate tungsten species, and tungsten-oxide clusters are studied with time-of-flight mass spectrometry (TOF-MS) as a function of HAB. Various WO x (OH) y are identified within the flame front. With increasing HAB, (WO3) n clusters with increasing cluster size appear in the burnt gases at the expense of the concentration of W1 species. Clusters with n =3-7 arise at 70mm HAB, followed by larger clusters at even larger heights. Clusters up to (WO3)38 were identified. The subsequent formation of nanoparticles is detected with particle mass spectrometry (PMS) and a quartz crystal microbalance (QCM) from 120mm HAB and the increasing particle size and mass flux have been determined. © 2016.
    view abstract10.1016/j.proci.2016.06.165
  • Micrometer-sized nano-structured silicon/carbon composites for lithium-ion battery anodes synthesized based on a three-step Hansen solubility parameter (HSP) concept
    Sehlleier, Y.H. and Dobrowolny, S. and Xiao, L. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Industrial and Engineering Chemistry 52 (2017)
    The processing towards Si/C composites, components and synthesis parameters were selected based on the concept of Hansen solubility parameters (HSP). Si/polymer composites were generated through modified bulk polymerization and subsequent pyrolysis transformed the polymer into the desired porous carbon matrix. Coulombic efficiencies (CE) in excess of 76% after the first cycle and 99.95% after solid electrolyte interphase (SEI) formation have been achieved. A notably high specific delithiation capacity of around 1600 mAh/g with an extremely stable cycling performance even after 400 cycles is obtained. This scalable and economical synthesis approach is readily applicable to the commercial production of anode materials. © 2017 The Korean Society of Industrial and Engineering Chemistry
    view abstract10.1016/j.jiec.2017.04.001
  • Novel Si-CNT/polyaniline nanocomposites as Lithium-ion battery anodes for improved cycling performance
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Materials Today: Proceedings 4 (2017)
    A novel nanocomposite consisting of gas-phased produced Si nanoparticles, carbon nanotubes (CNTs), and polyaniline (PANi) is developed as an anode material (Si-CNT/PANi) for lithium-ion batteries. This nanocomposite integrates the merits from its three components, where Si nanoparticles provide high capacity, CNTs act as an electrically conductive and mechanically flexible network, and PANi coating further enhances the electrical conductivity and protects the silicon structure. An anode made of this nanocomposite shows a high reversible capacity of 2430 mAh/g with good capacity retention over 500 cycles compared to pristine Si. The Si-CNT/PANi nanocomposite also demonstrated a high Coulombic efficiency and improved rate-capabilities. © 2017 Elsevier Ltd.
    view abstract10.1016/j.matpr.2017.09.197
  • Optical properties and pyrolysis of shock-heated gas-phase anisole
    Zabeti, S. and Aghsaee, M. and Fikri, M. and Welz, O. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    Anisole (C6H5OCH3) has previously been identified as a fluorescence tracer for fuel/air mixing studies based on laser-induced fluorescence (LIF) that provides stronger signal than the frequently-used toluene (Faust et al. (2013)). Reliable and quantitative application of anisole LIF at high temperature requires knowledge about its photophysical properties and its thermal stability. In this work, ultraviolet absorption and LIF of anisole were measured as a function of time in shock-heated gases at temperatures between 565 and 1620K and at pressures of 1.3 and 2.9bar. Absorption spectra were acquired with 50μs time resolution in the 240-310nm spectral range. LIF emission spectra of anisole were recorded at different fixed reaction times behind reflected shock waves using 266-nm laser excitation. Relative fluorescence quantum yields were determined that extend previously reported data beyond 980K. After the onset of pyrolysis of anisole at T >1000K, effective absorption cross-sections and the corresponding LIF signals after 266-nm excitation are reported. To aid the interpretation of these experiments, the products of anisole pyrolysis were investigated using a shock tube coupled to a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) for time-resolved multispecies measurements. Concentration-time profiles for anisole and products such as benzene, C2H4, and CO were measured and compared to simulations using two kinetics models from literature. © 2016.
    view abstract10.1016/j.proci.2016.06.156
  • Parasitic Reactions in Nanosized Silicon Anodes for Lithium-Ion Batteries
    Gao, H. and Xiao, L. and Plümel, I. and Xu, G.-L. and Ren, Y. and Zuo, X. and Liu, Y. and Schulz, C. and Wiggers, H. and Amine, K. and Chen, Z.
    Nano Letters 17 (2017)
    When designing nano-Si electrodes for lithium-ion batteries, the detrimental effect of the c-Li15Si4 phase formed upon full lithiation is often a concern. In this study, Si nanoparticles with controlled particle sizes and morphology were synthesized, and parasitic reactions of the metastable c-Li15Si4 phase with the nonaqueous electrolyte was investigated. The use of smaller Si nanoparticles (∼60 nm) and the addition of fluoroethylene carbonate additive played decisive roles in the parasitic reactions such that the c-Li15Si4 phase could disappear at the end of lithiation. This suppression of c-Li15Si4 improved the cycle life of the nano-Si electrodes but with a little loss of specific capacity. In addition, the characteristic c-Li15Si4 peak in the differential capacity (dQ/dV) plots can be used as an early-stage indicator of cell capacity fade during cycling. Our findings can contribute to the design guidelines of Si electrodes and allow us to quantify another factor to the performance of the Si electrodes. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.6b04551
  • Performance of photomultipliers in the context of laser-induced incandescence
    Mansmann, R. and Dreier, T. and Schulz, C.
    Applied Optics 56 (2017)
    Photomultiplier tubes (PMTs) are widely used as detectors for laser-induced incandescence (LII), a diagnostics method for gas-borne particles that requires signal detection over a large dynamic range with nanosecond time resolution around the signal peak. Especially when more than one PMT is used (i.e., for pyrometric temperature measurements) even small deviations from the linear detector response can lead to significant errors. Reasons for non-linearity observed in other PMT measurement techniques are summarized and strategies to identify nonlinear PMT operation in LII are outlined. To quantify the influence of the non-linear behavior, experiments at similar light levels as those encountered in LII measurements are carried out, and errors propagated in two-color pyrometry-derived temperatures are determined. As light sources, a calibrated broadband light source and light-emitting diodes (LEDs), centered at the bandpass filter wavelengths of the LII detectors, were used. The LEDs were operated in continuous and pulsed (<300 ns) mode, respectively, to simulate DC background radiation (e.g., from sooting flames) and similar pulsed signal traces as in typical LII measurements. A measured linearity deviation of up to 10% could bias the temperature determination by several hundred Kelvin. Guidelines are given for the design and the operation of LII setups, which allow users to identify and prevent errors. © 2017 Optical Society of America
    view abstract10.1364/AO.56.007849
  • Reaction-time-resolved measurements of laser-induced fluorescence in a shock tube with a single laser pulse
    Zabeti, S. and Fikri, M. and Schulz, C.
    Review of Scientific Instruments 88 (2017)
    Shock tubes allow for the study of ultra-fast gas-phase reactions on the microsecond time scale. Because the repetition rate of the experiments is low, it is crucial to gain as much information as possible from each individual measurement. While reaction-time-resolved species concentration and temperature measurements with fast absorption methods are established, conventional laser-induced fluorescence (LIF) measurements with pulsed lasers provide data only at a single reaction time. Therefore, fluorescence methods have rarely been used in shock-tube diagnostics. In this paper, a novel experimental concept is presented that allows reaction-time-resolved LIF measurements with one single laser pulse using a test section that is equipped with several optical ports. After the passage of the shock wave, the reactive mixture is excited along the center of the tube with a 266-nm laser beam directed through a window in the end wall of the shock tube. The emitted LIF signal is collected through elongated sidewall windows and focused onto the entrance slit of an imaging spectrometer coupled to an intensified CCD camera. The one-dimensional spatial resolution of the measurement translates into a reaction-time-resolved measurement while the species information can be gained from the spectral axis of the detected two-dimensional image. Anisole pyrolysis was selected as the benchmark reaction to demonstrate the new apparatus. © 2017 Author(s).
    view abstract10.1063/1.5010228
  • Self-quenching in toluene LIF
    Fuhrmann, D. and Benzler, T. and Fernando, S. and Endres, T. and Dreier, T. and Kaiser, S.A. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    Toluene is frequently used as laser-induced fluorescence (LIF) tracer for visualizing mixing processes, for example, in internal combustion engines. The signal evaluation relies on a linear dependence of the LIF signal on tracer concentration - which is not present in many practically relevant cases. This paper presents an investigation of the dependence of the LIF signal intensities on the toluene concentration, revealing a non-linear signal response already at concentrations approximately ten times below those given by the room-temperature vapor pressure. Toluene was vaporized in a mass-flow controlled evaporator and investigated in a free jet. Nitrogen was used as bath gas with a variable addition of oxygen. After excitation at 266. nm, an intensified CCD camera recorded the spectrally filtered fluorescence. In separate experiments, the effective fluorescence lifetime upon picosecond UV-laser excitation was determined. The results indicate that the fluorescence lifetime decreases with increasing tracer concentration due to self-quenching. Results from imaging and fluorescence lifetime measurements are consistent. The investigation reveals that the self-quenching of toluene is dominated by collisions with excited-state toluene molecules, which causes an additional dependence of the magnitude of self-quenching on the laser fluence. © 2016.
    view abstract10.1016/j.proci.2016.06.045
  • Sequential signal detection for high dynamic range time-resolved laser-induced incandescence
    Mansmann, R. and Thomson, K. and Smallwood, G. and Dreier, T. and Schulz, C.
    Optics Express 25 (2017)
    A new method for collecting time-resolved laser-induced incandescence (TiRe-LII) signals with high dynamic range is presented. Gated photomultiplier tubes (PMT) are used to detect temporal sections of the LII signal. This helps to overcome the limitations of PMTs caused by restricted maximum signal current at the strong initial signal and poor signal-to-noise ratios when the signal intensity approaches the noise level. We present a simple method for increasing the accuracy of two-color pyrometry at later decay times and two advanced strategies for getting high accuracy over the complete temperature trace or even achieve single-shot capability with high dynamic range. Validation measurements in a standardized flame show that the method is sensitive enough to even resolve the local increase in gas temperature as a consequence of heating the soot particles with a laser pulse. © 2017 Optical Society of America.
    view abstract10.1364/OE.25.002413
  • SiO multi-line laser-induced fluorescence for quantitative temperature imaging in flame-synthesis of nanoparticles
    Chrystie, R.S.M. and Feroughi, O.M. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 123 (2017)
    Silicon monoxide (SiO) is an intermediate in the gas-phase synthesis of SiO2 nanoparticles and coatings. We demonstrate a method for in situ imaging the gas-phase temperature via multi-line laser-induced fluorescence (LIF) using excitation in the A1Π–X1Σ+(0,0) band near 235 nm. A low-pressure lean (3 kPa, φ = 0.39) premixed hydrogen/oxygen flame was seeded with 210 ppm hexamethyldisiloxane (HMDSO) to produce SiO2 nanoparticles. Spectral regions with no interference from other species in the flame were located, and the excitation-spectral range that provides the best temperature sensitivity was determined from numerical experiments. Quenching rates of the selected transitions were also determined from fluorescence lifetime measurements, and found to be independent of the excited rotational state. Upon laser light-sheet excitation, images of fluorescence were recorded for a sequence of excitation wavelengths and pixel-wise multi-line fitting of the spectra yields temperature images. The results were compared against multi-line NO-LIF temperature imaging measurements using the A2Σ+–X2Π(0,0) band near 225 nm from 500 ppm NO added to the gas flow as a thermometry target. Both methods show good qualitative agreement with each other and demonstrate that temperature can be evaluated from the zone in the reactor where SiO is naturally present without adding tracers. SiO LIF exhibited high signal-to-noise ratios of the order of ten times that of NO LIF. © 2017, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-017-6692-0
  • Spectroscopic models for laser-heated silicon and copper nanoparticles
    Daun, K. and Menser, J. and Mansmann, R. and Moghaddam, S.T. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 197 (2017)
    Interpreting laser-induced incandescence (LII) measurements on aerosolized nanoparticles requires a spectroscopic model that relates the measured spectral incandescence to the temperature of the nanoparticles. We present spectroscopic models for molten silicon and copper nanoparticles, which are evaluated through extinction and incandescence measurements on nanoaerosols. Measurements on molten silicon nanoparticles are consistent with the Drude theory in the Rayleigh limit of Mie theory. The copper nanoparticles were initially assumed to coalesce into spheres, but the observed spectral incandescence does not show a surface plasmon polariton (SPP) peak in the vicinity of 600. nm expected of spheres. A simulation based on the discrete dipole approximation (DDA) suggests that this effect could be explained by the structure of the copper aggregates. © 2016.
    view abstract10.1016/j.jqsrt.2016.10.006
  • Ultraviolet absorption and laser-induced fluorescence of shock-heated acetylene
    Zabeti, S. and Fikri, M. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    Acetylene as a major intermediate in fuel pyrolysis and combustion has so far mostly been ignored as a contributor to UV absorption and fluorescence. Temperature-dependent ultraviolet absorption cross-sections of acetylene (C2H2) have been studied behind reflected shock waves between 565 and 1500K. Light from a deuterium lamp was transmitted through shock-heated gas mixtures and transmission spectra were recorded with ∼50μs time resolution. The absorption spectra strongly depend on temperature and show a strong red-shift with temperature. Above 900K, significant absorption is observed at 266nm. Laser-induced fluorescence (LIF) spectra were recorded in the shock tube using 266-nm laser excitation. In accord with absorption experiments, no LIF signal is observed below 900K. Above, the fluorescence intensity increases and the fluorescence spectra become broader over the studied temperature range. From fluorescence intensity and absorption cross-sections, relative effective fluorescence quantum yields are determined as a function of temperature. © 2016.
    view abstract10.1016/j.proci.2016.05.018
  • Uncertainty quantification and design-of-experiment in absorption-based aqueous film parameter measurements using Bayesian inference
    Pan, R. and Daun, K.J. and Dreier, T. and Schulz, C.
    Applied Optics 56 (2017)
    Diode laser-based multi-wavelength near-infrared (NIR) absorption in aqueous films is a promising diagnostic for making temporally resolved, simultaneous measurements of film thickness, temperature, and concentration of a solute. Our previous work in aqueous urea solutions aimed at determining simultaneously two of these system parameters, while the third one must be fixed or specified by additional measurements. The current work presents a simultaneous NIR absorption-based multi-parameter measurement of thickness, temperature, and solute concentration coupled with the Bayesian methodology that is used to infer probability densities for the obtained data. The Bayesian analysis is based on a temperature- and concentration-dependent spectral database generated with a Fourier transform infrared spectrometer in the range 5500-8000 cm-1 for water with variable temperature and urea concentration. The concept was first validated with measurements using a calibration cell. Probability densities in the measured parameters were quantified using a Markov chain Monte Carlo algorithm, which were used to derive credibility intervals. As a practical demonstration, the temporal variation of film thickness, urea concentration, and liquid temperature were recorded during evaporation of a liquid film deposited on a transparent heated quartz plate. © 2017 Optical Society of America.
    view abstract10.1364/AO.56.0000E1
  • UV absorption and fluorescence properties of gas-phase p-difluorobenzene
    Benzler, T. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 123 (2017)
    1,4-Difluorobenzene (p-DFB) is a promising aromatic tracer for determining concentration, temperature, and O2 partial pressure in mixing gas flows based on laser-induced fluorescence (LIF). Signal quantification requires the knowledge of absorption and fluorescence properties as a function of environmental conditions. We report absorption and fluorescence spectra as well as fluorescence lifetimes of p-DFB in the temperature, pressure, and oxygen partial pressure range that is relevant for many applications including internal combustion engines. The UV absorption cross section, investigated between 296 and 675 K, has a peak value close to 266 nm and decreases with temperature, while still exceeding other single-ring aromatics. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (296–1180 K), pressure (1–10 bar), and O2 partial pressure (0–210 mbar) using a streak camera (temporal resolution 50 ps) coupled to a spectrometer. The fluorescence spectra red-shift (~2 nm/100 K) and broaden (increase in full width at half maximum by 58% in the investigated temperature range) with temperature. In N2 as bath gas (1 bar), the fluorescence lifetime τeff decreases with temperature by a factor of about 20 (from 7 ns at 298 K down to 0.32 ns at 1180 K), while at 8 bar the shortest lifetime at 975 K is 0.4 ns. A noticeable pressure dependence (i.e., reduced τeff) is only visible at 675 K and above. Quenching of p-DFB LIF by O2 (for partial pressures up to 210 mbar) shortens the fluorescence lifetime significantly at room temperature (by a factor of 8), but much less at higher temperatures (by a factor of 1.8 at 970 K). For fixed O2 partial pressures (52 mbar and above), τeff shows a plateau region with temperature which shifts toward higher temperatures at the higher O2 partial pressures. O2 quenching is less prominent for p-DFB compared to other aromatic compounds investigated so far. The temperature dependence of O2 quenching can be approximately expressed by an exponential function. The influence of temperature, total pressure, and O2 partial pressure on absorption cross sections and fluorescence quantum yields are given as empirical functions that allow for interpolation. For typical applications, p-DFB LIF provides up to three orders of magnitude stronger signal compared to toluene LIF. © 2017, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-016-6612-8
  • UV absorption and fluorescence properties of gas-phase p-difluorobenzene
    Benzler, T. and Dreier, T. and Schulz, C.
    Applied Physics B-lasers and Optics 123 (2017)
    1,4-Difluorobenzene (p-DFB) is a promising aromatic tracer for determining concentration, temperature, and O-2 partial pressure in mixing gas flows based on laser-induced fluorescence (LIF). Signal quantification requires the knowledge of absorption and fluorescence properties as a function of environmental conditions. We report absorption and fluorescence spectra as well as fluorescence lifetimes of p-DFB in the temperature, pressure, and oxygen partial pressure range that is relevant for many applications including internal combustion engines. The UV absorption cross section, investigated between 296 and 675 K, has a peak value close to 266 nm and decreases with temperature, while still exceeding other single-ring aromatics. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (296-1180 K), pressure (1-10 bar), and O-2 partial pressure (0-210 mbar) using a streak camera (temporal resolution 50 ps) coupled to a spectrometer. The fluorescence spectra red-shift (similar to 2 nm/100 K) and broaden (increase in full width at half maximum by 58% in the investigated temperature range) with temperature. In N-2 as bath gas (1 bar), the fluorescence lifetime tau(eff) decreases with temperature by a factor of about 20 (from 7 ns at 298 K down to 0.32 ns at 1180 K), while at 8 bar the shortest lifetime at 975 K is 0.4 ns. A noticeable pressure dependence (i.e., reduced tau(eff)) is only visible at 675 K and above. Quenching of p-DFB LIF by O-2 (for partial pressures up to 210 mbar) shortens the fluorescence lifetime significantly at room temperature (by a factor of 8), but much less at higher temperatures (by a factor of 1.8 at 970 K). For fixed O-2 partial pressures (52 mbar and above), teff shows a plateau region with temperature which shifts toward higher temperatures at the higher O-2 partial pressures. O-2 quenching is less prominent for p-DFB compared to other aromatic compounds investigated so far. The temperature dependence of O-2 quenching can be approximately expressed by an exponential function. The influence of temperature, total pressure, and O-2 partial pressure on absorption cross sections and fluorescence quantum yields are given as empirical functions that allow for interpolation. For typical applications, p-DFB LIF provides up to three orders of magnitude stronger signal compared to toluene LIF.
    view abstract10.1007/s00340-016-6612-8
  • A novel magnetically-separable porous iron-oxide nanocomposite as an adsorbent for methylene blue (MB) dye
    Sehlleier, Y.H. and Hardt, S. and Schulz, C. and Wiggers, H.
    Journal of Environmental Chemical Engineering 4 (2016)
    In this study, efficient and magnetically separable adsorbents for the removal of organic pollutants from water are developed, which are both, environmental friendly and cheap to produce. A new type of porous iron-oxide/polymer nanocomposite was synthesized by a two-step process utilizing surface modification of gas-phase synthesized iron-oxide nanoparticles and a subsequent polymerization process. The potential of iron-oxide/polymer composite adsorbents with a large surface area for the removal of organic components was studied using methylene blue (MB) as a test substance. Adsorption isotherms fitted well with the Langmuir isotherm model and the adsorption capacity of MB on this adsorbent was found to be as high as 298 mg/g which is several times higher than the adsorption capacity of a number of recently reported potential adsorbents. Owing to its magnetic properties, the polluted adsorbent can be easily separated from aqueous solutions. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jece.2016.08.018
  • A single-pulse shock tube coupled with high-repetition-rate time-of-flight mass spectrometry and gas chromatography for high-temperature gas-phase kinetics studies
    Sela, P. and Shu, B. and Aghsaee, M. and Herzler, J. and Welz, O. and Fikri, M. and Schulz, C.
    Review of Scientific Instruments 87 (2016)
    Shock tubes are frequently used to investigate the kinetics of chemical reactions in the gas phase at high temperatures. Conventionally, two complementary arrangements are used where either time-resolved intermediate species measurements are conducted after the initiation of the reaction or where the product composition is determined after rapid initiation and quenching of the reaction through gas-dynamic processes. This paper presents a facility that combines both approaches to determine comprehensive information. A single-pulse shock tube is combined with high-sensitivity gas chromatography/mass spectrometry for product composition and concentration measurement as well as high-repetition-rate time-of-flight mass spectrometry for time-dependent intermediate concentration determination with 10 μs time resolution. Both methods can be applied simultaneously. The arrangement is validated with investigations of the well-documented thermal unimolecular decomposition of cyclohexene towards ethylene and 1,3-butadiene at temperatures between 1000 and 1500 K and pressures ranging from 0.8 to 2.4 bars. The comparison shows that the experimental results for both detections are in very good agreement with each other and with literature data. © 2016 Author(s).
    view abstract10.1063/1.4963844
  • Diode laser-based standoff absorption measurement of water film thickness in retro-reflection
    Pan, R. and Brocksieper, C. and Jeffries, J.B. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    A dual-wavelength diode laser-based absorption sensor for standoff point measurements of water film thickness on an opaque surface is presented. The sensor consists of a diode laser source, a foil as backscattering target, and off-axis paraboloids for collecting the fraction of the laser radiation transmitted through the liquid layer via retro-reflection. Laser wavelengths in the near infrared at 1412 and 1353 nm are used where the temperature dependence of the liquid water absorption cross section is known. The lasers are fiber coupled and the detection of the retro-reflected light was accomplished through a multimode fiber and a single photodiode using time-division multiplexing. The water film thickness at a given temperature was determined from measured transmittance ratios at the two laser wavelengths. The sensor concept was first validated with measurement using a temperature-controlled calibration cell providing liquid layers of variable and known thickness between 100 and 1000 µm. Subsequently, the sensor was demonstrated successfully during recording the time-varying thickness of evaporating water films at fixed temperatures. The film thickness was recorded as a function of time at three temperatures down to 50 µm. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-016-6524-7
  • High-yield and scalable synthesis of a Silicon/Aminosilane-functionalized Carbon NanoTubes/Carbon (Si/A-CNT/C) composite as a high-capacity anode for lithium-ion batteries
    Sehlleier, Y.H. and Dobrowolny, S. and Plümel, I. and Xiao, L. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Applied Electrochemistry 46 (2016)
    In this study, we present a novel anode architecture for high-performance lithium-ion batteries based on a Silicon/3-aminosilane-functionalized CNT/Carbon (Si/A-CNT/C) composite. A high-yield, low-cost approach has been developed to stabilize and support silicon as an active anode material. Silicon (Si) nanoparticles synthesized in a hot-wall reactor and aminosilane-functionalized carbon nanotubes (A-CNT) were dispersed in styrene and divinylbenzene (DVB) and subsequently polymerized forming a porous Si/A-CNT/C composite. Transmission electron microscopy showed that this method enables the interconnection and a uniform encapsulation of Si nanoparticles within a porous carbon matrix especially using aminosilane-functionalized CNT (A-CNT). Electrochemical characterization shows that this material can deliver a delithiation capacity of 2293 mAh g−1 with a capacity retention of more than 90 % after 200 cycles at lithiation and delithiation rate of 0.5 C. We conclude that the porous Si/A-CNT/C composite material can accommodate sufficient space for Si volume expansion and extraction and improve the electronic and ionic conduction. Excellent electrochemical performance during repeated cycling can thus be achieved. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10800-015-0897-x
  • Laser-based diagnostics in the gas-phase synthesis of inorganic nanoparticles
    Dreier, T. and Schulz, C.
    Powder Technology 287 (2016)
    As gas-phase methods for the synthesis of tailored nanomaterials become increasingly sophisticated, the need for in situ diagnostics of reaction conditions and particle properties grows correspondingly. Laser-based methods provide a wide range of capabilities which are reviewed in this article. © 2015.
    view abstract10.1016/j.powtec.2015.10.015
  • Laser-induced incandescence from laser-heated silicon nanoparticles
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    This work describes the application of temporally and spectrally resolved laser-induced incandescence to silicon nanoparticles synthesized in a microwave plasma reactor. Optical properties for bulk silicon presented in the literature were extended for nanostructured particles analyzed in this paper. Uncertainties of parameters in the evaporation submodel, as well as measurement noise, are incorporated into the inference process by Bayesian statistics. The inferred nanoparticle sizes agree with results from transmission electron microscopy, and the determined accommodation coefficient matches the values of the preceding study. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-016-6551-4
  • Measurements of liquid film thickness, concentration, and temperature of aqueous urea solution by NIR absorption spectroscopy
    Pan, R. and Jeffries, J.B. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    A multi-wavelength near-infrared (NIR) diode laser absorption sensor has been developed and demonstrated for real-time monitoring of the thickness, solute concentration, and temperature of thin films of urea–water solutions. The sensor monitors the transmittance of three near-infrared diode lasers through the thin liquid film. Film thickness, urea mass fraction, and liquid temperature were determined from measured transmittance ratios of suitable combinations of lasers. Available laser wavelengths were selected depending on the variation of the NIR absorption spectrum of the solution with temperature and solute concentration. The spectral database was measured by a Fourier transform infrared spectrometer in the range 5500–8000 cm−1 for urea solutions between 5 and 40 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was first validated with measurements using a calibration cell providing liquid layers of variable thickness (200–1500 μm), urea mass fraction (5–40 wt%) and temperature (298–318 K). Temporal variations of film thickness and urea concentration were captured during the constant-temperature evaporation of a liquid film deposited on an optically polished heated quartz flat. © Springer-Verlag Berlin Heidelberg 2016.
    view abstract10.1007/s00340-015-6290-y
  • Quantitative two-dimensional measurement of oil-film thickness by laser-induced fluorescence in a piston-ring model experiment
    Wigger, S. and Füßer, H.-J. and Fuhrmann, D. and Schulz, C. and Kaiser, S.A.
    Applied Optics 55 (2016)
    This paper describes advances in using laser-induced fluorescence of dyes for imaging the thickness of oil films in a rotating ring tribometer with optical access, an experiment representing a sliding piston ring in an internal combustion engine. A method for quantitative imaging of the oil-film thickness is developed that overcomes the main challenge, the accurate calibration of the detected fluorescence signal for film thicknesses in the micrometer range. The influence of the background material and its surface roughness is examined, and a method for flat-field correction is introduced. Experiments in the tribometer show that the method yields quantitative, physically plausible results, visualizing features with submicrometer thickness. © 2016 Optical Society of America.
    view abstract10.1364/AO.55.000269
  • Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
    Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
    Combustion and Flame 169 (2016)
    Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times &gt;4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
    view abstract10.1016/j.combustflame.2016.03.030
  • Time-resolved detection of temperature, concentration, and pressure in a shock tube by intracavity absorption spectroscopy
    Fjodorow, P. and Fikri, M. and Schulz, C. and Hellmig, O. and Baev, V.M.
    Applied Physics B: Lasers and Optics 122 (2016)
    In this paper, we demonstrate the first application of intracavity absorption spectroscopy (ICAS) for monitoring species concentration, total pressure, and temperature in shock-tube experiments. ICAS with a broadband Er3+-doped fiber laser is applied to time-resolved measurements of absorption spectra of shock-heated C2H2. The measurements are performed in a spectral range between 6512 and 6542 cm−1, including many absorption lines of C2H2, with a time resolution of 100 µs and an effective absorption path length of 15 m. Up to 18-times increase of the total pressure and a temperature rise of up to 1200 K have been monitored. Due to the ability of simultaneously recording many absorption lines in a broad spectral range, the presented technique can also be applied to multi-component analysis of transient single-shot processes in reactive gas mixtures in shock tubes, pulse detonation engines, or explosions. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-016-6434-8
  • A Genetic Algorithm-Based Method for the Optimization of Reduced Kinetics Mechanisms
    Sikalo, N. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I.
    International Journal of Chemical Kinetics 47 (2015)
    This paper describes an automatic method for the optimization of reaction rate constants of reduced reaction mechanisms. The optimization technique is based on a genetic algorithm that aims at finding new reaction rate coefficients that minimize the error introduced by the preceding reduction process. The error is defined by an objective function that covers regions of interest where the reduced mechanism may deviate from the original mechanism. The mechanism's performance is assessed for homogeneous reactor or laminar-flame simulations against the results obtained from a given reference - the original mechanism, another detailed mechanism, or experimental data, if available. The overall objective function directs the search towards more accurate reduced mechanisms that are valid for a given set of operating conditions. An optional feature to the objective function is a penalty term that permits to minimize the change to the reaction coefficients, keeping them as close as possible to the original value. This means that the penalty function can be used to constrain the reaction rates modifications during the optimization if needed. It is demonstrated that the penalty function is successful and can be combined with predefined uncertainty bounds for each reaction of the mechanism. In addition, the penalty function can be modified to achieve a further reduction of the mechanism. The algorithm is demonstrated for the optimization of a previously reduced variant of the GRI-Mech 3.0, a tert-butanol combustion mechanism by Sarathy et al. (Combust. Flame, 2012, 159, 2028-2055) and a hydrogen mechanism by Konnov (Combust. Flame, 2008, 152, 507-528), for which the complete uncertainty vector is known. The method has shown to be, robust, flexible, and suitable for a wide range of operating conditions by using multiple criteria simultaneously. © 2015 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20942
  • A standard burner for high pressure laminar premixed flames: Detailed soot diagnostics
    Leschowski, M. and Dreier, T. and Schulz, C.
    Zeitschrift fur Physikalische Chemie 229 (2015)
    Soot formation and oxidation in high-pressure combustion is of high practical relevance but still sparsely investigated because of its experimental complexity. In this work we present a high-pressure burner for studying sooting premixed flames at pressures up to 30?bar. An optically accessible vessel houses a burner that stabilizes a rich premixed ethylene/air flame on a porous sintered stainless-steel plate. The flame is surrounded by a non-sooting rich methane/air flame and an air coflow for reducing temperature gradients, buoyancy-induced instabilities, and heat loss of the innermost flame. Spectrally-resolved soot pyrometry was used for determining gas temperatures. These were introduced into model functions to fit the temporal signal decay curves obtained from two-color time-resolved laser-induced incandescence (TiRe-LII) measurements for extracting soot volume fractions and mean particle size as a function of height above burner and gas pressure. The derived mean particle sizes and soot concentrations were compared against thermophoretically sampled soot analyzed via transmission electron microscopy (TEM) and laser extinction measurements at 785?nm, respectively. Soot volume fractions derived from LII peak signal intensities need to be corrected for signal attenuation at the high soot concentrations present in the investigated flame. From the various heat conduction models employed in deriving mean soot particle diameters from TiRe-LII, the Fuchs model gave remarkably good agreement with TEM on sampled soot at various heights above the burner. © 2015 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/zpch-2014-0631
  • Assessment of soot particle-size imaging with LII at Diesel engine conditions
    Cenker, E. and Kondo, K. and Bruneaux, G. and Dreier, T. and Aizawa, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 119 (2015)
    Two-time-step laser-induced incandescence (LII) imaging was performed in Diesel engine-relevant combustion to investigate its applicability for spatially resolved measurements of soot primary particle sizes. The method is based on evaluating gated LII signals acquired with two cameras consecutively after the laser pulse and using LII modeling to deduce the particle size from the ratio of local signals. Based on a theoretical analysis, optimized detection times and durations were chosen to minimize measurement uncertainties. Experiments were conducted in a high-temperature high-pressure constant-volume pre-combustion vessel under the Engine Combustion Network’s “Spray A” conditions at 61–68 bar with additional parametric variations in injection pressure, gas temperature, and composition. The LII measurements were supported by pyrometric imaging measurements of particle heat-up temperatures. The results were compared to particle-size and size-dispersion measurements from transmission electron microscopy of soot thermophoretically sampled at multiple axial distances from the injector. The discrepancies between the two measurement techniques are discussed to analyze uncertainties and related error sources of the two diagnostics. It is found that in such environment where particles are small and pressure is high, LII signal decay times are such that LII with standard nanosecond laser and detector equipment suffers from a strong bias toward large particles. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6106-0
  • Calibration-free, high-speed, in-cylinder laser absorption sensor for cycle-resolved, absolute H2O measurements in a production IC engine
    Witzel, O. and Klein, A. and Meffert, C. and Schulz, C. and Kaiser, S.A. and Ebert, V.
    Proceedings of the Combustion Institute 35 (2015)
    The performance of a hygrometer based on calibration-free direct tunable diode laser absorption spectroscopy (dTDLAS) for in-cylinder H2O measurements is demonstrated in a nearly unmodified production internal combustion engine. The H2O concentration is a proxy for the residual gas fraction remaining in the cylinder after intake-valve closure. One challenge for in-cylinder measurements, especially in multi-cylinder engines, is to obtain optical access to the combustion chamber. The measurements here were performed in the flywheel-side cylinder of a four-cylinder engine with small access ports that were previously designed for endoscopic imaging. Due to their position these ports prohibit the usual collinear arrangement of the optical elements typical for line-of-sight measurement techniques. Therefore, we developed a new "angled" fiber-optical interface, which allows a trans-illumination of the engine at a 90° angle. The optical fiber interface uses a scattering target inside the combustion chamber with its 84 mm bore achieving an absorption length of about 70 mm. With this arrangement, crank-angle resolved measurements of the H2O concentration during early compression could be realized with a temporal resolution of 250 μs and a H2O detection limit of 0.074 vol.%. This allows detailed analysis of single engine cycles as needed for residual gas investigations. Measurements were performed over a range of loads (25-100 Nm) and speeds (1400-3650 rpm), over which the residual gas fraction was expected to vary significantly. H2O concentrations were measured between 3.3 and 5.0 vol.%. The results were compared with a simple model of residual gas content and were found to agree within the combined uncertainty of both methods, which gives an indication that dTDLAS can be used to validate more complex engine models beyond what is possible by pressure-trace analysis. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2014.06.038
  • Combination of LII and extinction measurements for determination of soot volume fraction and estimation of soot maturity in non-premixed laminar flames
    Leschowski, M. and Thomson, K.A. and Snelling, D.R. and Schulz, C. and Smallwood, G.J.
    Applied Physics B: Lasers and Optics 119 (2015)
    The measurement of soot and soot precursors is important for understanding the formation of soot particles in flames. In this paper, we use the difference between laser-induced incandescence (LII) and two-dimensional extinction measurements to assess the contribution of soot precursors to the extinction measurement. LII measurements are performed with a high spatial resolution of 100 µm to determine the soot volume fraction (f<inf>V</inf>) in a laminar ethylene/air non-premixed flame at the standard Gülder conditions. While LII is specific to mature soot only, the extinction data represent attenuation due to mature and young soot (absorption and elastic scattering) and also absorption by soot precursors. The difference between the two measurements indicates the contribution of soot precursors and allows a determination of the maturity of soot. This is important knowledge for those using extinction techniques to measure soot concentration, as the contribution from soot precursors may lead to an overestimation of the mature soot concentration. Further, regions with high soot-precursor concentrations, which lead to soot formation, can be identified. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6092-2
  • Determination of small soot particles in the presence of large ones from time‑resolved laser‑induced incandescence
    Cenker, E. and Bruneaux, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 118 (2015)
    A novel strategy for the analysis of time-resolved laser-induced incandescence (TiRe-LII), called two-exponential reverse ftting (TERF), is introduced. The method is based on combined monoexponential fts to the LII signal decay at various delay times and approximates the particle-size distribution as a weighted combination of one large and one small monodisperse equivalent mean particle size without requiring assumption on the particle-size distribution. The effects of particle size, heat-up temperature, aggregate size, and pressure on the uncertainty of this method are evaluated using numerical experiments for lognormal and bimodal size distributions. TERF is applied to TiRe-LII measured in an atmospheric pressure laminar non-premixed ethylene/air fame at various heights above burner. The results are compared to transmission electron microscopy (TEM) measurements of thermophoretically sampled soot. The particle size of the large particle-size class agreed well for both methods. The size of the small particle-size class and the relative contribution did not agree which is attributed to missing information in the TEM results for very small particles. These limitations of TEM measurements are discussed and the effect of the exposure time of the sampling grid is evaluated. © Springer-Verlag Berlin Heidelberg 2014.
    view abstract10.1007/s00340-014-5966-z
  • Direct self-assembly of Fe2O3/reduced graphene oxide nanocomposite for high-performance lithium-ion batteries
    Xiao, L. and Schroeder, M. and Kluge, S. and Balducci, A. and Hagemann, U. and Schulz, C. and Wiggers, H.
    Journal of Materials Chemistry A 3 (2015)
    In this study, Fe2O3/reduced graphene oxide (rGO) nanocomposites were prepared using a direct self-assembly of oppositely charged Fe2O3 nanoparticles (NPs) and graphene oxide (GO) sheets, followed with a lowerature hydrothermal reduction process. The characterization of the nanocomposite shows that Fe2O3 NPs with an average diameter of about 9 nm are uniformly distributed on well-exfoliated rGO layers. The nanocomposites show a high iron oxide mass loading of 63%. The electrical conductivity of the composite was significantly enhanced by about 6 orders of magnitude in comparison to pure Fe2O3 NPs. The characterization of the composite as an anode material for lithium-ion batteries (LIBs) demonstrated a strong positive synergistic effect with respect to its electrochemical performance. Fe2O3/rGO exhibited a capacity of 600 mA h g-1 at a current density of 0.1 A g-1, and even more than 180 mA h g-1 at 10 A g-1 (approx. 17 C), indicating its superior high-rate performance. In addition, it features high efficiency at high rates and very good cyclic stability over a long cycle life of more than 550 cycles. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c5ta02549d
  • Effect of fluctuations on time-averaged multi-line NO-LIF thermometry measurements of the gas-phase temperature
    Feroughi, O.M. and Kronemayer, H. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 120 (2015)
    Multi-line NO laser-induced fluorescence (LIF) thermometry enables accurate gas-phase temperature imaging in combustion systems through least-squares fitting of excitation spectra. The required excitation wavelength scan takes several minutes which systematic biases the results in case of temperature fluctuations. In this work, the effect of various types (linear, Gaussian and bimodal) and amplitudes of temperature fluctuations is quantified based on simulated NO-LIF excitation spectra. Temperature fluctuations of less than ±5 % result in a negligible error of less than ±1 % in temperature for all cases. Bimodal temperature distributions have the largest effect on the determined temperature. Symmetric temperature fluctuations around 900 K have a negligible effect. At lower mean temperatures, fluctuations cause a positive bias leading to over-predicted mean temperatures, while at higher temperatures the bias is negative. The results of the theoretical analysis were applied as a guide for interpreting experimental multi-line NO-LIF temperature measurements in a mildly turbulent pilot-plant scale flame reactor dedicated for nanoparticle synthesis. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6152-7
  • Endoscopic temperature imaging in a four-cylinder IC engine via two-color toluene fluorescence
    Gessenhardt, C. and Schulz, C. and Kaiser, S.A.
    Proceedings of the Combustion Institute 35 (2015)
    Building on the development of a large-aperture, UV-transparent endoscope designed specifically for use in IC engines, the gas-phase temperature in a fired, multi-cylinder engine was imaged based on laser-induced fluorescence (LIF) of a fuel tracer. Laser light at 266 nm was formed into a light sheet via a laser-input endoscope and excited fluorescence of toluene, port-fuel injected in a mixture with the base fuel iso-octane. The resulting UV-LIF signal was collected by an endoscope head in the combustion chamber, split into two wavelength-channels by a dichroic beam splitter, and detected on two separate cameras. Exploiting the temperature-dependence of the LIF spectrum, quantitative images of temperature were derived from the pixel-wise ratio between the two images. We describe the procedures for cross-registration of the two images and calibration of the LIF-temperature conversion, which are more challenging compared to fully optically-accessible engines. To assess the systematic error we performed a quasi-dimensional simulation matched in detail to engine data. Between intake-valve closure and mid-compression, the temperatures derived from the model and that from LIF agreed within 5 K, while the difference increased to 50 K at 28 CA before compression top-dead center. In addition to such quantitative imaging in the compression stroke, unburnt fuel was detected in the residual gas. This feature unexpectedly enabled qualitative imaging even after combustion. Predicted by the simulation, the initial back-flow of exhaust gas into the intake during gas exchange could be visualized on a single-shot basis. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2014.06.085
  • Experimental study of the kinetics of ethanol pyrolysis and oxidation behind reflected shock waves and in laminar flames
    Aghsaee, M. and Nativel, D. and Bozkurt, M. and Fikri, M. and Chaumeix, N. and Schulz, C.
    Proceedings of the Combustion Institute 35 (2015)
    The pyrolysis and oxidation of ethanol mixtures at high temperature is studied in a shock tube in the 1047-2518 K range at initial pressures of 1.06 and 2.07 bar. Pyrolysis and oxidation intermediates were investigated with high-repetition-rate time-of-flight mass spectrometry (TOF-MS). Ignition delay times were determined from chemiluminescence measurements and the OH concentration was determined with high time resolution from ring dye laser absorption measurements. Laminar flame speeds of ethanol in air were measured in a spherical bomb for initial temperatures between 318 and 473 K at 1, 2, and 5 bar and for equivalence ratios from 0.7 to 1.5. The measurements were compared to simulations based on various mechanisms from the literature. This comparison showed that the mechanism developed by Ranzi et al. (2012) provides the best agreement with the measured data for ethanol oxidation at high temperature.
    view abstract10.1016/j.proci.2014.05.063
  • Ignition delay times of diethyl ether measured in a high-pressure shock tube and a rapid compression machine
    Werler, M. and Cancino, L.R. and Schiessl, R. and Maas, U. and Schulz, C. and Fikri, M.
    Proceedings of the Combustion Institute 35 (2015)
    Ignition delay times of diethyl ether (DEE)/air/argon mixtures were studied in a shock tube in the temperature range from 900 to 1300 K at pressures of 10, 20, and 40 bar and in a rapid compression machine (RCM) at various equivalence ratios between 500 and 1060 K at pressures between 2.5 and 13 bar. Between 2.5 and 5.5 bar, the RCM results show that ignition delay times of DEE exhibit a region (between 590 and 800 K) where ignition delay times are weakly temperature dependent only, while above 833 K and below 590 K, the ignition delay times are strongly temperature dependent. Two-stage ignition was observed in the temperature range from 500 to 665 K in the RCM measurements. At the conditions of the shock tube, a strong pressure and temperature dependence of the ignition delay times was observed, but no non-thermal (NTC) behavior was found in the investigated temperature range. Simulations based on detailed chemistry using the mechanism of Yasunaga et al. (2010) [15] indicate that at high pressures ignition delay times show a high sensitivity towards the two H-atom abstraction reactions by HO2 from diethyl ether. By increasing the rate coefficients of these two reactions relative to the original values by a factor of five, the mechanism well describes our measurements and still well reproduces the original data of Yasunaga et al. (2010) [15].
    view abstract10.1016/j.proci.2014.06.143
  • Impact of ambient pressure on titania nanoparticle formation during spray-flame synthesis
    Hardt, S. and Wlokas, I. and Schulz, C. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 15 (2015)
    Nanocrystalline titania was synthesized via liquid-fed spray-flame synthesis in a hermetically closed system at various pressures. Titanium tetraisopropoxide dissolved in isopropanol was used as precursor. The size, crystal structure, degree of agglomeration, morphology and the band gap of the as-prepared particles were investigated ex situ by nitrogen adsorption, transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. In comparison to synthesis at atmospheric pressure it was found that decreasing pressure has a significant influence on the particle size distribution leading to smaller particles with reduced geometric standard deviation while particle morphology and crystal structure are not affected. Computational fluid dynamics simulations support the experimental findings also indicating a significant decrease in particle size at reduced pressure. Although it is well known that decreasing pressure leads to smaller particle sizes, it is (to our knowledge) the first time that this relation was investigated for spray-flame synthesis. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstract10.1166/jnn.2015.10607
  • Influence of carbon content, particle size, and partial manganese substitution on the electrochemical performance of LiFexMn1-xPO4/carbon composites
    Hamid, N.A. and Wennig, S. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Ionics 21 (2015)
    LiFePO<inf>4</inf>/C and LiFe<inf>x</inf>Mn<inf>1-x</inf>PO<inf>4</inf>/C (x = 0.7) nanocomposites were successfully synthesized via scalable spray-flame synthesis followed by solid-state reaction. A solution of iron (III) acetylacetonate and tributyl phosphate in toluene was used to produce amorphous, nanosized FePO<inf>4</inf>⋅H<inf>2</inf>O in a spray-flame reactor which was then milled with Li<inf>2</inf>CO<inf>3</inf> and glucose to produce a LiFePO<inf>4</inf>/C composite material in a solid-state reaction. The influence of calcination temperature and carbon content on the properties of the resulting material was investigated using specific surface area measurements (BET), X-ray diffraction (XRD), electron microscopy, and electrochemical characterization. The impact of manganese addition on the electrochemical behavior was analyzed using cyclic voltammetry (CV) and constant-current (CC) measurements. XRD shows that the combination of gas-phase synthesis and subsequent solid-state reaction yields highly pure LiFePO<inf>4</inf>/C. BET measurement revealed that the particle size of LiFePO<inf>4</inf> in the composite depends on the amount of glucose. A discharge capacity of more than 140 mAh/g at C/20 is achieved for LiFePO<inf>4</inf>/C with a carbon content of 6 wt%. This material supports high charge as well as discharge rates delivering more than 60 mAh/g at 16 C and sustains good cycle stability providing 115 mAh/g at 1 C. The energy density of the olivine increases about 10 % by substituting 30 mol% of iron by manganese while preserving the electrochemical performance of pure LiFePO<inf>4</inf>/C. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s11581-015-1366-6
  • Initial reaction steps during flame synthesis of iron-oxide nanoparticles
    Kluge, S. and Deng, L. and Feroughi, O. and Schneider, F. and Poliak, M. and Fomin, A. and Tsionsky, V. and Cheskis, S. and Wlokas, I. and Rahinov, I. and Dreier, T. and Kempf, A. and Wiggers, H. and Schulz, C.
    CrystEngComm 17 (2015)
    Premixed, laminar H2/O2/Ar and CH4/O2/N2 low-pressure flat flames doped with iron pentacarbonyl (Fe(CO)5) were used to investigate the initial steps towards the formation of iron-oxide nanoparticles. The particles were extracted from the flame using a molecular beam sampling probe and the mass flow rate of condensed material was measured by a quartz crystal microbalance (QCM). It was observed that particles are already formed on the cold side of the flame, and vanish quickly once they pass through the flame front. To understand the process and assess the perturbations caused by the sampling probe, spatially resolved laser-based measurements of temperature, Fe and FeO concentration as well as molecular-beam sampling with particle mass spectrometry (PMS) were carried out. Numerical flow simulations of the synthesis flames, the reactor, and the sampling were performed and the simulations confirmed the experimental findings of very early particle formation. The detailed knowledge of the perturbation caused by invasive probing enabled further insight into the iron-oxide nanoparticle formation mechanism. From the results it is concluded that neither Fe atoms nor FeO molecules belong to the growth species of iron-oxide nanoparticles from flame synthesis. © The Royal Society of Chemistry.
    view abstract10.1039/c5ce00456j
  • Laser-based in situ measurement and simulation of gas-phase temperature and iron atom concentration in a pilot-plant nanoparticle synthesis reactor
    Feroughi, O.M. and Hardt, S. and Wlokas, I. and Hülser, T. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Combustion Institute 35 (2015)
    A scaled-up flame reactor for nanoparticle synthesis was investigated through a combination of in-situ laser-induced fluorescence (LIF) measurements and computational fluid dynamics (CFD) simulations with detailed chemistry. Multi-line NO-LIF was used for imaging gas-temperature and Fe-LIF for measurement of iron atom concentration. Despite the challenging environment of production reactors in an industrial environment, various conditions for stable flames with different gas flows with and without adding Fe(CO)5 as precursor for the synthesis of iron-oxide nanoparticles were investigated. In contrast to previous measurements in laminar lab-scale flames, a second mechanism for forming iron oxide nanoparticles was found via intermediate formation of iron clusters and elemental iron particles in hot, oxygen-free gas streams followed by subsequent oxidation. © 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
    view abstract10.1016/j.proci.2014.05.039
  • Laser-induced incandescence: Particulate diagnostics for combustion, atmospheric, and industrial applications
    Michelsen, H.A. and Schulz, C. and Smallwood, G.J. and Will, S.
    Progress in Energy and Combustion Science 51 (2015)
    The understanding of soot formation in combustion processes and the optimization of practical combustion systems require in situ measurement techniques that can provide important characteristics, such as particle concentrations and sizes, under a variety of conditions. Of equal importance are techniques suitable for characterizing soot particles produced from incomplete combustion and emitted into the environment. Additionally, the production of engineered nanoparticles, such as carbon blacks, may benefit from techniques that allow for online monitoring of these processes. In this paper, we review the fundamentals and applications of laser-induced incandescence (LII) for particulate diagnostics in a variety of fields. The review takes into account two variants of LII, one that is based on pulsed-laser excitation and has been mainly used in combustion diagnostics and emissions measurements, and an alternate approach that relies on continuous-wave lasers and has become increasingly popular for measuring black carbon in environmental applications. We also review the state of the art in the determination of physical parameters central to the processes that contribute to the non-equilibrium nanoscale heat and mass balances of laser-heated particles; these parameters are important for LII-signal analysis and simulation. Awareness of the significance of particle aggregation and coatings has increased recently, and the effects of these characteristics on the LII technique are discussed. Because of the range of experimental constraints in the variety of applications for which laser-induced incandescence is suited, many implementation approaches have been developed. This review discusses considerations for selection of laser and detection characteristics to address application-specific needs. The benefits of using LII for measurements of a range of nanoparticles in the fields mentioned above are demonstrated with some typical examples, covering simple flames, internal-combustion engines, exhaust emissions, the ambient atmosphere, and nanoparticle production. We also remark on less well-known studies employing LII for particles suspended in liquids. An important aspect of the paper is to critically assess the improvement in the understanding of the fundamental physical mechanisms at the nanoscale and the determination of underlying parameters; we also identify further research needs in these contexts. Building on this enhanced capability in describing the underlying complex processes, LII has become a workhorse of particulate measurement in a variety of fields, and its utility continues to be expanding. When coupled with complementary methods, such as light scattering, probe-sampling, molecular-beam techniques, and other nanoparticle instrumentation, new directions for research and applications with LII continue to materialize. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.pecs.2015.07.001
  • Low-pressure effective fluorescence lifetimes and photo-physical rate constants of one- and two-ring aromatics
    Benzler, T. and Faust, S. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 121 (2015)
    One- and two-ring aromatics such as toluene and naphthalene are frequently used molecular tracer species in laser-induced fluorescence (LIF) imaging diagnostics. Quantifying LIF signal intensities requires knowledge of the photo-physical processes that determine the fluorescence quantum yield. Collision-induced and intramolecular energy transfer processes in the excited electronic state closely interact under practical conditions. They can be separated through experiments at variable low pressures. Effective fluorescence lifetimes of gaseous toluene, 1,2,4-trimethylbenzene, anisole, naphthalene, and 1-methylnaphthalene diluted in CO2 were measured after picosecond laser excitation at 266 nm and time-resolved detection of fluorescence intensities. Measurements in an optically accessible externally heated cell between 296 and 475 K and 0.010–1 bar showed that effective fluorescence lifetimes generally decrease with temperature, while the influence of the bath-gas pressure depends on the respective target species and temperature. The results provide non-radiative and fluorescence rate constants and experimentally validate the effect of photo-induced cooling. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6271-1
  • Sensitivity analysis for soot particle size imaging with laser-induced incandescence at high pressure
    Cenker, E. and Bruneaux, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 119 (2015)
    Soot particle sizes can be determined from time-resolved laser-induced incandescence (LII) in point measurements where full signal traces are detected. For instantaneous imaging, strategies are required that must cope with time-gated information and that rely on assumptions on the local boundary conditions. A model-based analysis is performed to identify the dependence of LII particle-size imaging on the assumed boundary conditions such as bath gas temperature, pressure, particle heat-up temperature, accommodation coefficients, and soot aggregate size. Various laser-fluence regimes and gas pressures are considered. For 60 bar, fluences that lead to particle heat-up temperatures of 3,400–3,900 K provided the lowest sensitivity on particle sizing. Effects of laser attenuation are evaluated. A combination of one detection gate starting at the signal peak and the other starting with 5 ns delay was found to provide the highest sensitivity at 60 bar. The optimum gate delays for different pressures are shown. The effects of timing jitter, polydispersity, and signal noise are investigated. Systematic errors in pyrometry imaging at 60 bar is evaluated. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6009-0
  • Si-CNT/rGO Nanoheterostructures as High-Performance Lithium-Ion-Battery Anodes
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Orthner, H. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    ChemElectroChem 2 (2015)
    A robust and electrochemically stable 3D nanoheterostructure consisting of Si nanoparticles (NPs), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) is developed as an anode material (Si-CNT/rGO) for lithium-ion batteries (LIBs). It integrates the benefits from its three building blocks of Si NPs, CNTs, and rGO; Si NPs offer high capacity, CNTs act as a mechanical, electrically conductive support to connect Si NPs, and highly electrically conductive and flexible rGO provides a robust matrix with enough void space to accommodate the volume changes of Si NPs upon lithiation/delithiation and to simultaneously assure good electric contact. The composite material shows a high reversible capacity of 1665mAhg-1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2Ag-1) where 755mAhg-1 are delivered, thus indicating promising characteristics of this material for high-performance LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500323
  • Temporally and spectrally resolved UV absorption and laser-induced fluorescence measurements during the pyrolysis of toluene behind reflected shock waves
    Zabeti, S. and Drakon, A. and Faust, S. and Dreier, T. and Welz, O. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 118 (2015)
    Toluene is frequently used as fluorescence tracer in high-temperature combustion applications. A quantitative analysis of laser-induced fluorescence (LIF) signals requires the knowledge of photophysical properties and decomposition kinetics. Using spectrally and temporally resolved ultraviolet absorption and LIF measurements, we studied the spectral properties of toluene and its pyrolysis products behind shock waves between 810 and 1,755 K. Transient absorption spectra were acquired between 220 and 300 nm. The temporal behavior of the absorption at 266 nm was compared to simulations based on literature kinetics models of toluene pyrolysis and available high-temperature absorption cross-sections of toluene, benzyl radicals, and C<inf>7</inf>H<inf>6</inf> as a product from benzyl decomposition. Experiment and simulation agree well at the beginning of the pyrolysis process, whereas for longer reaction times deviations occur presumably due to the build-up of high molecular weight species, which contribute to the observed absorption but have unknown spectral properties. Additionally, LIF emission spectra were recorded following 266-nm excitation at selected reaction times. From measurements up to 1,220 K, the relative fluo-rescence quantum yield of toluene was derived, extending existing data to higher temperatures. Products from toluene pyrolysis were found to be the major contributors to the LIF signal at higher temperatures. © Springer-Verlag Berlin Heidelberg 2014.
    view abstract10.1007/s00340-014-5986-8
  • Two-tracer LIF imaging of preferential evaporation of multi-component gasoline fuel sprays under engine conditions
    Itani, L.M. and Bruneaux, G. and Di Lella, A. and Schulz, C.
    Proceedings of the Combustion Institute 35 (2015)
    A laser-induced fluorescence (LIF) technique capable of imaging the effects of preferential evaporation of multi-component fuels was developed based on the simultaneous detection of two aromatic fluorescence tracers with complementary evaporation characteristics matched to different components of a multi-component fuel. Relative variations in the spatial distribution of fuel components as a consequence of preferential evaporation were determined from the ratio of LIF-signals measured within two distinct spectral bands. The accuracy and precision of the method was characterized from determining the LIF-signal ratio within two identical spectral bands. Measurements were performed in a high-pressure high-temperature vessel equipped with a hollow-cone injector. Experimental conditions with temperatures up to 700 K were chosen that are representative for engine environments and favor preferential evaporation. The effects of preferential evaporation were analyzed based on the comparison of instantaneous and mean images of LIF ratios obtained at various temperatures. Variations in the spatial distribution of the fuel volatility classes were observed up to 550 K. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2014.06.108
  • A comparison of selected organic tracers for quantitative scalar imaging in the gas phase via laser-induced fluorescence
    Faust, S. and Goschütz, M. and Kaiser, S.A. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 117 (2014)
    This paper compares three of the tracers most commonly used for laser-induced fluorescence in gaseous flows, toluene, naphthalene, and acetone. Additionally, anisole (methoxybenzene, CH<inf>3</inf>OC<inf>6</inf>H<inf>5</inf>) is included in the comparison. Each tracer is employed to image the scalar field in the same nonreacting transient impinging turbulent jet. The jet fluid is seeded with tracer vapor in a bubbler, excitation is at 266 nm, and both air and nitrogen are used as bath gases. Measured signals are compared to theoretical predictions based on fluorescence quantum yield, absorption cross-section, and vapor pressure. We find that anisole shows the highest total signal intensity of all investigated species, while naphthalene features the highest signal per molecule. Acetone has the advantage of being insensitive to quenching by oxygen and that its fluorescence is partly at visible wavelengths. In addition to this volatility-limited scenario at room temperature, we also compare the expected relative signals for elevated temperatures and for a hypothetical case in which the amount of admissible tracer seeding is limited. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-014-5818-x
  • A genetic algorithm-based method for the automatic reduction of reaction mechanisms
    Sikalo, N. and Hasemann, O. and Schulz, C. and Kempf, A. and Wlokas, I.
    International Journal of Chemical Kinetics 46 (2014)
    An automatic method for the reduction of chemical kinetic mechanisms under specific physical or thermodynamic conditions is presented. The method relies on the genetic algorithm search logic to gradually reduce the number of reactions from the detailed mechanism while still preserving its ability to describe the overall chemistry at an acceptable error. Accuracy of the reduced mechanism is determined by comparing its solution to the solution obtained with the full mechanism under the same initial and/or physical conditions. However, not only the chemical accuracy and the size of the mechanism are considered but also the time for its solution which helps to avoid stiff and slow converging mechanisms, thus preferring the fast solutions. The reduction method is demonstrated for a detailed mechanism for methane combustion, GRI-Mech 3.0, which was reduced from 325 reactions and 53 species to 58 reactions and 26 species, and for an iron oxide formation mechanism from iron pentacarbonyl doped flames by Wlokas et al. (Int J Chem Kinet 2013, 45(8), 487-498), originally consisting of 144 reactions and 34 species, which was reduced to 37 reactions and 24 species. The performance of the reduced mechanisms is shown for homogeneous constant pressure reactors and for burner-stabilized flames. The results show a good agreement between reduced and full mechanisms for both the reactor and flame cases. The presented method is flexible and can be easily adjusted to either yield more accurate (but bigger) or smaller (but less accurate) reduced mechanisms, depending on the user's preference. © 2013 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20826
  • An automated thermophoretic soot sampling device for laboratory-scale high-pressure flames
    Leschowski, M. and Dreier, T. and Schulz, C.
    Review of Scientific Instruments 85 (2014)
    Studying soot particle morphology in high-pressure flames via thermophoretic sampling critically depends on sampling precision, speed, and reproducibility. This is mainly limited by the challenges of applying pneumatically driven devices for burner chamber pressures higher than the pneumatic pressure. We present a pneumatically driven device for high-pressure applications up to 90 bars. The novelty is to separate the pneumatic driver section from the high-pressure environment in the burner chamber. The device was tested by sampling soot from a laminar high-pressure flame at 20 bars. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4868970
  • Endoscopic chemiluminescence measurements as a robust experimental tool in high-pressure gas turbine combustion tests
    Goers, S. and Witzel, B. and Heinze, J. and Stockhausen, G. and Van Kampen, J. and Schulz, C. and Willert, C. and Fleing, C.
    Proceedings of the ASME Turbo Expo 4B (2014)
    The development process for gas turbine combustion systems includes single-burner high-pressure combustion tests as an important validation step. In these tests the performance of a combustor is investigated at realistic gas turbine conditions. Measurement techniques that are typically used in these tests include mass flow meters, thermocouples, pressure transducers, and probes for exhaust-gas composition measurements. These measurement techniques, however, do not provide direct information of the flame behavior. Copyright © 2014 by Siemens Energy, Inc.
    view abstract10.1115/GT2014-26977
  • Endoscopic Imaging of Early Flame Propagation in a Near-Production Engine
    Goschütz, M. and Schulz, C. and Kaiser, S.A.
    SAE International Journal of Engines 7 (2014)
    UV-chemiluminescence from the excited hydroxyl-radical (OH*) has been used as a marker for the high-temperature reacting zone in spark-ignited engines for quite some time. In research engines with large optical access, sensitive camera systems make it possible to obtain images of the flame that can be used for, e.g., determining the flame-front's propagation speed [Aleiferis et al., Combust. Flame 136 (2004) 283-302]. However, on one hand such optical engines are limited in their speed and load range, on the other, typical UV endoscopes make wide-field imaging at low light levels challenging. Here, a large-aperture UV endoscope is used to capture sequences of OH* chemiluminescence during early flame propagation in a nearly unmodified production engine. We compare three imaging systems: phase-locked single-shot imaging, phase-locked double-frame imaging, and "high-speed" cinematography at kHz repetition rates. The four-cylinder spark-ignition engine can be operated at speeds and loads significantly exceeding the limits of most fully optically accessible engines. During the first 20Ŷ crank-angle after ignition, the phase-locked endoscopic images almost match the image quality reported from experiments in a dedicated optically-accessible engine. For later acquisition timings, the flame often exceeds the field of view. From phase-locked imaging the instantaneous size of the apparent burnt area (ABA) can be identified by thresholding after filtering. Its single-shot variant allows only computation of the multi-cycle average of the apparent flame speed (AFS). Acquisition of two successive frames in a single cycle enables determining the instantaneous AFS. High-speed imaging can follow a single cycle and thus the time-resolved ABA can be estimated, but the instantaneous shape of the flame cannot be imaged with much detail, because the detector hardware is less mature. Copyright © 2014 SAE International.
    view abstract10.4271/2014-01-1178
  • Formaldehyde laser-induced fluorescence imaging with a multi-band transmission filter
    Thering, H. and Beckmann, L. and Jördens, C. and Röder, M. and Dreier, T. and Schulz, C.
    Optics Letters 39 (2014)
    A method for laser-induced fluorescence (LIF) imaging of formaldehyde (CH2O) that discriminates against the interfering signal from polycyclic aromatic hydrocarbons (PAHs) is presented. This technique uses an interference filter with 11 transmission bands that closely match the most prominent fluorescence features of CH2O upon excitation at 355 nm. The signal increases by a factor of 12 with the multi-band filter compared to a single-band filter. Slight angle-tuning of the filter shifts the transmission maxima to the minima in between the CH2O-LIF features. The pixel-by-pixel difference between the images collected at the two filter angles thus provides CH2O images free of PAH interference, providing the capability for selective single-pulse CH2O-LIF imaging in engine combustion even under fuel-rich conditions. © 2014 Optical Society of America.
    view abstract10.1364/OL.39.001873
  • Ignition delay times of shock-heated tetraethoxysilane, hexamethyldisiloxane, and titanium tetraisopropoxide
    Abdali, A. and Fikri, M. and Orthner, H. and Wiggers, H. and Schulz, C.
    Chemical Physics Letters 601 (2014)
    Ignition delay times of tetraethoxysilane (TEOS), hexamethyldisiloxane (HMDSO) and titanium tetraisopropoxide (TTIP) were determined from the onset of chemiluminescence in shock-tube experiments behind reflected shock waves in dry as well as in humid gas mixtures. Additionally, the ignition delay times of TEOS and HMDSO have been investigated in humid air and as a function of water vapor concentration in the initial gas mixture. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cplett.2014.03.079
  • In situ nanoparticle size measurements of gas-borne silicon nanoparticles by time-resolved laser-induced incandescence
    Sipkens, T.A. and Mansmann, R. and Daun, K.J. and Petermann, N. and Titantah, J.T. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 116 (2014)
    This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated nanoparticles equilibrate with their surroundings. Models of the free molecular conduction and evaporation are derived, including a thermal accommodation coefficient found through molecular dynamics. The model is used to analyze TiRe-LII measurements made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon and hydrogen. Nanoparticle sizes inferred from the TiRe-LII data agree with the results of a Brunauer-Emmett-Teller analysis. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-013-5745-2
  • In Situ Particle Size Measurements of Gas-borne Silicon Nanoparticles by Time-resolved Laser-induced Incandescence
    Sipkens, T. A. and Petermann, N. and Daun, K. J. and Titantah, J. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Asme Summer Heat Transfer Conference - 2013, Vol 1 (2014)
    The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.
    view abstract10.1115/HT2013-17246
  • Influence of molecular hydrogen on acetylene pyrolysis: Experiment and modeling
    Aghsaee, M. and Dürrstein, S.H. and Herzler, J. and Böhm, H. and Fikri, M. and Schulz, C.
    Combustion and Flame 161 (2014)
    The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration-time profiles for various species with a time resolution of 10μs. Concentration histories of reactants and polyacetylene intermediates (C2xH2, x=1-4) are measured during the pyrolysis of acetylene with and without H2 added to the gas mixture for a wide range of conditions. In the 1760-2565K temperature range, reasonable agreement between the experiment and the model predictions for C2H2, C4H2, C6H2, and C8H2 is achieved. H2 addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H2, which are required for the fast build-up of carbonaceous material. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.03.012
  • Liquid film thickness measurement by two-line TDLAS
    Yang, H. and Chen, J. and Cai, X. and Greszik, D. and Dreier, T. and Schulz, C.
    AIP Conference Proceedings 1592 (2014)
    A fiber-based two-line tunable diode-laser absorption sensor with two near-infrared (NIR) distributed-feedback (DFB) diode lasers at ∼1.4μm was used for non-intrusive time-resolved liquid water film thickness measurement. When probing the liquid film at two different wavelengths with significantly different absorption cross-sections, the additional signal losses due to surface fowling, reflection and beam steering can be eliminated. In this work, the evaporation process of a liquid film on transparent quartz plate was tracked and large fluctuations of film thickness were found at the end of the evaporation. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4872109
  • Mixing processes in a compressible accelerated nozzle flow with blunt-body wakes
    Wohler, A. and Weigand, B. and Mohri, K. and Schulz, C.
    AIAA Journal 52 (2014)
    The mixing processes in an accelerated duct flow from subsonic to supersonic flow speed with blunt-body wakes were investigated experimentally. Four different injectors, all with their trailing edges located in the subsonic part of the flow and designed as wing bodies, were compared.Atoluene/nitrogen mixture was added into the flow through the injectors, and the flow structures behind the injector trailing edges were visualized applying laser-induced fluorescence imaging. The injector flow was parallel to the duct flow and at angles of 45 deg and 90 deg to the ambient air flow. Furthermore, one injector was designed with ramps on the upper and lower surface. Instantaneous images showed that the mixing process was dominated by separated shear layers behind the injectors, which rolled up to vortices being shed from the blunt trailing edge. The injector flow spread out more with increasing injection angle and the edges of the vortices were frayed. In the case of the ramp injector, additional streamwise vortices were generated, and thus the mixing took place in the core region of the flow for a longer distance. Scaling properties such as the halfwidth, the gradient of the growing wake, and the virtual origin were calculated. This showed that the half-width of the intensity profiles followed the 1/2-power-law. Furthermore, the intensity at the duct centerline decreased asymptotically, and the wake showed a self-preserving state. © 2013 by the American Institute of Aeronautics and Astronautics, Inc.
    view abstract10.2514/1.J052493
  • Silicon/Polyaniline nanocomposites as anode material for Lithium ion batteries
    Kummer, M. and Badillo, J.P. and Schmitz, A. and Bremes, H.-G. and Winter, M. and Schulz, C. and Wiggers, H.
    Journal of the Electrochemical Society 161 (2014)
    Due to of its high Li storage capacity, silicon is a promising anode material for lithium ion batteries. Unfortunately, this high specific capacity leads to extreme volume expansion of about 300% during lithiation and delithiation, that may lead to mechanical disintegration of the electrode and poor cycle life. To improve the cycling behavior, we combined nano-silicon (n-Si) active material with an inactive material that acts as a binder and buffering matrix. Stability, flexibility and conductivity are the main requirements for such matrix material. Polyaniline (PANi), a conducting polymer, meets all these requirements. With a theoretical capacity of 643 mAh g -1, the prepared n-Si/PANi sample showed a higher capacity in respect to the commonly used anode material, graphite. The electrochemical performance of the n-Si/PANi composite is stable compared to the performance of nano-silicon without PANi. After 300 cycles the composite still retains more than 60% of its theoretical capacity. © 2013 The Electrochemical Society.
    view abstract10.1149/2.020401jes
  • Surface functionalization of microwave plasma-synthesized silica nanoparticles for enhancing the stability of dispersions
    Sehlleier, Y.H. and Abdali, A. and Schnurre, S.M. and Wiggers, H. and Schulz, C.
    Journal of Nanoparticle Research 16 (2014)
    Gas phase-synthesized silica nanoparticles were functionalized with three different silane coupling agents (SCAs) including amine, amine/phosphonate and octyltriethoxy functional groups and the stability of dispersions in polar and non-polar dispersing media such as water, ethanol, methanol, chloroform, benzene, and toluene was studied. Fourier transform infrared spectroscopy showed that all three SCAs are chemically attached to the surface of silica nanoparticles. Amine-functionalized particles using steric dispersion stabilization alone showed limited stability. Thus, an additional SCA with sufficiently long hydrocarbon chains and strong positively charged phosphonate groups was introduced in order to achieve electrosteric stabilization. Steric stabilization was successful with hydrophobic octyltriethoxy-functionalized silica nanoparticles in non-polar solvents. The results from dynamic light scattering measurements showed that in dispersions of amine/phosphonate- and octyltriethoxy-functionalized silica particles are dispersed on a primary particle level. Stable dispersions were successfully prepared from initially agglomerated nanoparticles synthesized in a microwave plasma reactor by designing the surface functionalization. © 2014 Springer Science+Business Media.
    view abstract10.1007/s11051-014-2557-1
  • Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors
    Weise, C. and Faccinetto, A. and Kluge, S. and Kasper, T. and Wiggers, H. and Schulz, C. and Wlokas, I. and Kempf, A.
    Combustion Theory and Modelling 17 (2013)
    Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, 'safe' operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/13647830.2013.781224
  • Experimental investigation and modeling of the kinetics of CCl4 pyrolysis behind reflected shock waves using high-repetition-rate time-of-flight mass spectrometry
    Aghsaee, M. and Drakon, A. and Eremin, A. and Dürrstein, S.H. and Böhm, H. and Somnitz, H. and Fikri, M. and Schulz, C.
    Physical Chemistry Chemical Physics 15 (2013)
    The pyrolysis kinetics of CCl4 behind reflected shock waves was studied with high-repetition-rate time-of-flight mass spectrometry. For modeling, quantum mechanical calculations were performed to evaluate the dissociation energies of CCl bonds for the different CClx (x = 1 to 4) radicals. Good agreement with the JANAF thermochemical table was found. With the reaction mechanism developed for CCl4 decomposition satisfactory agreement with experimental results was obtained. The investigations show the importance of C2Cl2 formation for understanding the processes of carbon cluster growth leading to carbonaceous particle formation.© 2013 the Owner Societies.
    view abstract10.1039/c2cp42574b
  • High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels
    Fikri, M. and Cancino, L.R. and Hartmann, M. and Schulz, C.
    Proceedings of the Combustion Institute 34 (2013)
    Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p5 = 10, 20, and 40 bar) for equivalence ratios φ = 0.5 and 1.0 and over a wide temperature range 690 K < T5 < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, φ = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2012.05.101
  • Low temperature diffusion of Li atoms into Si nanoparticles and surfaces
    Nienhaus, H. and Karacuban, H. and Krix, D. and Becker, F. and Hagemann, U. and Steeger, D. and Bywalez, R. and Schulz, C. and Wiggers, H.
    Journal of Applied Physics 114 (2013)
    The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the electron beam induces a reaction between Li and SiO2, leading to LiOx and elemental Si floating on the surface. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4813872
  • Mechanism of iron oxide formation from iron pentacarbonyl-doped low-pressure hydrogen/oxygen flames
    Wlokas, I. and Faccinetto, A. and Tribalet, B. and Schulz, C. and Kempf, A.
    International Journal of Chemical Kinetics 45 (2013)
    A chemical reaction mechanism was developed for the formation of iron oxide (Fe2O3) from iron pentacarbonyl (Fe(CO)5) in a low-pressure hydrogen-oxygen flame reactor. In this paper, we describe an extensive approach for the flame-precursor chemistry and the development of a novel model for the formation of Fe2O3 from the gas phase. The detailed reaction mechanism is reduced for the implementation in two-dimensional, reacting flow simulations. The comprehensive simulation approach is completed by a model for the formation and growth of the iron oxide nanoparticles. The exhaustive and compact reaction mechanism is validated using experimental data from iron-atom laser-induced fluorescence imaging. The particle formation and growth model are verified with new measurements from particle mass spectrometry. © 2013 Wiley Periodicals, Inc.
    view abstract10.1002/kin.20786
  • Photo-physical properties of anisole: Temperature, pressure, and bath gas composition dependence of fluorescence spectra and lifetimes
    Faust, S. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 112 (2013)
    Anisole is a promising candidate for use as fluorescent tracer for gas-phase imaging diagnostics. Its high-fluorescence quantum yield (FQY) and its large Stokes shift lead to improved signal intensity (up to 100 times stronger) compared with the often used toluene. Fluorescence spectra and effective fluorescence lifetimes of gaseous anisole were investigated after picosecond laser excitation at 266 nm as a function of temperature (296-977 K) and bath gas composition (varying amounts of N2 and O2) at total pressures in the range of 1-10 bar to provide spectroscopic data and FQY for applications, e.g., in in-cylinder measurements in internal combustion engines. Fluorescence spectra of anisole extend from roughly 270-360 nm with a peak close to 290 nm at 296 K. The spectra show a red-shift with increasing temperature (0.03 nm/K) and O2 partial pressure (5 nm from N2 to air). In the investigated temperature range and in pure N2 at 1 bar total pressure the effective fluorescence lifetime drops with increasing temperature from 13.3 ± 0.5 to 0.05 ± 0.01 ns. Increasing the total pressure of N2 leads to a small decrease of the lifetime at temperatures above 400 K (e.g., at 525 K from 4.2 ± 0.2 ns at 1 bar to 2.7 ± 0.2 ns at 10 bar). At constant temperature and in the presence of O2 the lifetimes decrease significantly (e.g., at 296 K from 13.3 ± 0.5 ns in N2 to 0.40 ± 0.02 ns in air), with this trend diminishing with increasing temperature (e.g., at 675 K from 1.02 ± 0.08 ns in N 2 to 0.25 ± 0.05 ns in air). A phenomenological model that predicts fluorescence lifetimes, i.e., relative quantum yields as a function of temperature, pressure, and O2 concentration is presented. The photophysics of anisole is discussed in comparison with other aromatics. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-013-5420-7
  • Simultaneous measurement of localized heat-release with OH/CH 2O-LIF imaging and spatially integrated OH- chemiluminescence in turbulent swirl flames
    Röder, M. and Dreier, T. and Schulz, C.
    Proceedings of the Combustion Institute 34 (2013)
    In practical flames such as gas turbine combustors, spatially-integrated OH- chemiluminescence (CL) is frequently used as a heat release rate (HRR) indicator-which has been questioned by some authors to be restricted to flames of a limited range of equivalence ratios and low Reynolds numbers-while in lab flames the approach of combined detection of OH and H2CO via LIF is an accepted diagnostic technique. Even when using specialized optics with limited acceptance angle the first method is spatially integrating while the second one allows for spatially resolved imaging. In the present work we retrieved simultaneously HRR-based information via both techniques from the same spatial flame volume, i.e., OH--CL radiation is collected exclusively from within the light sheet volume cutting through the flame for LIF imaging. Turbulent premixed swirl flames were investigated with a thermal power up to 30 kW to shed light on the still unresolved question if correlations exist between signal intensities derived from both methods in turbulent flames. Measurements were performed in methane/air flames with Reynolds numbers between 6900 and 10,000, equivalence ratios between 0.8 and 1.2, and with a replacement of 20 vol% of methane by hydrogen. Although scatter plots of HRR vs. CL intensities cluster in certain regions depending on flame conditions, their large scatter shows that correlations are weak, probably caused by flame stretch and curvature. Depending on flame conditions, correlation coefficients to characterize the scatter plots range between 0.45 and 0.81. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2012.06.102
  • Study of soot formation and oxidation in the engine combustion network (ECN), spray A: Effects of ambient temperature and oxygen concentration
    Cenker, E. and Bruneaux, G. and Pickett, L. and Schulz, C.
    SAE International Journal of Engines 6 (2013)
    Within the Engine Combustion Network (ECN) spray combustion research frame, simultaneous line-of-sight laser extinction measurements and laser-induced incandescence (LII) imaging were performed to derive the soot volume fraction (fv). Experiments are conducted at engine-relevant high-temperature and high-pressure conditions in a constantvolume pre-combustion type vessel. The target condition, called "Spray A", uses well-defined ambient (900 K, 60 bar, 22.8 kg/m3, 15% oxygen) and injector conditions (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, ndodecane, 363 K). Extinction measurements are used to calibrate LII images for quantitative soot distribution measurements at cross sections intersecting the spray axis. LII images are taken after the start of injection where quasi-stationary combustion is already established. In addition, by changing the LII timing relative to the injection, the temporal variation of the soot cloud is observed from initial soot formation until soot oxidization. OH-chemiluminescence imaging was used to determine the lift-off length, relative to the soot-forming region and used to interpret the soot measurements. Results show that Spray A is a moderately sooting flame where signal trapping is not significant, aiding the potential for quantitative soot diagnostics. Maximum soot volume fractions around 2-3 ppm are obtained at the nominal ambient temperature defined for Spray A (i.e. 900 K) that rise to 12 ppm at elevated temperature (1030 K). At 1.5 ms nominal injection duration the Spray A soot cloud is mainly transient. Therefore, an extended injection duration of 4 ms at identical rail pressure was used to characterize the soot structure in quasi-steady mode. Variations of ambient temperature and oxygen concentration are carried out showing effects on soot formation and oxidation that are consistent with the literature. Copyright © 2013 SAE International.
    view abstract10.4271/2013-01-0901
  • Synthesis of small carbon nanoparticles in a microwave plasma flow reactor
    Gurentsov, E. and Priemchenko, K. and Grimm, H. and Orthner, H. and Wiggers, H. and Borchers, C. and Jander, H. and Eremin, A. and Schulz, C.
    Zeitschrift fur Physikalische Chemie 227 (2013)
    Unusually small carbon nanoparticles were synthesized in a microwave plasma flow-reactor by pyrolysis of 0.3-1.2% CH4, C2H 4, and C2H2 with 0.3-3.6% addition of molecular hydrogen in argon. Final particle sizes were analyzed by in-line particle-mass spectrometry (PMS) and by transmission electron microscopy (TEM). TEM measurements of primary particle sizes were found to be in a good agreement with PMS data. The carbon particles formed in the plasma generated by a 2.45 GHz magnetron with an applied power of 180 W and a total pressure of 13 mbar have diameters of 4-6 nm. The type of hydrocarbon precursor and 0.3-3.6% of hydrogen addition did not noticeably influence the final particle sizes. The formation of such small particles is attributed to the low pressure and the comparably low operation power. This method of small carbon nanoparticles synthesis could be useful for the production of carbon black material, where large surface area is important. © by Oldenbourg Wissenschaftsverlag.
    view abstract10.1524/zpch.2013.0369
  • Temperature, pressure, and bath gas composition dependence of fluorescence spectra and fluorescence lifetimes of toluene and naphthalene
    Faust, S. and Tea, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 110 (2013)
    Time-resolved fluorescence spectra of gas-phase toluene and naphthalene were investigated upon picosecond laser excitation at 266 nm as a function of temperature (toluene 296-1,025 K, naphthalene 374-1,123 K), pressure (1-10 bar), and bath gas composition (varying concentrations of N2, O 2, and CO2) with a temporal resolution of 50 ps. In the investigated temperature range, the fluorescence spectra of both toluene and naphthalene show a significant red-shift, whereas the fluorescence lifetime decreases with increasing temperature, more pronounced for toluene than for naphthalene. Increasing the total pressure of either N2 or CO 2 from atmospheric to 10 bar leads to an increase by about 20 % (naphthalene at 373 K) and a decrease by 60 % (toluene at 575 K) in fluorescence lifetimes, respectively. As expected, at atmospheric pressure collisions with O2 shorten the fluorescence lifetime of both toluene and naphthalene significantly, e.g., by a factor of 30 and 90 when changing O2 partial pressure at 373 K from 0 to 0.21 bar, respectively. The fluorescence model of Koban et al. (Appl Phys B 80: 777, 2005) for the dependence of the toluene quantum yield on temperature and O2 partial pressure at atmospheric pressure describes toluene fluorescence lifetimes well within its range of validity. The model is modified to satisfactorily predict effective toluene fluorescence lifetimes in N2 at pressures up to 10 bar. However, it still fails to predict the dependence at simultaneously elevated temperatures and pressures in air as bath gas. Similarly, an empirical model is presented for predicting (relative) fluorescence quantum yields and lifetimes of naphthalene. Although the fitting models have their shortcomings this publication presents a data set of great importance for practical LIF applications, e.g., in-cylinder mixture formation diagnostics in internal combustion engines. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-012-5254-8
  • Thermal stratification in an internal combustion engine due to wall heat transfer measured by laser-induced fluorescence
    Kaiser, S.A. and Schild, M. and Schulz, C.
    Proceedings of the Combustion Institute 34 (2013)
    Laser-induced fluorescence of toluene was used to image spatial fluctuations of gas temperature in an optically accessible engine. These thermal inhomogeneities develop due to wall heat-transfer and convection during the compression stroke. They are known to be important for slowing heat release when operating the engine in compression auto-ignition (CAI) mode. The engine had a pent-roof four-valve head typical for automotive spark-ignited engines and a flat-top piston with a window. Measurements were performed in the central vertical symmetry plane of the cylinder with the engine motored and fed with nitrogen. Toluene was seeded homogeneously into the intake gas. Fluorescence was excited at 248 nm and detected spectrally integrated. Toluene fluorescence decreases strongly with increasing temperature. Estimating the absolute in-cylinder temperature from isentropic compression along the measured pressure trace, we found the magnitude of this decrease in the engine to be consistent with literature data from heated flow-cell measurements. This calibration allowed for determination of the spatial fluctuations in temperature against the multi-cycle average temperature. Precision error, mostly from laser mode fluctuations, was between 1.6 and 4.0 K depending on crank angle. The results show that the temperature-fluctuation field transitions during the compression stroke from small, evenly distributed inhomogeneities to much greater fluctuations, mostly localized near walls, but convecting into the cylinder center very late in the stroke. There are subtle differences in the spatial structure of the near-wall fluctuation field between cylinder head and piston top. Compiled from the entire imaged area, at top dead-center the standard deviation of the fluctuations was 1.9% of the temperature differential between gas phase and wall, consistent with corresponding literature data. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2012.05.059
  • VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines
    Witzel, O. and Klein, A. and Meffert, C. and Wagner, S. and Kaiser, S. and Schulz, C. and Ebert, V.
    Optics Express 21 (2013)
    We report the first application of a vertical-cavity surfaceemitting laser (VCSEL) for calibration- and sampling-free, high-speed, in situ H2O concentration measurements in IC engines using direct TDLAS (tunable diode laser absorption spectroscopy). Measurements were performed in a single-cylinder research engine operated under motored conditions with a time resolution down to 100 μs (i.e., 1.2 crank angle degrees at 2000 rpm). Signal-to-noise ratios (1σ) up to 29 were achieved, corresponding to a H2O precision of 0.046 vol.% H2O or 39 ppm·m. The modulation frequency dependence of the performance was investigated at different engine operating points in order to quantify the advantages of VCSEL against DFB lasers. ©2013 Optical Society of America.
    view abstract10.1364/OE.21.019951
  • Application of endoscopic OH*-chemiluminescence measurements at a full-scale high-pressure gas turbine combustion test rig
    Witzel, B. and Heinze, J. and Kock, B.F. and Stockhausen, G. and Streb, H. and Van Kampen, J. and Schulz, C. and Willert, C. and Fleing, C.
    Proceedings of the ASME Turbo Expo 1 (2012)
    Single burner combustion tests play a key role in the Siemens gas turbine combustion system development process. The main scope of these tests is to assess the performance of combustor design variants in terms emissions or combustion stability at gas turbine relevant operation conditions. Both emissions and combustion stability strongly depend on the flame front and flame position. A pragmatic approach to investigate the flame is to detect the chemiluminescence signal of the combustion intermediate species OH*. Thus, the OH*- chemiluminescence signal was recorded at high-pressure combustion tests to get more insight in the complex interactions between combustor design, operation conditions and combustion performance. To minimize the impact of the measurement system on the combustion behavior, the optical access to the test rig was realized by using a water-cooled probe with an UV-transparent endoscope. The probe was located in the test rig side-wall, downstream of the burner outlet, viewing towards the burner with a 90° angle relative to the endoscope orientation. The experimental setup was completed by a combination of bandpass filters and an ICCD camera. During the experiments acoustic pressure oscillations inside the combustion chamber were recorded simultaneously to the chemiluminescence images to allow for phase-sorting of the recorded images during the image post-processing. The postprocessed images then were correlated with the pressure oscillations to investigate the relationship of the heat release to the pressure oscillations. The measurements were carried out during single burner gas turbine combustion tests at realistic gas turbine operation conditions at a scaled pressure of 9 bar. This paper presents selected test results and discusses how they give new insight in the complex combustion processes at full-scale high-pressure gas turbine combustion tests. Copyright © 2012 by ASME.
    view abstract10.1115/GT2012-68965
  • Autoignition of surrogate biodiesel fuel (B30) at high pressures: Experimental and modeling kinetic study
    Ramirez Lancheros, H.P. and Fikri, M. and Cancino, L.R. and Moréac, G. and Schulz, C. and Dagaut, P.
    Combustion and Flame 159 (2012)
    Ignition delay times of surrogate biodiesel fuels were measured in a high-pressure shock tube over a wide range of experimental conditions (pressures of 20 and 40. bar, equivalence ratios in the range 0.5-1.5, and temperatures ranging from 700 to 1200. K). A detailed chemical kinetic mechanism developed for the oxidation of a biodiesel fuel and a B30 biodiesel surrogate (49% n-decane, 21% 1-methylnaphthalene, and 30% methyloctanoate in mol%) was used to simulate the present experiments. Cross reactions between radicals from the three fuel components and reactions of methylnaphthalene oxidation recently proposed in the literature were introduced into the model in order to improve ignition delay time predictions at low temperatures. The new scheme (7865 reversible reactions and 1975 species) yields improved model predictions of concentration profiles measured earlier in a jet-stirred reactor, and also represents fairly well the present experimental data over the entire range of conditions of this study. Sensitivity analyses and reaction path analyses were used to rationalize the results. © 2011 The Combustion Institute.
    view abstract10.1016/j.combustflame.2011.10.006
  • Comparison of micro- and nanoscale Fe +3-containing (Hematite) particles for their toxicological properties in human lung cells in vitro
    Bhattacharya, K. and Hoffmann, E. and Schins, R.F.P. and Boertz, J. and Prantl, E.-M. and Alink, G.M. and Byrne, H.J. and Kuhlbusch, T.A.J. and Rahman, Q. and Wiggers, H. and Schulz, C. and Dopp, E.
    Toxicological Sciences 126 (2012)
    The specific properties of nanoscale particles, large surface-to-mass ratios and highly reactive surfaces, have increased their commercial application in many fields. However, the same properties are also important for the interaction and bioaccumulation of the nonbiodegradable nanoscale particles in a biological system and are a cause for concern. Hematite (α-Fe 2O 3), being a mineral form of Fe(III) oxide, is one of the most used iron oxides besides magnetite. The aim of our study was the characterization and comparison of biophysical reactivity and toxicological effects of α-Fe 2O 3 nano- (d < 100 nm) and microscale (d < 5 μm) particles in human lung cells. Our study demonstrates that the surface reactivity of nanoscale α-Fe 2O 3 differs from that of microscale particles with respect to the state of agglomeration, radical formation potential, and cellular toxicity. The presence of proteins in culture medium and agglomeration were found to affect the catalytic properties of the hematite nano- and microscale particles. Both the nano- and microscale α-Fe 2O 3 particles were actively taken up by human lung cells in vitro, although they were not found in the nuclei and mitochondria. Significant genotoxic effects were only found at very high particle concentrations (> 50 μg/ml). The nanoscale particles were slightly more potent in causing cyto- and genotoxicity as compared with their microscale counterparts. Both types of particles induced intracellular generation of reactive oxygen species. This study underlines that α-Fe 2O 3 nanoscale particles trigger different toxicological reaction pathways than microscale particles. However, the immediate environment of the particles (biomolecules, physiological properties of medium) modulates their toxicity on the basis of agglomeration rather than their actual size. © The Author 2012. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved.
    view abstract10.1093/toxsci/kfs014
  • Diffractive/refractive (hybrid) UV-imaging system for minimally invasive metrology: Design, performance, and application experiments
    Reichle, R. and Pruss, C. and Gessenhardt, C. and Schulz, C. and Osten, W.
    Applied Optics 51 (2012)
    A hybrid imaging system was developed to enable the application of laser-based measurement techniques like UV laser-induced fluorescence in near-production engines with small access ports. For this task, wide-angle characteristics and high lens speed are required in combination with small engine-bound optics able to survive in harsh environmental conditions. Our approach combines a simple and robust access lens with refractive/diffractive (hybrid) imaging stages away from the engine that are customized for individual wavelength bands. We give a detailed insight into the design strategy, including the integration of diffractive optics and the performance of the system with analysis of the modulation transfer function (MTF), lens speed, and stray light. Finally, results from applications in an actual engine are shown. © 2012 Optical Society of America.
    view abstract10.1364/AO.51.001982
  • Experimental and modeling study of carbon suboxide decomposition behind reflected shock waves
    Aghsaee, M. and Böhm, H. and Dürrstein, S.H. and Fikri, M. and Schulz, C.
    Physical Chemistry Chemical Physics 14 (2012)
    At temperatures between 1150 and 2000 K and pressures between 0.1 and 0.2 MPa, the thermal decomposition of carbon suboxide (C3O2) behind reflected shock waves was investigated with a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) connected to the end flange of a shock tube enabling rapid repetitive (100 kHz) measurements of the gas-phase composition. Concentration-time profiles for C3O2 and CO were measured and compared to simulations based on an improved mechanism for C3O2 decomposition and carbon cluster growth. In addition, relative concentrations of C atoms and C2 molecules were detected and related to model predictions. For temperatures up to 1800 K, satisfactory agreement between experimental data and calculations was obtained. At higher temperatures, measurements and simulations differed noticeably. The importance of C2 for the growth of carbon clusters was confirmed. This journal is © the Owner Societies.
    view abstract10.1039/c1cp22044f
  • Functionalization of SiO2 nanoparticles and their superhydrophobic surface coating
    Sehlleier, Y. H. and Abdali, A. and Hulser, T. and Wiggers, H. and Schulz, C.
    Nanoformulation (2012)
    view abstract10.1039/9781849735247-00113
  • High-capacity cathodes for lithium-ion batteries from nanostructured LiFePO 4 synthesized by highly-flexible and scalable flame spray pyrolysis
    Hamid, N.A. and Wennig, S. and Hardt, S. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Power Sources 216 (2012)
    Olivine, LiFePO 4 is a promising cathode material for lithium-ion batteries due to its low cost, environmental acceptability and high stability. Its low electric conductivity prevented it for a long time from being used in large-scale applications. Decreasing its particle size along with carbon coating significantly improves electronic conductivity and lithium diffusion. With respect to the controlled formation of very small particles with large specific surface, gas-phase synthesis opens an economic and flexible route towards high-quality battery materials. Amorphous FePO 4 was synthesized as precursor material for LiFePO 4 by flame spray pyrolysis of a solution of iron acetylacetonate and tributyl phosphate in toluene. The pristine FePO 4 with a specific surface from 126-218 m 2 g -1 was post-processed to LiFePO 4/C composite material via a solid-state reaction using Li 2CO 3 and glucose. The final olivine LiFePO 4/C particles still showed a large specific surface of 24 m 2 g -1 and were characterized using X-ray diffraction (XRD), electron microscopy, X-ray photoelectron spectrocopy (XPS) and elemental analysis. Electrochemical investigations of the final LiFePO 4/C composites show reversible capacities of more than 145 mAh g -1 (about 115 mAh g -1 with respect to the total coating mass). The material supports high drain rates at 16 C while delivering 40 mAh g -1 and causes excellent cycle stability. © 2012 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2012.05.047
  • High-speed tunable diode laser absorption spectroscopy for sampling-free in-cylinder water vapor concentration measurements in an optical IC engine
    Witzel, O. and Klein, A. and Wagner, S. and Meffert, C. and Schulz, C. and Ebert, V.
    Applied Physics B: Lasers and Optics 109 (2012)
    A novel, fiber-optic in situ laser hygrometer was developed to measure water vapor with microsecond time resolution directly inside an internal combustion (IC) engine. The instrument is intended for sampling-free quantification of recirculated exhaust gas in combustion engines. Direct tunable diode laser absorption spectroscopy was employed to allow absolute and self-calibrating H2O measurements. The compact and user-friendly instrument combines a fiber-coupled, 1.37 μm distributed feedback diode laser with kHz-fast, continuous wavelength scanning. Only small, typically 10 mm, optical access ports in the engine are needed. The new in situ hygrometer was tested via measurements in a motored optical research engine operated on ambient air, without any artificial humidification. Scanning the laser at 4 kHz resulted in a time resolution of 250 μs (i.e., 3 crank angle at 2,000 rpm), while the DC-coupled detector signals are digitized with a 4MSamples/s 16-bit data acquisition system. Absolute water vapor concentrations around 1 vol.% could be measured and quantified during the full compression stroke, i.e., over a pressure/temperature range of 0.07-0.52 MPa/300-500 K. Without any scan averaging or bandwidth filtering we could demonstrate signal-to-noise ratios between 51 (at p = 0.1 MPa) and 33 (at p = 0.4 MPa), which corresponds to H 2O detection limits between 0.02 and 0.035 vol.% or length and bandwidth normalized detectivities of 285 and 477 ppb m Hz-, respectively. Comparison of the dynamic H2O behavior during the compression stroke across several engine cycles and different operating conditions showed good reproducibility and absolute accuracy of the results, consistent with the boundary conditions, i.e., motored air operation. This new sensor therefore opens up new possibilities for engine cycle-resolved, calibration-free in situ AGR quantification and optimization in engine applications. © 2012 Springer-Verlag.
    view abstract10.1007/s00340-012-5225-0
  • In-cylinder temperature measurements via time-correlated single-photon counting of toluene laser-induced fluorescence through a fiber-based sensor
    Friesen, E. and Gessenhardt, C. and Kaiser, S.A. and Dreier, T. and Schulz, C.
    Optics Letters 37 (2012)
    In a near-production internal combustion engine, the effective fluorescence lifetime of toluene was determined by time-correlated single-photon counting with a minimally invasive fiber-optic spark-plug sensor. The lifetime measurement provided continuous crank-angle-resolved measurements of gas temperature. Proof-of-concept experiments in a motored four-cylinder spark-ignition engine were evaluated with a time resolution of 500 μs, yielding temperature precision of 25 K (standard deviation) at top-dead center. In these experiments, 10% toluene was added to the nonfluorescent base fuel iso-octane. Fluorescence lifetimes were related to temperature via calibration measurements in a high temperature pressure vessel, with the data fitted to a functional dependence derived from a previously published phenomenological model. © 2012 Optical Society of America.
    view abstract10.1364/OL.37.005244
  • Investigation of the kinetics of OH* and CH* chemiluminescence in hydrocarbon oxidation behind reflected shock waves
    Bozkurt, M. and Fikri, M. and Schulz, C.
    Applied Physics B: Lasers and Optics 107 (2012)
    The temporal variation of chemiluminescence emission from OH*(A 2 ∑+) and CH*(A2 Δ) in reacting Ar-diluted H2/O2/CH4, C2H 2/O2 and C2H2/N2O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH* in hydrocarbon combustion was studied with CH4 as benchmark fuel. Three reaction pathways leading to CH* were studied with C2H2 as fuel. Based on well-validated ground-state chemistry models from literature, sub-mechanisms for OH* and CH* were developed. For the main OH*-forming reaction CH + O2 = OH* + CO, a rate coefficient of k2 = (8.0 ± 2.6) × 1010 cm3 mol-1 s -1 was determined. For CH* formation, best agreement was achieved when incorporating reactions C2 + OH = CH* + CO (k5 = 2.0 × 1014 cm3 mol-1 s-1) and C2H + O = CH* + CO (k 6 = 3.6 × 1012 exp(-10.9 kJ mol-1/RT) cm3 mol-1 s-1) and neglecting the C2H + O 2 = CH* + CO2 reaction. © 2012 Springer-Verlag.
    view abstract10.1007/s00340-012-5012-y
  • Nanoparticles from the Gas Phase: Formation, Structure, Properties
    Lorke, A. and Winterer, M. and Schmechel, R. and Schulz, C.
    NanoScience and Technology 79 (2012)
    view abstract10.1007/978-3-642-28546-2
  • On the effect of molecular and hydrocarbon-bonded hydrogen on carbon particle formation in C 3O 2 pyrolysis behind shock waves
    Böhm, H. and Emelianov, A. and Eremin, A. and Schulz, C. and Jander, H.
    Combustion and Flame 159 (2012)
    The effect of H 2 and C 2H 2 addition on particle formation in the pyrolysis of C 3O 2/Ar mixtures was studied behind reflected shock waves. An existing reaction mechanism for the pyrolysis of highly-diluted C 3O 2 in argon was expanded to conditions with higher C 3O 2 concentrations (up to 33volume%) at elevated pressures and high temperatures and was validated against experimental data. The simulations for the gas-phase chemistry were performed with the program CHEMKIN. The heterogeneous particle formation was modeled by post-processing using the program PREDICI relying on the Galerkin method. It was found that in C 3O 2/H 2/Ar pyrolysis, the induction times and rate constants of particle formation do not differ significantly from those of pure C 3O 2/Ar pyrolysis. However, the presence of H 2 reduced the particle volume fraction, the mean diameter of particles, the particle number density, and the maximum temperature rise of the mixture. Hydrocarbon-bonded hydrogen in C 3O 2/C 2H 2/Ar pyrolysis caused significantly increased induction times for particle formation, decreased particle volume fractions, and decreased temperature rises. The different reaction channels for carbon particle formation were identified in view of the role of hydrogen. An alternating reaction channel including C 2 species played an important role in forming polycyclic aromatic hydrocarbons (PAH) in the mixtures. © 2011 The Combustion Institute.
    view abstract10.1016/j.combustflame.2011.09.012
  • Selected papers about chemiluminescence of flames
    Brockhinke, A. and Schulz, C.
    Applied Physics B: Lasers and Optics 107 (2012)
    view abstract10.1007/s00340-012-5092-8
  • Simultaneous measurement of localized heat release with OH/CH 2O-LIF imaging and spatially integrated OH* chemiluminescence in turbulent swirl flames
    Röder, M. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 107 (2012)
    The in-situ and localized observation of heat release in turbulent flames is important for the validation of computational modeling of turbulent flows with combustion. In the present work we obtain localized information on heat release rate (HRR) by the commonly accepted technique of the simultaneous and single-shot planar imaging of OH and CH2O concentrations by laser-induced fluorescence (LIF). Additionally, we combine this with the simultaneous line-of-sight and temporally resolved chemiluminescence detection of OH*, spatially integrated within the flame volume, interrogated by the laser sheets used for the HRR imaging technique. The combined diagnostic methods are demonstrated for a swirl-stabilized, premixed turbulent methane/air flame of 30-kW thermal power, and they show the existence of correlations between both HRR-sensitive diagnostic techniques. © 2012 Springer-Verlag.
    view abstract10.1007/s00340-012-4990-0
  • Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid
    Bywalez, R. and Karacuban, H. and Nienhaus, H. and Schulz, C. and Wiggers, H.
    Nanoscale Research Letters 7 (2012)
    We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months. © 2012 Bywalez et al.
    view abstract10.1186/1556-276X-7-76
  • Strain rate and fuel composition dependence of chemiluminescent species profiles in non-premixed counterflow flames: Comparison with model results
    Prabasena, B. and Röder, M. and Kathrotia, T. and Riedel, U. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 107 (2012)
    A detailed comparison has been conducted between chemiluminescence (CL) species profiles of OH*, CH*, and C2*, obtained experimentally and from detailed flame kinetics modeling, respectively, of atmospheric pressure non-premixed flames formed in the forward stagnation region of a fuel flow ejected from a porous cylinder and an air counterflow. Both pure methane and mixtures of methane with hydrogen (between 10 and 30 % by volume) were used as fuels. By varying the air-flow velocities methane flames were operated at strain rates between 100 and 350 s-1, while for methane/hydrogen flames the strain rate was fixed at 200 s-1. Spatial profiles perpendicular to the flame front were extracted from spectrograms recorded with a spectrometer/CCD camera system and evaluating each spectral band individually. Flame kinetics modeling was accomplished with an in-house chemical mechanism including C1-C4 chemistry, as well as elementary steps for the formation, removal, and electronic quenching of all measured active species. In the CH4/air flames, experiments and model results agree with respect to trends in profile peak intensity and position. For the CH4/H2/air flames, with increasing H2 content in the fuel the experimental CL peak intensities decrease slightly and their peak positions shift towards the fuel side, while for the model the drop in mole fraction is much stronger and the peak positions move closer to the fuel side. For both fuel compositions the modeled profiles peak closer to the fuel side than in the experiments. The discrepancies can only partly be attributed to the limited attainable spatial resolution but may also necessitate revised reaction mechanisms for predicting CL species in this type of flame. © 2012 Springer-Verlag.
    view abstract10.1007/s00340-012-4989-6
  • Synthesis of tailored nanoparticles in flames: Chemical kinetics, in situ diagnostics, numerical simulation, and process development
    Wiggers, H. and Fikri, M. and Wlokas, I. and Roth, P. and Schulz, C.
    NanoScience and Technology 79 (2012)
    Flame synthesis of nanoparticles provides access to a wide variety of metal oxide nanoparticles. Detailed understanding of the underlying fundamental processes is a prerequisite for the synthesis of specific materials with well-defined properties. Multiple steps from gas-phase chemistry, inception of first particles and particle growth are thus investigated in detail to provide the information required for setting up chemistry and particle dynamics models that allow simulating particle synthesis apparatus. Experiments are carried out in shock wave and flow reactors with in situ optical diagnostics, such as absorption, laser-induced fluorescence, and laser-induced incandescence, with in-line sampling via mass spectrometry as well as with thermophoretic sampling for ex situ microscopic analysis and electronic characterization. Focus is on tuning particle size as well as crystallinity and stoichiometry, with a specific focus on sub-stoichiometric materials with tunable composition. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_1
  • The autoignition of practical fuels at HCCI conditions: High-pressure shock tube experiments and phenomenological modeling
    Vandersickel, A. and Hartmann, M. and Vogel, K. and Wright, Y.M. and Fikri, M. and Starke, R. and Schulz, C. and Boulouchos, K.
    Fuel 93 (2012)
    Delay times for first- and second-stage ignition of n-heptane and two practical kerosene-like fuels have been measured in a heated high-pressure shock tube at conditions similar to those found in homogeneous charge compression ignition (HCCI) engines. Initial reflected shock conditions covered temperatures from 700 to 1100 K, pressures from 20 to 65 bar, equivalence ratios of 0.5, 0.67, and 1, and exhaust gas recirculation rates (EGR) of 0%, 30%, and 50%. EGR is simulated by introducing additional N 2 in the test gas mixture. Because detailed chemical kinetics models are not available for practical fuels so far, we propose a global ignition time correlation that is fitted to the measured data. The empirical model describes both first- and second-stage ignition delay as a function of temperature, pressure, equivalence ratio and EGR. It is based on a three-stage Arrhenius approach, which has been extended to capture the first-stage ignition times and the influence of EGR. For each of the fuels, even for the fuels that contain cyclo-alkanes and aromatics, good agreement between model and experiment is observed, especially at lean and high-EGR conditions relevant for HCCI. Based on this extensive set of ignition data, interesting correlations between fuel composition and ignition behavior could be identified, which may prove useful in matching the fuel to the engine application. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2011.10.062
  • Two-dimensional cycle-resolved exhaust valve temperature measurements in an optically accessible internal combustion engine using thermographic phosphors
    Fuhrmann, N. and Schild, M. and Bensing, D. and Kaiser, S.A. and Schulz, C. and Brübach, J. and Dreizler, A.
    Applied Physics B: Lasers and Optics 106 (2012)
    Phosphor thermometry was employed to measure the temperature distribution of the exhaust valves in an optically accessible direct injection internal combustion engine. A CMOS high-speed camera was used to two-dimensionally resolve the temperature dependent luminescence decay of the phosphor Gd 3Ga 5O 12:Cr. Measurements were performed under motored and fired conditions for several degrees crank angle to determine the temperature distributions within cycles. Additionally, several binders have been tested in terms of survivability and signal strength to guarantee ideal phosphor coating. © Springer-Verlag 2012.
    view abstract10.1007/s00340-011-4819-2
  • Visualization of the gas flow in fuel cell bipolar plates using molecular flow seeding and micro-particle image velocimetry
    Hecht, C. and Van Der Schoot, N. and Kronemayer, H. and Wlokas, I. and Lindken, R. and Schulz, C.
    Experiments in Fluids 52 (2012)
    Main components of proton exchange membrane fuel cells are bipolar plates that electrically connect the electrodes and provide a gas flow to the membrane. We investigate the flow in the channel structures of bipolar plates. Flow seeding is used to visualize the propagating and mixing gas stream. It is shown that a part of the gas is transported perpendicularly to the channel structure. An analysis of the diffusion compared with the convection shows different transport behavior for both flow directions. Additionally, the convective flow field is investigated in detail near the channel wall using Micro-PIV in a Reynolds-number-scaled liquid fluid system. For a more exact comparison of the experimental setups, flow seeding in both gas and liquid systems is performed. © Springer-Verlag 2011.
    view abstract10.1007/s00348-011-1112-4
  • A shock tube with a high-repetition-rate time-of-flight mass spectrometer for investigations of complex reaction systems
    Drrstein, S.H. and Aghsaee, M. and Jerig, L. and Fikri, M. and Schulz, C.
    Review of Scientific Instruments 82 (2011)
    A conventional membrane-type stainless steel shock tube has been coupled to a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) to be used to study complex reaction systems such as the formation of pollutants in combustion processes or formation of nanoparticles from metal containing organic compounds. Opposed to other TOF-MS shock tubes, our instrument is equipped with a modular sampling unit that allows to sample with or without a skimmer. The skimmer unit can be mounted or removed in less than 10 min. Thus, it is possible to adjust the sampling procedure, namely, the mass flux into the ionization chamber of the HRR-TOF-MS, to the experimental situation imposed by species-specific ionization cross sections and vapor pressures. The whole sampling section was optimized with respect to a minimal distance between the nozzle tip inside the shock tube and the ion source inside the TOF-MS. The design of the apparatus is presented and the influence of the skimmer on the measured spectra is demonstrated by comparing data from both operation modes for conditions typical for chemical kinetics experiments. The well-studied thermal decomposition of acetylene has been used as a test system to validate the new setup against kinetics mechanisms reported in literature. © 2011 American Institute of Physics.
    view abstract10.1063/1.3627573
  • Auto-ignition of toluene-doped n-heptane and iso-octane/air mixtures: High-pressure shock-tube experiments and kinetics modeling
    Hartmann, M. and Gushterova, I. and Fikri, M. and Schulz, C. and Schießl, R. and Maas, U.
    Combustion and Flame 158 (2011)
    Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times τign were measured behind reflected shock waves in mixtures with air at φ=1.0 and 0.5 at p=40bar, over a temperature range of T=700-1200K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on τign on the base fuels as function of temperature. For typical toluene tracer concentrations ≤10%, the ignition delay time τign changes by less than 10% in the relevant pressure and temperature range. © 2010 The Combustion Institute.
    view abstract10.1016/j.combustflame.2010.08.005
  • Gas-temperature imaging in a microwave-plasma nanoparticle-synthesis reactor using multi-line NO-LIF thermometry
    Hecht, C. and Abdali, A. and Dreier, T. and Schulz, C.
    Zeitschrift fur Physikalische Chemie 225 (2011)
    Multi-line NO-LIF thermometry is used to determine the two-dimensional temperature distribution inside a low-pressure plasma reactor. The applicability of multi-line temperature measurements to non-equilibrium plasma environments was evaluated. Temperatures between 300 and 3000K have been observed, while microwave power, and pressure show a strong effect on the temperature distribution. Metal-organic precursors added for particle synthesis additionally influence the temperature through the heat release during particle formation and the oxidation of organic ligands. © by Oldenbourg Wissenschaftsverlag, München.
    view abstract10.1524/zpch.2011.0162
  • High temperature shock-tube study of the reaction of gallium with ammonia
    Fikri, M. and Bozkurt, M. and Somnitz, H. and Schulz, C.
    Physical Chemistry Chemical Physics 13 (2011)
    The gas-phase reaction of Ga atoms with NH3 was studied behind reflected shock waves in the temperature range of 1380 to 1870 K at pressures of 1.4 to 4.0 bar. Atomic-resonance-absorption spectroscopy (ARAS) at 403.299 nm was applied for the time-resolved determination of the Ga-atom concentration. Trimethylgallium (Ga(CH3)3) was used as a precursor of Ga atoms. After the initial increase in Ga concentration due to Ga(CH 3)3 decomposition, the Ga concentration decreases rapidly in the presence of NH3. For the simulation of the measured Ga-atom concentration profiles from the studied reaction, additional knowledge about the thermal decomposition of Ga(CH3)3 is required. The rate coefficient k4 of the reaction Ga + NH3 → products (R4) was determined from the Ga-atom concentration profiles under pseudo-first-order assumption and found to be k4(T) = 10 14.1±0.4 exp(-11900 ± 700 K/T) cm3 mol -1 s-1 (error limits at the one standard deviation level). No significant pressure dependence was noticeable within the scatter of the data at the investigated pressure range. © the Owner Societies 2011.
    view abstract10.1039/c0cp01373k
  • Ignition delay times of ethanol-containing multi-component gasoline surrogates: Shock-tube experiments and detailed modeling
    Cancino, L.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C.
    Fuel 90 (2011)
    Ignition delay times for binary (ethanol/iso-octane, 25%/75% by liquid volume) and quinary (iso-octane/toluene/n-heptane/diisobutylene/ethanol, 30%/25%/22%/13%/10%) gasoline surrogate fuels in air were measured under stoichiometric conditions behind reflected shock waves. The investigated post-shock temperature ranges from 720 to 1220 K at pressures of 10 bar for the binary mixture and 10 bar and 30 bar for the quinary mixture. Ignition delay times were evaluated using side-wall detection of CH* chemiluminescence (λ = 431.5 nm). Multiple regression analysis of the data indicates global activation energy of ∼124 kJ/mol for the binary mixture and ∼101 kJ/mol for the quinary mixture and a pressure dependence exponent of -1.0 was obtained for the quinary mixture. The measurements were compared to predictions using a proposed detailed kinetics model for multicomponent mixtures that is based on the reference fuels (PRF) model as a kernel and incorporates sub-mechanisms to account for the chemistry of ethanol, toluene and diisobutylene. The model was tested using the measured ignition delay times for the surrogate fuels. Additional comparisons are based on literature data for other fuel combinations of the single constituents forming the quinary surrogate to insure that the modified mechanism still correctly predicts the behavior of simple fuels. The proposed model reproduces the trend of the experimental data for all pure fuels and blends investigated in this work, including the pressure dependence. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2010.11.003
  • Imaging of the oxygen distribution in an isothermal turbulent free jet using two-color toluene LIF imaging
    Mohri, K. and Luong, M. and Vanhove, G. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 103 (2011)
    The results of a novel technique for the quantification of oxygen in an isothermal turbulent free jet using toluene laser induced fluorescence (LIF) are presented. This method relies on the red-shift of the toluene LIF emission spectrum with increasing oxygen concentration. Evaluating the LIF signal ratio from two different wavelength regions simultaneously produces results that depend only on the local oxygen concentration. From calibration data, obtained from repeated tests, the oxygen sensitivity of the two-color LIF technique is best for oxygen partial pressures pO2 120 mbar in the current setup. Quantified images of oxygen distribution are presented for 40.4, 60.5, 80.5, and 103 mbar pO2 in the toluene-seeded jet flow that is shielded by a toluene-seeded nitrogen co-flow at atmospheric pressure and temperature. Based on the average oxygen concentration images (obtained from 100 instantaneous oxygen images), the error in accuracy of measuring the oxygen concentration was 0.8, 3.0, 7.7, and 7.3% with a precision of ± 8.6, 5.5, 13.3, and 11.6% for the jet pO2 = 40.4, 60.5, 80.5 and 103 mbar cases, respectively. The main jet flow characteristics have been captured by the technique as determined from the measured oxygen distribution images. Centerline profiles of average oxygen concentration, normalized to the value at the nozzle exit, demonstrate self-similar behavior from 5 mm above the nozzle exit. Radial oxygen concentration profiles exhibit a Gaussian-type distribution that broadens with distance above the nozzle exit, in agreement with literature. © 2011 Springer-Verlag.
    view abstract10.1007/s00340-011-4564-6
  • Laser-based diagnostics for the measurement of liquid water film thickness
    Greszik, D. and Yang, H. and Dreier, T. and Schulz, C.
    Applied Optics 50 (2011)
    Three different diagnostic techniques are investigated for measurement of the thickness of liquid water films deposited on a transparent quartz plate. The methods are based on laser-induced fluorescence (LIF) from low concentrations of a dissolved tracer substance and spontaneous Raman scattering of liquid water, respectively, both excited with 266nm of radiation, and diode laser absorption spectroscopy (DLAS) in the near-infrared spectral region. Signal intensities are calibrated using liquid layers of known thickness between 0 and 1000 μm. When applied to evaporating liquid water films, the thickness values derived from the direct DLAS and Raman scattering measurements correlate well with each other as a function of time after the start of data recording, while the LIF signal derived thickness values decrease faster with time due to selective tracer evaporation from the liquid. The simultaneous application of the LIF with a tracer-free detection technique can serve as an in situ reference for quantitative film thickness measurements. © 2010 Optical Society of America.
    view abstract10.1364/AO.50.000A60
  • Measurement of water film thickness by laser-induced fluorescence and Raman imaging
    Greszik, D. and Yang, H. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 102 (2011)
    We present two non-intrusive, laser-based imaging techniques for the quantitative measurement of water fluid film thickness. The diagnostics methods are based on laser-induced fluorescence (LIF) of the organic tracer ethyl acetoacetate added to the liquid in sub-percent (by mass) concentration levels, and on spontaneous Raman scattering of liquid water, respectively, both with excitation at 266 nm. Signal intensities were calibrated with measurements on liquid layers of known thickness in a range between 0 and 500 μm. Detection via an image doubler and appropriate filtering in both light paths enabled the simultaneous detection of two-dimensional liquid film thickness information from both methods. The thickness of water films on transparent quartz glass plates was determined with an accuracy of 9% for the tracer LIF and 15% for the Raman scattering technique, respectively. The combined LIF/Raman measurements also revealed a preferential evaporation of the current tracer during the time-resolved recording of film evaporation. © 2010 Springer-Verlag.
    view abstract10.1007/s00340-010-4200-x
  • Plasma synthesis of nanostructures for improved thermoelectric properties
    Petermann, N. and Stein, N. and Schierning, G. and Theissmann, R. and Stoib, B. and Brandt, M.S. and Hecht, C. and Schulz, C. and Wiggers, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The utilization of silicon-based materials for thermoelectrics is studied with respect to the synthesis and processing of doped silicon nanoparticles from gas phase plasma synthesis. It is found that plasma synthesis enables the formation of spherical, highly crystalline and soft-agglomerated materials. We discuss the requirements for the formation of dense sintered bodies, while keeping the crystallite size small. Small particles a few tens of nanometres and below that are easily achievable from plasma synthesis, and a weak surface oxidation, both lead to a pronounced sinter activity about 350 K below the temperature usually needed for the successful densification of silicon. The thermoelectric properties of our sintered materials are comparable to the best results found for nanocrystalline silicon prepared by methods other than plasma synthesis. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/44/17/174034
  • Recent activities in silicon hydride research in Europe
    Hidding, B. and Lorenz, P. and Klaus, F. and Hundsdörfer, C. and Kornath, A. and Kaufmann, A. and Hadjizadeh, S.M. and Soltner, T. and Klapötke, T. and Fikri, M. and Bozkurt, M. and Dürrstein, S.H. and Schulz, C. and Zhu, X. and Möller, M. and Bruno, C. and Simone, D. and Vergine, F. and Eisenreich, N. and Scharlemann, C.A. and Pfitzner, M. and Lang, M. and Langel, G. and Ellerbrock, H. and Delis, J. and Wagner, R.
    17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference 2011 (2011)
    Silicon hydrides have potential for chemical propulsion of hypersonic vehicles and for various other applications such as the semiconductor industry. Research activity in Europe in these fields is increasing, and various activities have recently been finished, are on-going or planned. It is reported on a range of these projects, the results of which continue to increase the attractiveness of silanes for hypersonic propulsion applications, for example in combination with liquid hydrocarbon fuels as a liquid, hypergolic hydrocarbon/silicon hydride scramjet fuel blend.
    view abstract10.2514/6.2011-2287
  • Synthesis of tailored WO3 and WOx (2.9 \< x \< 3) nanoparticles by adjusting the combustion conditions in a H2/O 2/Ar premixed flame reactor
    Gupta, A. and Ifeacho, P. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute 33 (2011)
    Flame synthesis of WO3 and WOx (2.9 < x < 3) nanoparticles is carried out by adding a dilute concentration of WF6 as precursor in a low-pressure H2/O2/Ar premixed flame reactor. The reactor is equipped with molecular-beam sampling and particle mass spectroscopy (PMS) to determine particle composition and sizes as a function of height above burner. Varying the H2/O2 ratio allowed us to tune the stoichiometry of the product. With a H2/O2 ratio of 0.67 white colored stoichiometric WO3 is formed, whereas the H2/O2 ratio &gt;0.8 yields blue colored non-stoichiometric WOx (2.9 < x < 3) nanoparticles. The size of nanoparticles can be controlled by varying the residence time in the high-temperature zone of the reactor as observed by molecular-beam sampling with subsequent analysis using PMS. Transmission electron microscopy (TEM) images of as-synthesized nanoparticles show that particles are non-agglomerated and have an almost spherical morphology. The X-ray diffraction (XRD) pattern of the as-synthesized material indicates that the powders exhibit poor crystallinity, however, subsequent thermal annealing of the sample in air changes its structure from amorphous to crystalline phase. It is observed that particles with sub-stoichiometric composition (WOx) show higher conductivity compared to the stoichiometric WO3 sample. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2010.06.162
  • Temperature and bath gas composition dependence of effective fluorescence lifetimes of toluene excited at 266 nm
    Faust, S. and Dreier, T. and Schulz, C.
    Chemical Physics 383 (2011)
    Time-resolved fluorescence spectra of gas-phase toluene upon picosecond excitation at 266 nm were investigated as a function of temperature (296-1074 K) and bath gas composition (varying amounts of N2, O2, and CO2) at 1 bar total pressure with a temporal resolution of 50 ps. In the investigated temperature range the effective fluorescence lifetime drops with increasing temperature from 46 ± 3 ns to 0.05 ± 0.01 ns in N2 and CO2. In the presence of O2 at constant temperature the lifetimes also decrease significantly (e.g., from 46 ± 3 ns without O2 to 0.63 ± 0.05 ns in air at room temperature), whereas lifetimes are independent on the CO2 concentration. The implications of the results for the existing phenomenological model of predicting temporally integrated fluorescence intensities in toluene [W. Koban, J.D. Koch, R.K. Hanson, C. Schulz, Appl. Phys. B 80 (2005) 777] are discussed. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.chemphys.2011.03.013
  • Tunable diode laser absorption sensor for the simultaneous measurement of water film thickness, liquid- and vapor-phase temperature
    Yang, H. and Greszik, D. and Wlokas, I. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 104 (2011)
    A four-wavelength near-infrared (NIR) tunable diode laser sensor has been developed for the simultaneous measurement of liquid water film thickness, liquid-phase temperature and vapor-phase temperature above the film. This work is an important improvement of a three-wavelength concept previously introduced by Yang et al. (Appl. Phys. B 99:385, 2010), which measured the film thickness in environments with known temperature only. In the new sensor, an optimized combination of four wavelengths is chosen based on a sensitivity analysis with regard to the temperature dependence of the liquid water absorption cross section around 1.4 μm. The temperature of liquid water and the film thickness are calculated from absorbance ratios taken at three wavelength positions assessing the broad-band spectral signature of liquid water. The vapor-phase temperature is determined from the absorbance ratio of two lasers rapidly tuned across two narrow-band gas-phase water absorption transitions. The performance of the sensor was demonstrated in a calibration cell providing liquid layers of variable thickness and temperature with uncertainties smaller than 5% for thickness measurements and 1.5% for liquid-phase temperatures, respectively. Experiments are also presented for time-resolved thickness and temperature measurements of evaporating water films on a quartz plate. © 2011 Springer-Verlag.
    view abstract10.1007/s00340-011-4643-8
  • Unburned gas temperature measurements in a surrogate Diesel jet via two-color toluene-LIF imaging
    Tea, G. and Bruneaux, G. and Kashdan, J.T. and Schulz, C.
    Proceedings of the Combustion Institute 33 (2011)
    Non-intrusive temperature measurements of the unburned fuel/air mixture in vaporized Diesel jets have been performed using two-color toluene laser-induced fluorescence (LIF). This diagnostics technique exploits the temperature- dependent spectral shift of the LIF signal which occurs after ultraviolet (UV) excitation of toluene that is added as tracer to a non-fluorescing base fuel. The method requires the determination of the ratio of LIF intensities collected by two detectors separate spectral bands. In the current study, measurements were performed in a high-pressure, high-temperature cell capable of reproducing the thermodynamic conditions in the combustion chamber of a Diesel engine during the injection event. Various aspects of the experimental set-up and the data evaluation were optimized. The temperature sensitivity of the measurement strategy is optimum at temperatures below 700 K. Temperature data acquired from two-color LIF thermometry were compared to single-color toluene-LIF measurements using an adiabatic mixing model. The latter is determined from toluene LIF-based fuel concentration measurements, the evaporation enthalpy, and thermocouple measurements of the bath-gas/ambient cell temperature prior to fuel injection. Based on simultaneous measurements with two cameras using identical optical filters a methodology to optimize the image superposition and to minimize the statistical error was developed. These measurements also allowed to determine the 1 - σ precision of the two-color LIF measurement to be in the 20-40 K range. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstract10.1016/j.proci.2010.05.074
  • Measurement and chemical kinetics modeling of shock-induced ignition of ethanol-air mixtures
    Cancino, L.R. and Fikri, M. and Oliveira, A.A.M. and Schulz, C.
    Energy and Fuels 24 (2010)
    A detailed kinetics model for the thermal oxidation of ethanol-air mixtures at intermediate temperatures and high pressures is proposed and validated against ignition delay times measured in a shock tube under stoichiometric conditions at 10, 30, and 50 bar and for lean mixtures (φ = 0.3) at 30 bar in the 650-1220 K temperature range. The measurements showed a typical decrease of the ignition delay at low temperatures and a reduced sensitivity to pressure for higher pressures. All data were scaled to 30 bar by a multiple linear regression, yielding τ = τ30(p/30)-0.88. A temperature dependence of τ/(p/bar)-0.88 = 10-3.21 exp(139 kJ/mol/RT) μs was derived for the stoichiometric mixture. The chemical kinetics model was built upon sub-mechanisms for ethanol (Marinov, N. M. Int. J. Chem. Kinet. 1999, 31, 183 -220) and C3 oxidation (Konnov, A. A. Combust. Flame 2009, 156, 2093-2105). Additional key reactions obtained from computational chemistry were included. The model was validated in the 650-1600 K temperature range at stoichiometric composition for 10, 30, and 50 bar, at an equivalence ratio φ = 0.3 for 30 bar, and in the 1200-1600 K range at 0.25 ≤ φ ≤ 2.0 in the 2.0-4.6 bar pressure range by comparing the predictions against these measurements and models of Dunphy and Simmie(Dunphy, M. P.; Simmie, J. M. J. Chem. Soc., Faraday Trans. 1991, 87, 1691-1696). Sensitivity coefficients for temperature and OH, H2O 2, and C2H5OH concentrations were determined using a time-dependent homogeneous reactor assumption at 800, 950, and 1100 K. The sensitivity analysis identified a set of important reactions involving hydrogen-atom abstraction from the ethanol molecule by the hydroperoxy radical (HO2), giving CH3CHOH, acetaldehyde, and H 2O2. For higher pressures, the model presents good agreement with the temperature dependence. At lower pressures, the model overpredicts the value of the apparent ignition delay activation energy obtained from the measurements by 34%. Overall, the model predicts well the global trend of ignition delay times on pressure. © 2010 American Chemical Society.
    view abstract10.1021/ef100076w
  • Simultaneous measurement of liquid water film thickness and vapor temperature using near-infrared tunable diode laser spectroscopy
    Yang, H. and Greszik, D. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 99 (2010)
    A fiber-based multiplexed tunable diode-laser absorption sensor with three near-infrared distributed-feedback diode lasers at ∼1.4 μm is used for simultaneous nonintrusive measurements of liquid water film thickness and vaporphase temperature. Water film thicknesses are derived from broad-band absorption determined at two fixed wavelengths while gas-phase temperature above the film is obtained via two-line thermometry using the fast wavelength tuning with line-integrating absorption. Probing the liquid film at two wavelengths with significantly different liquid-phase absorption cross sections allows discriminating against additional signal losses due to surface fowling, reflection, and beam steering. The technique is demonstrated for liquid layers of defined thicknesses and in time-resolved measurements of evaporating films. © Springer-Verlag 2010.
    view abstract10.1007/s00340-010-3980-3
  • Study of the H+O+M reaction forming OH*: Kinetics of OH* chemiluminescence in hydrogen combustion systems
    Kathrotia, T. and Fikri, M. and Bozkurt, M. and Hartmann, M. and Riedel, U. and Schulz, C.
    Combustion and Flame 157 (2010)
    The temporal variation of OH* (A2Σ+) chemiluminescence in hydrogen oxidation chemistry has been studied in a shock tube behind reflected shock waves at temperatures of 1400-3300K and at a pressure of 1bar. The aim of the present work is to obtain a validated reaction scheme to describe OH* formation in the H2/O2 system. Temporal OH* emission profiles and ignition delay times for lean and stoichiometric H2/O2 mixtures diluted in 97-98% argon were obtained from the shock-tube experiments. Based on a literature review for the hydrogen combustion system, the key reaction considered was H+O+M=OH*+M (R1). The temperature dependence of the measured peak OH* emission from the shock tube and the peak OH* concentration from a homogeneous closed reactor model are compared. Based on these results a reaction rate coefficient of k1=(1.5±0.4)×1013 exp(-25kJmol-1/RT) cm6mol-2s-1 was found for the forward reaction (R1) which is slightly higher than the rate coefficient suggested by Hidaka et al. (1982). The comparison of measured and simulated absolute concentrations shows good agreement. Additionally, a one-dimensional laminar premixed low-pressure flame calculation was performed for where absolute OH* concentration measurements have been reported by Smith et al. (2005). The absolute peak OH* concentration is fairly well reproduced if the above mentioned rate coefficient is used in the simulation. © 2010 The Combustion Institute.
    view abstract10.1016/j.combustflame.2010.04.003
  • Temperature and species measurement in a quenching boundary layer on a flat-flame burner
    Fuyuto, T. and Kronemayer, H. and Lewerich, B. and Brübach, J. and Fujikawa, T. and Akihama, K. and Dreier, T. and Schulz, C.
    Experiments in Fluids 49 (2010)
    A detailed understanding of transport phenomena and reactions in near-wall boundary layers of combustion chambers is essential for further reducing pollutant emissions and improving thermal efficiencies of internal combustion engines. In a model experiment, the potential of laser-induced fluorescence (LIF) was investigated for measurements inside the boundary layer connected to flame-wall interaction at atmospheric pressure. Temperature and species distributions were measured in the quenching boundary layer formed close to a cooled metal surface located parallel to the flow of a premixed methane/air flat flame. Multi-line NO-LIF thermometry provided gas-phase temperature distributions. In addition, flame species OH, CH2O and CO were monitored by single-photon (OH, CH2O) and two-photon (CO) excitation LIF, respectively. The temperature dependence of the OH-LIF signal intensities was corrected for using the measured gas-phase temperature distributions. The spatial line-pair resolution of the imaging system was 22 μm determined by imaging microscopic line pairs printed on a resolution target. The experimental results show the expected flame quenching behavior in the boundary layer and they reveal the potential and limitations of the applied diagnostics techniques. Limitations in spatial resolution are attributed to refraction of fluorescence radiation propagating through steep temperature gradients in the boundary layer. For the present experimental arrangements, the applied diagnostics techniques are applicable as close to the wall as 200 μm with measurement precision then exceeding the 15-25% limit for species detection, with estimates of double this value for the case of H2CO due to the unknown effect of the Boltzmann fraction corrections not included in the data evaluation process. Temperature measurements are believed to be accurate within 50 K in the near-wall zone, which amounts to roughly 10% at the lower temperatures encountered in this region of the flames. © 2010 Springer-Verlag.
    view abstract10.1007/s00348-010-0917-x
  • absorption spectroscopy

  • chemical kinetics

  • combustion

  • fluorescence

  • gas phase

  • internal combustion engines

  • laser induced incandescence

  • laser-based gas-phase and aerosol diagnostics

  • lasers

  • nanoparticles

  • plasmas

  • reactive flows

  • shock tubes

  • synthesis

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