Dr.-Ing. Irenäus Wlokas

Institute for Combustion and Gasdynamics
University of Duisburg-Essen


  • Insights into the mechanism of combustion synthesis of iron oxide nanoparticles gained by laser diagnostics, mass spectrometry, and numerical simulations: A mini-review
    Rahinov, I. and Sellmann, J. and Lalanne, M.R. and Nanjaiah, M. and Dreier, T. and Cheskis, S. and Wlokas, I.
    Energy and Fuels (2021)
    To fully master a scaled-up combustion synthesis of nanoparticles toward a wide library of materials with tailored functionalities, a detailed understanding of the underlying kinetic mechanism is required. In this respect, flame synthesis of iron oxide nanoparticles is a model case, being one of the better understood systems and guiding the way how other material synthesis systems could be advanced. In this mini-review, we highlight, on the example of an iron oxide system, an approach combining laser spectroscopy and mass spectrometry with detailed simulations. The experiments deliver information on time-temperature history and concentration field data for gas-phase species and condensable matter under well-defined conditions. The simulations, which can be considered as in silico experiments, combining detailed kinetic modeling with computational fluid dynamics, serve both for mechanism validation via comparison to experimental observables as well as for shedding light on quantities inaccessible by experiments. This approach shed light on precursor decomposition, initial stages of iron oxide particle formation, and precursor role in flame inhibition and provided insights into the effect of temperature-residence time history on nanoparticle formation, properties, and flame structure. © XXXX American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03561
  • 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
  • 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
  • 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 model flow reactor design for the study of nanoparticle structure formation under well-defined conditions
    Rosenberger, T. and Sellmann, J. and Wlokas, I. and Kruis, F.E.
    Review of Scientific Instruments 91 (2020)
    Structure formation models describe the change of the particle structure, e.g., by sintering or coating, as a function of the residence time and temperature. For the validation of these models, precise experimental data are required. The precise determination of the required data is difficult due to simultaneously acting mechanisms leading to particle structure formation as well as their dependency on various particle properties and process conditions in the reactor. In this work, a model flow reactor (MFR) is designed and optimized, supported by a validated computational fluid dynamic simulation, to determine the structure formation of nanoparticles under well-defined conditions. Online instrumentation is used to measure the particle mass and different equivalent diameter to detect changes of the particle shape and to calculate the particle structure, defined by the primary particle size, the number of primary particles per agglomerate, coating thickness, effective density, and fractal dimension, by means of structural models. High precision is achieved by examining size-selected particles in a low number concentration and a laminar flow field. Coagulation can be neglected due to the low particle number concentration. Structure formation is restricted to a defined region by direct particle trajectories from the water-cooled aerosol inlet to the water-cooled outlet. A preheated sheath gas is used to concentrate the aerosol on the centerline. The simulated particle trajectories exhibit a well-defined and narrow temperature residence time distribution. Residence times of at least 1 s in the temperature range from 500 K to 1400 K are achieved. The operation of the MFR is demonstrated by the sintering of size-selected FexOy agglomerates with measurements of the particle size and mass distribution as a function of the temperature. An increase of the effective density, resulting from the decreasing particle size at constant particle mass, is observed. © 2020 Author(s).
    view abstract10.1063/5.0018880
  • 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
  • Direct numerical simulations of nanoparticle formation in premixed and non-premixed flame-vortex interactions
    Cifuentes, L. and Sellmann, J. and Wlokas, I. and Kempf, A.
    Physics of Fluids 32 (2020)
    Direct numerical simulations (DNSs) of nanoparticle formation in reactive flows are challenging, and only greatly simplified DNS test-cases are possible, which help clarify the turbulence-particle-dynamics interaction and guide the necessary modeling efforts. As a basis for such studies, a new DNS database is introduced, which resolves the smallest relevant scales of the nanoparticle concentration field to obtain insights into the statistics of nanoparticle formation in reactive flows. Formation and evolution of iron oxide nanoparticles in premixed and non-premixed flames wrapped-up by a vortex have been investigated using the sectional model and direct chemistry. The DNSs capture the "engulfing"and local dilution of the particle fields. Different zones of high particle number concentration have been found in every flame, and it was shown that the thickness of these zones decreases with increasing Schmidt number, which confirms that in simulations of nanoparticle-forming turbulent reacting flows, the grid resolution has to be very fine to resolve the characteristic scale for high sections. The contributions to the change in particle concentration due to diffusion, coagulation, and nucleation have been analyzed in detail, and dominant contributions across the particle number concentration layers and across the flames have been identified. This analysis has also been carried out in terms of flat, concave, and convex iso-surface geometries, induced by the flame-vortex interaction and characterized by the curvature of the particle number concentration fields and also by the flame curvature. The results demonstrate that the flame curvature effects cannot be ignored in modeling strategies. The probability density functions for the particle number concentrations have been analyzed and quantified in terms of Shannon information entropy, which illustrates the effect of fast diffusion (and entropy production) of the smaller particles and slow diffusion (and entropy production) of the largest particles with high Schmidt numbers. In addition, the unclosed filtered or averaged agglomeration term was evaluated as a basis for future modeling efforts, showing that agglomeration rates will be underestimated by orders of magnitude unless suitable models are developed. © 2020 Author(s).
    view abstract10.1063/5.0020979
  • 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
  • Experimental and numerical study on the influence of equivalence ratio on key intermediates and silica nanoparticles in flame synthesis
    Karakaya, Y. and Janbazi, H. and Wlokas, I. and Levish, A. and Winterer, M. and Kasper, T.
    Proceedings of the Combustion Institute (2020)
    Tetramethylsilane is a precursor often used for the production of flame-synthesized silica nanoparticles or coatings. This study investigates the chemical reaction mechanism of tetramethylsilane in a series of H 2 /O 2 /Ar low-pressure (p = 30 mbar) flames from fuel-lean to slightly fuel-rich flame conditions (ϕ= 0.8, 1.0 and 1.2). Mole fraction profiles are obtained by molecular-beam mass spectrometry. The experimental data are compared to simulations using a recently published reaction mechanism. The present study reveals the influence of the flame composition on the depletion of the precursor TMS, the formation of its main carbon-containing products (e.g. CO 2 and CO) and the main silicon-containing intermediates (e.g. Si(CH 3) 3 (CH 2)OO), Si(OH) 4, SiO 2, Si 4 O 10 H 4) appearing along the routes of particle formation. TEM images of synthesized particles reveal that the nanoparticles obtained from the gas-phase synthesis are spheres with a low degree of agglomeration. The particle size distribution appears to be dependent on the equivalence ratio of the synthesis flames and the changes can tentatively be traced to different particle formation pathways. The data set provided in this work can serve a basis for improvements to the reaction mechanisms of the Si/C/H/O system that are urgently needed to improve particle synthesis processes. © 2020 The Combustion Institute. Published by Elsevier Inc.
    view abstract10.1016/j.proci.2020.06.096
  • Investigation of a High Karlovitz, High Pressure Premixed Jet Flame with Heat Losses by LES
    Gruhlke, P. and Janbazi, H. and Wlokas, I. and Beck, C. and Kempf, A.M.
    Combustion Science and Technology 192 (2020)
    Large-eddy simulations (LES) are presented for a lean preheated high pressure jet flame experiment for which detailed in situ data is available, using a finite rate chemistry (FRC) approach in a gas-turbine model combustor at high Karlovitz number. The impact of the different combustion models on the flame stabilization in the simulation is investigated and the predicted carbon monoxide (CO) and nitric oxide (NO x) emissions are analyzed. For the FRC approach, the DRM19 reaction mechanism and a new inhouse skeletal mechanism are applied. The more detailed DRM19 mechanism is extended to include OH* species, the new skeletal mechanism includes CO and NO x reaction paths. An industry relevant tabulated chemistry approach is assessed on the ability to predict this lifted flame, where the flamelet tables are calculated from the detailed GRI-3.0 reaction mechanism. A dynamic thickened flame approach is applied to resolve the flame on the numerical grid including a model for the turbulence chemistry interaction. Adiabatic and non-adiabatic simulations are compared, where the impact of heat losses due to chamber cooling and thermal radiation are considered. Velocities, temperatures, fuel mass fractions and CO and NO x mass fractions at different axial locations are in good agreement to the experiments when heat losses are considered. The significant flame lift was correctly predicted by the FRC approach with DRM19 chemistry when non-adiabatic boundary conditions were applied. This provides evidence that the flame is stabilized by flame propagation assisted by auto ignition and that ignition-delay times of mixtures composed of fresh and burnt gases need to be captured by the applied models. © 2020 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2020.1781101
  • 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
  • Thermophysical Properties of Mixtures of Titanium(IV) Isopropoxide (TTIP) and p-Xylene
    Keller, A. and Wlokas, I. and Kohns, M. and Hasse, H.
    Journal of Chemical and Engineering Data 65 (2020)
    Titanium(IV) isopropoxide (TTIP) is an important precursor for the production of nanoparticles by spray flame processes. In these processes, the precursor is provided in a solution in a combustible solvent, which is p-xylene here. As no thermophysical data for solutions of TTIP in p-xylene were available in the literature, they were measured in the present work. The vapor-liquid equilibrium was measured at pressures ranging from 20 to 80 kPa. The specific density, viscosity, thermal conductivity, molar isobaric heat capacity, and self-diffusion coefficients were determined experimentally at 101.3 kPa at temperatures between 293.15 and 373.15 K. Sample compositions cover the range from pure TTIP to pure p-xylene. Chemical reactions in the studied system were considered. The experiments were carried out in such a way that they do not compromise the results for the thermophysical properties. The vapor-liquid equilibrium data were correlated using the NRTL model. Empirical correlations were established for the other properties. The results provide a rational basis for spray flame process design. © 2020 American Chemical Society.
    view abstract10.1021/acs.jced.9b01059
  • Thermophysical Properties of Solutions of Iron(III) Nitrate Nonahydrate in Mixtures of 1-Propanol and Water
    Keller, A. and Wlokas, I. and Kohns, M. and Hasse, H.
    Journal of Chemical and Engineering Data 65 (2020)
    The simulation of spray flame processes for the production of high-quality nanoparticles relies on thermophysical properties of the precursor solutions, for which literature data are scarce. Here, we report experimental thermophysical data of solutions of iron(III) nitrate nonahydrate (INN) in (1-propanol + water) mixed solvents. The specific density, viscosity, thermal conductivity, and isobaric heat capacity of the solutions were measured at 101.3 kPa between 288.15 and 333.15 K, solvent compositions ranging from 0.73 mol mol-1 1-propanol to pure water, and INN molalities up to 1.3 mol kg-1. Empirical correlations of the experimental data are provided. © 2020 American Chemical Society.
    view abstract10.1021/acs.jced.0c00531
  • Thermophysical Properties of Solutions of Iron(III) Nitrate-Nonahydrate in Mixtures of Ethanol and Water
    Keller, A. and Wlokas, I. and Kohns, M. and Hasse, H.
    Journal of Chemical and Engineering Data 65 (2020)
    The quality of nanoparticles that are obtained by spray flame synthesis depends strongly on the thermophysical properties of the precursor solutions. Solutions of iron(III) nitrate-nonahydrate (INN) in ethanol are interesting precursor solutions for the production of iron oxide nanoparticles in these processes. However, no data on the thermophysical properties of solutions of INN in ethanol are available in the literature. Therefore, in the present work, the specific density, viscosity, thermal conductivity, and molar isobaric heat capacity of solutions of INN in solvent mixtures of ethanol and water were measured at 101.3 kPa between 288.15 and 333.15 K, solvent compositions ranging from pure ethanol to pure water, and INN molalities up to 1.3 mol kg-1. Empirical correlations of the experimental data are provided. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jced.0c00105
  • 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
  • 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
  • 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
  • Analysis of mild ignition in a shock tube using a highly resolved 3D-LES and high-order shock-capturing schemes
    Lipkowicz, J.T. and Wlokas, I. and Kempf, A.M.
    Shock Waves (2018)
    A highly resolved three-dimensional large-eddy simulation (LES) is presented for a shock tube containing a stoichiometric hydrogen–oxygen (H 2/O 2) mixture, and the results are compared against experimental results. A parametric study is conducted to test the effects of grid resolution, numerical scheme, and initial conditions before the 3D simulations are presented in detail. An approximate Riemann solver and a high-order interpolation scheme are used to solve the conservation equations of the viscous, compressible fluid and to account for turbulence behind the reflected shock. Chemical source terms are calculated by a finite-rate model. Simultaneous results of pseudo-Schlieren, temperature, pressure, and species are presented. The ignition delay time is predicted in agreement with the experiments by the three-dimensional simulations. The mechanism of mild ignition is analysed by Lagrangian tracer particles, tracking temperature histories of material particles. We observed strongly increased temperatures in the core region away from the end wall, explaining the very early occurrence of mild ignition in this case. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00193-018-0867-4
  • 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
  • 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
  • Dilute suspensions in annular shear flow under gravity: Simulation and experiment
    Schröer, K. and Kurzeja, P. and Schulz, S. and Brockmann, P. and Hussong, J. and Janas, P. and Wlokas, I. and Kempf, A. and Wolf, D.E.
    EPJ Web of Conferences 140 (2017)
    A dilute suspension in annular shear flow under gravity was simulated using multi-particle collision dynamics (MPC) and compared to experimental data. The focus of the analysis is the local particle velocity and density distribution under the influence of the rotational and gravitational forces. The results are further supported by a deterministic approximation of a single-particle trajectory and OpenFOAM CFD estimations of the overcritical frequency range. Good qualitative agreement is observed for single-particle trajectories between the statistical mean of MPC simulations and the deterministic approximation. Wall contact and detachment however occur earlier in the MPC simulation, which can be explained by the inherent thermal noise of the method. The multi-particle system is investigated at the point of highest particle accumulation that is found at 2/3 of the particle revolution, starting from the top of the annular gap. The combination of shear flow and a slowly rotating volumetric force leads to strong local accumulation in this section that increases the particle volume fraction from overall 0.7% to 4.7% at the outer boundary. MPC simulations and experimental observations agree well in terms of particle distribution and a close to linear velocity profile in radial direction. © The Authors, published by EDP Sciences, 2017.
    view abstract10.1051/epjconf/201714009034
  • 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
  • Large eddy simulations of nanoparticle synthesis from flame spray pyrolysis
    Rittler, A. and Deng, L. and Wlokas, I. and Kempf, A.M.
    Proceedings of the Combustion Institute 36 (2017)
    Large eddy simulations of the nanoparticle synthesis from flame spray pyrolysis are presented. A standard reactor is investigated, with ethanol/hexamethyldisiloxane (HMDSO) mixture as spray/precursor composition and oxygen as dispersion gas for the production of silica nanoparticles. Spray evaporation, ignition and stabilisation of the flame are achieved by a premixed methane/oxygen pilot flame. The gas, spray and nanoparticle phases are modelled with Eulerian, Lagrangian and Eulerian approaches, respectively. A modified tabulated chemistry model, adapted from the premixed flamelet generated manifold approach (PFGM) with artificial flame thickening (ATF) is proposed, tested and applied for the system. The control variables are the element mass fractions of hydrogen and carbon together with a joint progress variable. The population balance equation of the nanoparticles is modelled in terms of number, volume and surface area concentration, its subfilter distribution is modelled with a delta function. The combustion of HMDSO and formation of silica particle monomers is described by a two-step global mechanism. The nucleation source term is tabulated as a function of the control variables. The submodels for spray and combustion are validated separately to compensate for the shortage in detailed experimental data for nanoparticle spray flames. Subsequently, simulation results for the particles are presented and discussed, in particular the polydisperse particle size distributions resulting from turbulence. © 2016 The Combustion Institute.
    view abstract10.1016/j.proci.2016.08.005
  • On the Evolution of the Flow Field in a Spark Ignition Engine
    Janas, P. and Wlokas, I. and Böhm, B. and Kempf, A.
    Flow, Turbulence and Combustion 98 (2017)
    The development of the turbulent flow field inside a spark ignition engine is examined by large-eddy simulation (LES), from the intake flow to the tumble break-down. Ten consecutive cold flow engine cycles on a coarse and twenty cycles on a fine grid are simulated and compared to experiments of the same engine. The turbulent subgrid scales are modeled by the standard Smagorinsky and by the recently developed Sigma model. A comparison of the intake flow is made against Particle Image Velocimetry (PIV) measurements along horizontal and vertical lines and to an LES simulation performed by the Darmstadt group. Furthermore, we show the first LES comparison to Magnetic Resonance Velocimetry (MRV conducted by Freudenhammer et al.) measurements, which provided the 3D flow field inside a full scale dummy of the entire upper cylinder head including the valve seat region, at a time which mimics inflow conditions of the corresponding engine. Our LES is in good qualitative and quantitative agreement with the simulation and the experiments, with the notable exception of the measured in-cylinder pressure, which is discussed in detail and compared to 0D simulations and simulations from other groups. A criterion is proposed for estimating the number of cycles needed in a simulation, if experimental data is available. We put emphasis on the flow in the valve seat region, where turbulence is generated, and discuss the formation of the large scale tumble motion, including a comparison of the radial velocity fields on rolled-up planes around the valve seat. Here, spots of high velocities were found in the under flow region, which cannot been seen by the ensemble averaged MRV measurement. Within the compression stroke, a 2D vortex center identification algorithm is applied on slices inside the combustion chamber, yielding a 3D visualization of the tumble vortex, which is found to have a “croissant-like” shape. The tumble vortex trajectory is plotted on the symmetry plane and compared to measurements. Finally, we consider a modified definition of the (turbulent) integral length scale that provided further insight to the tumble break-down process. © 2016, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-016-9744-3
  • Large Eddy Simulation of an Internal Combustion Engine Using an Efficient Immersed Boundary Technique
    Nguyen, T.M. and Proch, F. and Wlokas, I. and Kempf, A.M.
    Flow, Turbulence and Combustion 97 (2016)
    This paper presents highly resolved large eddy simulations (LES) of an internal combustion engine (ICE) using an immersed boundary method (IBM), which can describe moving and stationary boundaries in a simple and efficient manner. In this novel approach, the motion of the valves and the piston is modeled by Lagrangian particles, whilst the stationary parts of the engine are described by a computationally efficient IBM. The proposed mesh-free technique of boundary representation is simple for parallelization and suitable for high performance computing (HPC). To demonstrate the method, LES results are presented for the flow and the combustion in an internal combustion engine. The Favre-filtered Navier-Stokes equations are solved for a compressible flow employing a finite volume method on Cartesian grids. Non-reflecting boundary conditions are applied at the intake and the exhaust ports. Combustion is described using a flame surface density (FSD) model with an algebraic reaction rate closure. A simplified engine with a fixed axisymmetric valve (see Appendix A) is employed to show the correctness of the method while avoiding the uncertainties which may be induced by the complex engine geometry. Three test-cases using a real engine geometry are investigated on different grids to evaluate the impact of the cell size and the filter width. The simulation results are compared against the experimental data. A good overall agreement was found between the measurements and the simulation data. The presented method has particular advantages in the efficient generation of the grid, high resolution and low numerical dissipation throughout the domain and an excellent suitability for massively parallel simulations. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10494-015-9683-4
  • 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
  • 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
  • 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
  • Investigation of the sampling nozzle effect on laminar flat flames
    Deng, L. and Kempf, A. and Hasemann, O. and Korobeinichev, O.P. and Wlokas, I.
    Combustion and Flame 162 (2015)
    Sampling probes used for the mass spectrometric sampling of a flame can affect the flame's flow field. Although this effect is already compensated for by heuristic correction functions, state of the art 3-D simulations may permit an even better consideration of this effect. This work has investigated the perturbations induced by sampling probes in burner-stabilized, laminar, flat flames using numerical simulations. Any deviations in the flow and temperature fields from the ideal, one-dimensional flat flame were generated here by a perforated burner plate; they are also examined. Corresponding mass spectrometric measurements were performed in flames of CH4/O2/Ar and H2/O2/N2, burning under atmospheric conditions. In the present study, heat transfer from the flame to the sampling nozzle was studied with a conjugate heat transfer model. Combustion was described using a finite rate chemistry model, employing a detailed reaction mechanism for a H2/O2/N2 flame and a reduced mechanism for a CH4/O2/Ar flame. Compared to the ideal, one-dimensional, and unperturbed flame, the probe was found to affect the measurements of the concentrations of some species by up to 50%. The results highlight the value of supporting numerical simulations of both the flow and combustion for such measurements with invasive probing. © 2014 The Combustion Institute.
    view abstract10.1016/j.combustflame.2014.11.035
  • 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
  • Microwave plasma synthesis of Si/Ge and Si/WSi2 nanoparticles for thermoelectric applications
    Petermann, N. and Schneider, T. and Stötzel, J. and Stein, N. and Weise, C. and Wlokas, I. and Schierning, G. and Wiggers, H.
    Journal of Physics D: Applied Physics 48 (2015)
    The utilization of microwave-based plasma systems enables a contamination-free synthesis of highly specific nanoparticles in the gas phase. A reactor setup allowing stable, long-term operation was developed with the support of computational fluid dynamics. This paper highlights the prospects of gas-phase plasma synthesis to produce specific materials for bulk thermoelectrics. Taking advantage of specific plasma reactor properties such as Coulomb repulsion in combination with gas temperatures considerably higher than 1000 K, spherical and non-aggregated nanoparticles of multiple compositions are accessible. Different strategies towards various nanostructured composites and alloys are discussed. It is shown that, based on doped silicon/germanium alloys and composites, thermoelectric materials with zT values up to almost unity can be synthesized in one step. First experimental results concerning silicon/tungsten silicide thermoelectrics applying the nanoparticle-in-alloy idea are presented indicating that this concept might work. However, it is found that tungsten silicides show a surprising sinter activity more than 1000 K below their melting temperature. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/31/314010
  • Mobility size and mass of nascent soot particles in a benchmark premixed ethylene flame
    Camacho, J. and Liu, C. and Gu, C. and Lin, H. and Huang, Z. and Tang, Q. and You, X. and Saggese, C. and Li, Y. and Jung, H. and Deng, L. and Wlokas, I. and Wang, H.
    Combustion and Flame 162 (2015)
    The burner stabilized stagnation flame technique coupled with micro-orifice probe sampling and mobility sizing has evolved into a useful tool for examining the evolution of the particle size distribution of nascent soot in laminar premixed flames. Several key aspects of this technique are examined through a multi-university collaborative study that involves both experimental measurement and computational modeling. Key issues examined include (a) data reproducibility and facility effects using four burners of different sizes and makers over three different facilities, (b) the mobility diameter and particle mass relationship, and (c) the degree to which the finite orifice flow rate affects the validity of the boundary condition in a pseudo one dimensional stagnation flow flame formulation. The results indicate that different burners across facilities yield nearly identical results after special attention is paid to a range of experimental details, including a proper selection of the sample dilution ratio and quantification of the experimental flame boundary conditions. The mobility size and mass relationship probed by tandem mass and mobility measurement shows that nascent soot with mobility diameter as small as 15 nm can deviate drastically from the spherical shape. Various non-spherical morphology models using a mass density value of 1.5 g/cm3 can reconcile this discrepancy in nascent soot mass. Lastly, two-dimensional axisymmetric simulations of the experimental flame with and without the sample orifice flow reveal several problems of the pseudo one-dimensional stagnation flow flame approximation. The impact of the orifice flow on the flame and soot sampled, although small, is not negligible. Specific suggestions are provided as to how to treat the non-ideality of the experimental setup in experiment and model comparisons. © 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.combustflame.2015.07.018
  • Numerical investigation of the process steps in a spray flame reactor for nanoparticle synthesis
    Weise, C. and Menser, J. and Kaiser, S.A. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 35 (2015)
    The synthesis of titanium dioxide nanoparticles from titanium tetraisopropoxide (TTIP) in a nanoparticle spray flame reactor was investigated. The nanoparticle properties are affected by different processes: (a) the break-up of the liquid jet from the spray nozzle, (b) the combustion of the spray and in the pilot flame and (c) the formation and growth of the nanoparticles. The spray process of the injected liquid was analyzed by volume of fluid (VOF) calculations and validated by shadowgraphy imaging which provided the size distribution and the mean velocity of the droplets. The spray angle was determined by a side illuminated long exposure image of the spray. The resulting spray properties (droplet sizes, velocity, and spray angle) served as injector boundary conditions for the downstream combustion simulations. Spray and gas phase of the flame were simulated using an Euler-Lagrange approach, turbulence was modeled by the RNG k-epsilon model, and turbulent combustion was described as a partially stirred reactor (PaSR). For the formation and growth of the nanoparticles within the synthesis reactor, the population balance equation was solved coupled to the spray combustion using a monodisperse model. The findings from experiment and simulation are discussed in terms of flow, species, temperature, and nanoparticle formation inside the reactor. The effect of the spray droplet properties as droplet size, angle, mean velocity and the dispersion behavior on the nanoparticle synthesis process are investigated and discussed, confirming the observation that this type of spray reactor is a robust design overall. © 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
    view abstract10.1016/j.proci.2014.05.037
  • On the mechanism of nanoparticle formation in a flame doped by iron pentacarbonyl
    Poliak, M. and Fomin, A. and Tsionsky, V. and Cheskis, S. and Wlokas, I. and Rahinov, I.
    Physical Chemistry Chemical Physics 17 (2015)
    In this work we have investigated the mechanism of nanoparticle synthesis in a low pressure, premixed, laminar flat flame of CH4-O2, doped with iron pentacarbonyl using a combined quartz-crystal-microbalance-particle-mass-spectrometry apparatus. We have unambiguously demonstrated that the formation of nanoparticles in iron pentacarbonyl-doped flames occurs very early, in close proximity to the burner surface, prior to the flame front. This early rise of nanoparticle mass concentration is followed by a sharp drop in nanoparticle concentration at the high temperature flame front. This "prompt" nanoparticle generation is consistent with kinetic models describing iron cluster formation. The observation of this phenomenon in a quasi-one-dimensional premixed flat flame strengthens our previous findings and points out that the "prompt" nanoparticle formation is a general phenomenon, not limited to diffusion flames. It presents a challenge and a trigger for further development of the existing mechanisms for gas phase synthesis of iron oxide particles in flames. This journal is © the Owner Societies 2015.
    view abstract10.1039/c4cp04454a
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • discretizations

  • gas phase reactions

  • modelling and simulation

  • nanoparticle synthesis

  • numerical methods

  • transport

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