Laminar Flame Calculations Research Articles

Reduction of a detailed reaction mechanism for hydrogen combustion under gas turbine conditions

The aim of this study is to find a reduced mechanism that accurately represents chemical kinetics for lean hydrogen combustion at elevated pressures, as present in a typical gas turbine combustor. Calculations of autoignition, extinction, and laminar premixed flames are used to identify the most relevant species and reactions and to compare the results of several reduced mechanisms with those of a detailed reaction mechanism. The investigations show that the species OH and H are generally the radicals with the highest concentrations, followed by the O radical. However, the accumulation of the radical pool in autoignition is dominated by HO 2 for temperatures above, and by H 2O 2 below the crossover temperature. The influence of H 2O 2 reactions is negligible for laminar flames and extinction, but becomes significant for autoignition. At least 11 elementary reactions are necessary for a satisfactory prediction of the processes of ignition, extinction, and laminar flame propagation under gas turbine conditions. A 4-step reduced mechanism using steady-state approximations for HO 2 and H 2O 2 yields good results for laminar flame speed and extinction limits, but fails to predict ignition delay at low temperatures. A further reduction to three steps using a steady-state approximation for O leads to significant errors in the prediction of the laminar flame speed and extinction limit.

Comparison of direct numerical simulation of lean premixed methane–air flames with strained laminar flame calculations

Direct numerical simulation (DNS) with complex chemistry was used to study statistics of displacement and consumption speeds in turbulent lean premixed methane–air flames. The main focus of the study is an evaluation of the extent to which a turbulent flame in the thin reaction zones regime can be described by an ensemble of strained laminar flames. Conditional averages with respect to strain for displacement and consumption speeds are presented over a wide range of strain typically encountered in a turbulent flame, compared with previous studies that either made local pointwise comparisons or conditioned the data on small strain and curvature. The conditional averages for positive strains are compared with calculated data from two different canonical strained laminar configurations to determine which is the optimal representation of a laminar flame structure embedded in a turbulent flame: the reactant-to-product (R-to-P) configuration or the symmetric twin flame configuration. Displacement speed statistics are compared for the progress-variable isosurface of maximum reaction rate and an isosurface toward the fresh gases, which are relevant for both modeling and interpretation of experiment results. Displacement speeds in the inner reaction layer are found to agree very well with the laminar R-to-P calculations over a wide range of strain for higher Damköhler number conditions, well beyond the regime in which agreement was expected. For lower Damköhler numbers, a reduced response to strain is observed, consistent with previous studies and theoretical expectations. Compared with the inner layer, broader and shifted probability density functions (PDFs) of displacement speed were observed in the fresh gases, and the agreement with the R-to-P calculations deteriorated. Consumption speeds show a poorer agreement with strained laminar calculations, which is attributed to multidimensional effects and a more attenuated unsteady response to strain fluctuations; however, they also show less departure from the unstrained laminar value, suggesting that detailed modeling of this quantity may not be critical for the conditions considered. For all quantities investigated, including CO production, the R-to-P laminar configuration provides an improved description relative to the twin flame configuration, which predicts qualitatively incorrect trends and overestimates extinction.

Three-dimensional temperature gradients in premixed turbulent flamelets via crossed-plane Rayleigh imaging

A new technique for obtaining instantaneous, high-resolution, three-dimensional thermal structure data from turbulent flames, crossed-plane Rayleigh imaging is described and then demonstrated. Quantitative Rayleigh imaging measurements are made simultaneously in two orthogonal, intersecting laser-sheet illumination planes. At points along the line of intersection of the two laser sheets, instantaneous, three-dimensional temperature gradient data are measured. The technique has higher resolution than parallel plane measurement techniques, which have limited resolution in the direction orthogonal to the parallel planes. The technique is used to measure temperature gradient data for a lean, premixed, methane–air turbulent V-flame with an equivalence ratio of Φ = 0.7 , and normalized turbulence intensity ( u ′ / S L 0 ) = 1 , where u ′ is the turbulence intensity and S L 0 is the unstretched laminar flame speed. Measurements are also presented for a laminar V-flame and a laminar Bunsen flame for comparison. Finally, an unstretched laminar flame calculation is made. Quantitative estimates of the experimental uncertainty are presented. The primary source of uncertainty in the data is due to shot noise. Measured temperature gradient data for laminar flames differ from that of the unstretched laminar flame calculation, especially in the oxidation layer. Turbulent flame temperature gradient data indicate that the turbulent V-flame thermal structure is not significantly perturbed from the measured laminar V-flame structure. For the flame studied, the flamelet approximation is valid if the flamelet used is based on the measured laminar flame structure of the V-flame. Isothermal surface orientation data are presented and are close to parallel for most realizations. Isothermal surface density is calculated from the distribution of isothermal surface orientations and from conditional averages of the magnitude of the temperature gradient. Isothermal surface density does not vary for different isothermal surfaces.

Experimental analysis of local flame extinction in a turbulent jet diffusion flame by high repetition 2-D laser techniques and multi-scalar measurements

In this paper, we present a detailed experimental study of turbulence chemistry interactions in the “DLR_B” turbulent jet diffusion flame. The flame operates on mixtures of CH 4, H 2, and N 2 in the fuel stream at Re = 22,800 and is a target flame within the TNF workshop. Extinction and re-ignition events can be tracked in real time and related to the underlying flow field phenomena and temperature fields. Time resolved measurements of OH radical concentration fields are performed in combination with temperature and velocity field measurements. For this purpose, we combined high repetition rate (33 kHz) PLIF imaging with stereoscopic PIV and double pulse Rayleigh imaging techniques. Comparisons are made with results from multi-scalar Raman/Rayleigh/LIF point measurements that reveal the thermochemical state of the flame. The large deviations from equilibrium observed on resulting OH/temperature joint pdfs could be related to strain rate and Damköhler number variations caused by turbulent flow structures leading to frequent extinctions. The 2D measurement series uniquely reveal the underlying mechanism that can lead to such events. Finally, comparisons are made to strained laminar flame calculations, which are generally found to be in good agreement with the measured data.

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling ☆

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.

Premixed flame front structure in intense turbulence

Six methane/air flames of equivalence ratio 0.7, Ka =1–17 stabilized on a intense turbulence low-swirlburner of were investigated by Rayleigh laser sheet measurements of the gas density within the preheat zone (approximately 400–1000 K). Lean premixed turbulent flames with Karlovitz numbers Ka , greater than unity are becoming of increasing practical importance but only limited detailed experimental data are available. This regime is often called the distributed reaction zone and a new approach to analyzing flame front broadening was developed to address issues of flame three dimensionality. Instantaneous progress variable, c , contour sets constitute the basic data and a statistical analysis of three aspects of this data are presented: (1) c contour spacing (2) flame front curvature, and (3) flame surface density. A detailed investigation revealed little change of these parameters with turbulence, even at the highest Ka numbers: the contour spacings were similar to those derived from laminar flame calculations and neither the flame front curvature nor the flame surface density varied significantly with c . Geometric effects of turbulence on flame front structure still predominate at these turbulence levels. The primary effect of increasing turbulence is increasingly to convolute the flame front and so increase the flame surface density, reduce the scalar length scales, and increase the burning rate. This finding is also reflected in the significant broadening of the flame front curvature distributions with increasing u′/S L . The data presented here indicate that the diagrams often used to delimit the various regimes of premixed turbulent combustion and hence guide modeling are based on parameters which need to be carefully investigated to determine their appropriateness. The results are consistent with an interpretation of combustion/turbulence interactions which sees the reaction zone thickness as the significant scalar length scale.

Modeling soot formation in turbulent methane–air jet diffusion flames

The modeling of soot formation and oxidation by the conditional moment closure (CMC) method is considered. It is particularly focused on the influence of differential diffusion of the soot particles on soot predictions. Most importantly, no changes are made to the soot models that were derived from laminar flame experiments and calculations. Good to excellent predictions are achieved in lightly sooting turbulent methane–air jet diffusion flames at atmospheric and elevated pressure when differential diffusion is taken into account. Unity Lewis number assumptions yield underpredictions of soot volume fractions by about 40%. Soot oxidation by OH and O 2 can be treated accurately and both oxidation mechanisms are found to be important for soot burnout in downstream regions. © 2000 by The Combustion Institute

Experiments on the scalar structure of turbulent CO/H 2/N 2 jet flames

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H 2/N 2 (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneous measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separately. This paper focuses on the scalar results but includes some limited velocity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise distance is scaled by nozzle diameter. However, OH mole fractions are lower and NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residence times. Turbulent flame measurements are compared with steady strained laminar flame calculations. Laminar calculations show remarkably close agreement with measured conditional means of the major species when all diffusivities are set equal to the thermal diffusivity. In contrast, laminar flame calculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in determining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differential diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometric simplicity of the unpiloted jet flame in coflow, while including a chemical kinetic system of intermediate complexity between hydrogen flames and the simplest hydrocarbon flames. Aspects of the measurements, including Favre-averaged profiles, conditional statistics, mixture fraction pdf’s, and departures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The complete data are available on the World Wide Web for use in model validation studies.

The inevitability of engine-out NO x emissions from spark-ignited and diesel engines

Spark-ignition and diesel engine combustion experiments were carried out for fuels including natural gas, propane, and indolene, showing that flame extinction and high unburned hydrocarbon emissions result when in-cylinder combustion temperatures fall below a critical threshold value of approximately 1920 K. Laminar flame calculations were then carried out for methane and propane to relate these experimental limiting temperatures to those associated with lean-limit conditions for flame propagation. Model laminar flame results for pressures above 50 atm produced a lean limit at an equivalence ratio of 0.6, where the adiabatic flame temperature was about 1900 K. These results show that at the elevated pressures of engine combustion, the lean-limit product temperature for these hydrocarbon fuels is considerably higher than that obtained previously for lean-limit flames at atmospheric pressure and that these much higher temperatures will produce significant amounts of NO x . These compled studies indicate that under conditions of pressure and temperature in both spark-ignition and diesel internal combustion engines, high product gas temperatures are required and relatively high NO x producton rates are the inevitable result. Thus, it may be impossible to reduce NO x production to very low levels if flame propagation at high pressures is involved, and it will be necessary to provide postcombustion aftetreatment to reduce NO x emissions to legislated levels.

Fuel-dilution effect on differential molecular diffusion in laminar hydrogen diffusion flames

Laminar flame calculations have been made for a Tsuji counterflow geometry to investigate salient features caused by the differential diffusion effect in nitrogen-diluted hydrogen diffusion flames. A strong dependence of the differential diffusion parameter zH on fuel dilution is found, where zH is the difference of the mixture fractions based on H and O elements. The strain rate, however, appears to have a relatively minor impact on zH. A simplified transport equation for the zH parameter has been derived to explain qualitatively the behaviours exhibited in the numerical solutions. Two source terms of zH are identified in the transport equation; one is due to mixing among species of different diffusion coefficients and the other one is associated with chemical reactions of H2. More importantly, the second source term is found to be dominant in reacting flows, and it increases with inert gas dilution. This feature causes the differential diffusion parameter to increase with the amount of fuel dilution. The zH values at the stoichiometric position are shown to correlate well with the ratio, YH2O|max/(ZH,1−ZH,2), which may be useful for quantifying the influence of chemical reactions on the differential diffusion effect. For flames at low strain rates, the scalar dissipation rate exhibits a local minimum near the stoichiometric position. This peculiar feature is found to be caused by the differential diffusion effect modulated by chemical reactions. The local minimum in the scalar dissipation rate disappears at high strain rates when the convective transport overwhelms the molecular diffusion.

Automatically simplified chemical kinetics and molecular transport and its applications in premixed and non-premixed laminar flame calculations

Among many other methods for simplifying chemical kinetics for laminar and turbulent flame calculations, the method of intrinsic low-dimensional manifolds (ILDM) has shown to be an efficient tool for the development of reduced kinetic schemes. Based on a numerical analysis, it identifies and decouples the fast relaxing timescales of the chemical system. The results, for example, the thermokinetic state of the system or the reaction rates, are then stored in terms of a small number of parameters (mixture fraction, reaction progress variables) for subsequent use in reacting flow calculations. Furthermore, together with the reduced mechanism, information about the coupling of the chemical kinetics with the physical processes (molecular transport, turbulent mixing) is obtained. In this paper, we present a method that allows an efficient implementation of the ILDM method in flame calculations and overcomes several problems that had been discussed in previous work. It is based on three ingredients: A robust numerical method to calculate the ILDM, a storage scheme that allows an easy implementation in CFD codes, and a model for the coupling of the chemical kinetics with transport processes. In this way, not only the chemistry can be calculated beforehand but also a reduced set of diffusion coefficients. The method is verified by simulations of laminar syngas-air flames with an emphasis on the non-premixed case.

Three-scalar imaging in turbulent non-premixed flames of methane

Raman scattering from nitrogen in laminar (Re=1600) and turbulent (Re=15,000) non-premixed flames of diluted methane has been investigated as a means of complementing the two-scalar mixture-fraction computation based on measurements of fuel mass fraction and temperature (ξ} FT ). Using a diluent consisting of argon and oxygen with an overall volumetric dilution ratio of 3/1 (diluent/fuel) allows sufficient variation in the measured nitrogen concentration for its use as a passive scalar. The present experimental setup requires only a single laser (532 nm) in a high-power intracavity configuration. Mixture-fraction profiles calculated using independent fuel-temperature and nitrogen-temperature two-scalar approaches show excellent agreement in the laminar flame. For the turbulent flame, a 100 single-shot average of mixture fraction shows reasonable agreement between the two approaches. Discrepancies are most evident in single-shot images in regions of large mixture fraction (ξ>ξ s ) where the nitrogen Raman signal is noise dominated. The location of the stoichiometric contour is consistently determined by both two-scalar approaches. A simple modification of the functional dependence for parameters appearing in the fuel-temperature mixture-fraction formulation is shown to correct deficiencies in the approach predicted by laminar flame calculations, most notably for values of ξ FT around stoichiometric and into lean regimes.

Mechanism of nitric oxide formation in oxygen-natural gas combustion

This study is an investigation, both experimental and numerical, on the formation mechanisms of nitric oxide in argon-diluted oxygen-natural gas diffusion flames. Turbulent flames were investigated experimentally with argon or helium dilution to vary the adiabatic flame temperature from 2100 to 3000 K. Species concentrations are measured using a standard gas-sampling technique. Results showed an increase in nitric oxide concentration with decreasing dilution, both inside and outside the flame zone. This is attributable to the adiabatic flame temperature increase. Furthermore, a marked transition in the evolution of the NO content occurs for a dilution of about 50%: Above this level, NO varies slowly with dilution, whereas below this level, NO increases rapidly with decreasing dilution. This feature is the evidence of a transition in the NO formation mechanism within the flame from prompt and N 2 O-path NO at high dilution (low adiabatic flame temperatures) to thermal NO at low dilution (high adiabatic flame temperature). Numerical calculations of counterflow diffusion laminar flames using a detailed kinetic mechanism for methane combustion and nitric oxide formation confirm the experimental results: At low dilution (high temperatures), O+N 2 =NO+O is the main N 2 -consuming step, and at high dilution (low temperatures), CH+N 2 =HCN+N and O+N 2 +M=N 2 O+M dominate. Differences due to the type of diluent (argon versus helium) are identified and are explained on the basis of differences in thermal conductivities.

Planar laser-induced fluorescence imaging of flame heat release rate

Local heat release rate represents one of the most interesting experimental observables in the study of unsteady reacting flows. The direct measure of burning or heat release rate as a field variable is not possible. Numerous experimental investigations have relied on inferring this type of information as well as flame-front topology from indirect measures that are presumed to be correlated. A recent study has brought into question many of the commonly used flame-front marker and burning-rate diagnostics. This same study found that the concentration of formyl radical offers the best possibility for measuring flame burning rate. However, primarily due to low concentrations, the fluorescence signal level from formyl is too weak to employ this diagnostic for single-pulse measurements of turbulent-reacting flows. In this paper, we describe and demonstrate a new fluorescence-based reaction-front imaging diagnostic suitable for single-shot applications. The measurement is based on taking the pixel-by-pixel product of OH and CH 2 O planar laser-induced fluorescence (PLIF) images to yield an image closely related to a reaction rate. The spectroscopic and collisional processes affecting the measured signals are discussed, and the foundation of the diagnostic, as based on laminar and unsteady flame calculations, is presented. We report the results of applying this diagnostic to the study of a laminar premixed flame subject to an interaction with an isolated line-vortex pair.

Experimental and numerical study of a highly diluted turbulent diffusion flame close to blowout

The nature of stability, finite rate chemistry, and differential diffusion inherent in a highly diluted turbulent non-premixed hydrogen flame close to blowout is investigated. Major-species concentrations and temperature are measured by Raman/Rayleigh spectroscopy. OH concentration is simultaneously measured by laser induced fluorescence (LIF), and laser Doppler velocimetry (LDV) is used to determine velocities. The experimental data are used to assess the performance of a probability density function (PDF) simulation with a five-step reduced chemistry mechanism and differential diffusion effects. Comparison is made in terms of averaged quantities, scatter plots, and conditional averaged data. First- and second-order statistics of predictions and experimental data are found in reasonably good agreement. The significant amount of finite rate chemistry observed in the flame as well as the relaxation to equilibrium with downstream distance is well reproduced by the PDF calculation. PDF model predictions of differential diffusion compare favorable with experimentally determined values. Numerical results reveal that differential diffusion may have a major impact on the flame stability, although the average deviation from the equal diffusivity limit is small. Computed laminar extinction limits of laminar flame calculations with multicomponent diffusion and equal diffusivity are in agreement with this observation.

Simultaneous rayleigh, raman, and LIF measurements in turbulent premixed methane-air flames

Instantaneous measurements of temperature, major species, OH, CO, and NO are performed in turbulent premixed flames using simultaneous Rayleigh scattering, Raman scattering, and laser-induced fluorescence (LIF). Temperature is determined from Rayleigh scattering, and concentrations of CH 4 , O 2 , N 2 , H 2 O, CO 2 , and H 2 are obtained from Raman scattering. Linear LIF is used to measure OH and NO, while two-photon LIF is used to determine CO concentrations. The two-photon CO-LIF system provides significant improvements over CO-Raman measurements. This combination of diagnostic techniques is used to investigate the detailed compositional structure of turbulent lean premixed methane-air flames. Three turbulent flames having different equivalence ratios are considered. The ratio of rms velocity to laminar flame speed ranges from approximately 6 to 17. The correlation of major species, OH, and CO concentrations with temperature are similar to those predicted by one-dimensional laminar flame calculations. However, NO concentrations in the leaner flames are higher than the predicted values.

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.

Calculations of the effect of nitrogen vibrational kinetics on laminar flame temperature profiles

Calculations of several premixed and nonpremixed laminar flames have been performed using two chemical kinetics mechanisms: (1) a vibrational kinetics mechanism treating nitrogen as a number of distinct species corresponding to different vibrational energy levels, with reactions representing transitions between energy levels; and (2) a traditional mechanism that treats nitrogen as being in vibrational equilibrium. In the vibrational kinetics calculations, translational/vibrational and vibrational/vibrational energy transfers are included, as is the effect of collisions with CO 2 and H 2O on N 2 vibrational excitation. Vibrational temperatures are calculated from the populations of various N 2 species. For a stoichiometric, atmospheric-pressure premixed methane/air flame, the vibrational temperature is 40 K lower than the rotational/translational temperature in the region of high temperature gradient. The lag in filling upper vibrational energy levels of nitrogen also results in a lower effective heat capacity for the mixture in the vibrational kinetics case than in the vibrational equilibrium case. Rotational/translational temperatures exceed those calculated with the traditional mechanism by as much as 15 K in the region of steep temperature gradient. For diffusion flames over the range of strain rates investigated here, the effect of vibrational kinetics is much smaller. Sensitivity analysis indicates that, among the vibrational kinetics reactions, the initial vibrational excitation of N 2 by CO 2 and H 2O has the greatest impact on the temperature results.

Simultaneous Imaging of Temperature and OH Number Density in Turbulent Diffusion Flames

A single laser source and a single intensified CCD camera have been used to image temperature and the hydroxyl radical, OH in turbulent nonpremixed flames. Images are simultaneous and quantitative. Temperature is derived from Rayleigh scattering. Laser Induced Fluorescence (LlF) is used to image OH. Correction of the LIF image for quenching effects is done using a combination of laminar flame calculations and the temperature measurements. Images for flames of the following fuels are presented: CH3OH, C2H5OH, CNG (91% CH4 by volume) and CNG-H2(31% CNG, 69% H2 by volume). The OH profile is used as a measure of reaction zone thickness. Number densities of OH are found to be in superflamelet levels for all flames investigated here. Temperatures agree well with laminar flame computations. Flames show evidence of partial or full premixing of the fluid in the reignition zone at high velocities downstream of regions of high local extinction. The OH profiles are seen to increase in width and this is consistant with an increase in calculated reaction zone width and an increase in stoichiometric mixture fraction. An increase in Reynolds number alone does not change the OH profile thickness.