apl. Prof. Dr. Thomas Dreier

Institute for Combustion and Gas Dynamics – Reactive Fluids
University of Duisburg-Essen

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  • 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
  • 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
  • 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
  • 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
  • Simultaneous measurement of liquid-film thickness and solute concentration of aqueous solutions of two urea derivatives using NIR absorption
    Lubnow, M. and Dreier, T. and Schulz, C. and Endres, T.
    Applied Optics 60 (2021)
    We present a method to simultaneously measure the film thickness and individual concentrations of two urea derivates (urea CH4N2O and dimethylurea C3H8N2O) mixed in an aqueous solution at constant temperature using near-infrared (NIR) absorption at multiple specific wavelengths. Fourier transform infrared (FTIR) spectra of aqueous mixtures of urea and dimethylurea solutions were recorded in the 1250-2500 nm wavelength range in thin-layer quartz cuvettes at room temperature. The spectra reveal suitable detection wavelengths, i.e., 1450, 1933, 2200, and 2270 nm, for which both the absorption coefficient and its variation with the species concentration are large enough to achieve satisfactory detection sensitivity and selectivity. For validation measurements, samples were prepared in thin-layer quartz transmission cells with known path lengths and mixture compositions in the range 100-1000 _m and 0-40 wt.%, respectively. Film thickness and mass fractions of both species were determined from measured absorbance ratios in the determined characteristic wavelength bands. © 2021 Optical Society of America.
    view abstract10.1364/AO.440465
  • 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
  • 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
  • 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
  • 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
  • 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
  • Laser applications to chemical, security, and environmental analysis: Introduction to the feature issue
    Kiefer, J. and Seeger, T. and Dreier, T. and Chen, W. and Stauffer, H. and Meier, W.
    Applied Optics 58 (2019)
    view abstract10.1364/AO.58.00LAC1
  • 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
  • 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
  • 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
  • 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
  • Multi-pulse shadowgraphic RGB illumination and detection for flow tracking
    Menser, J. and Schneider, F. and Dreier, T. and Kaiser, S.A.
    Experiments in Fluids 59 (2018)
    This work demonstrates the application of a multi-color LED and a consumer color camera for visualizing phase boundaries in two-phase flows, in particular for particle tracking velocimetry. The LED emits a sequence of short light pulses, red, green, then blue (RGB), and through its color-filter array, the camera captures all three pulses on a single RGB frame. In a backlit configuration, liquid droplets appear as shadows in each color channel. Color reversal and color cross-talk correction yield a series of three frozen-flow images that can be used for further analysis, e.g., determining the droplet velocity by particle tracking. Three example flows are presented, solid particles suspended in water, the penetrating front of a gasoline direct-injection spray, and the liquid break-up region of an “air-assisted” nozzle. Because of the shadowgraphic arrangement, long path lengths through scattering media lower image contrast, while visualization of phase boundaries with high resolution is a strength of this method. Apart from a pulse-and-delay generator, the overall system cost is very low. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00348-018-2541-0
  • 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
  • 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
  • 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
  • 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 applications to chemical, security, and environmental analysis: Introduction to the feature issue
    Seeger, T. and Dreier, T. and Chen, W. and Kearny, S. and Kulatilaka, W.
    Applied Optics 56 (2017)
    view abstract10.1364/AO.56.00LAC1
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • chemiluminescence

  • laser

  • nanoparticles

  • plasmas

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