Mixture Fraction Profiles Research Articles

Local extinction and near-field structure in piloted turbulent CH4/air jet flames with inhomogeneous inlets

Line-imaged measurements of temperature and major species, based on well-established Raman/Rayleigh/CO-LIF techniques, are used to better understand the scalar structure of piloted CH4/air jet flames with inhomogeneous inlets. Recent studies using a variant of the Sydney piloted burner have demonstrated that the blowoff velocity of a partially-premixed jet flame of CNG or CH4 and air can be increased significantly by tailoring the mixture fraction profile at the burner exit. This is done by adding a small, retractable central tube within the main tube of the burner and separately supplying fuel and air for partial premixing. Both tubes are located within the pilot annulus. When the central tube is retracted far upstream of the burner exit, the flame has the same stability as that of the original burner with homogeneous fuel–air composition at the jet exit. However, when the inner tube supplies fuel and is retracted an optimal distance (10–13 times the main tube diameter) the blowoff velocity is increased by nearly 40%. Previous results indicated that combustion very close to the burner exit occurs in a stratified-premixed mode, augmenting the stabilizing effect of the pilot. This is followed by transition to a diffusion-dominated mode of burning within the first ten main tube diameters.The present paper provides a detailed examination of a series of piloted CH4/air jet flames with different inlet conditions and with a pilot flame that matches the composition and adiabatic flame temperature of CH4/air. It addresses trends in local extinction as well as differences in near-field flame structure. The evolution in the local mode of combustion is traced using instantaneous line-imaged realizations where the change in mixture fraction across a 1000 K interval is used as a conditioning variable. Doubly conditioned means of selected species mass fractions confirm that the flames with inhomogeneous inlet profiles undergo transition from a stratified-premixed mode of combustion to a diffusion-dominated mode of combustion within the first ten jet diameters. Calculations of strained laminar partially-premixed flames are used to gain insight on the effects of strain rate and fuel-side equivalence ratio on flame structure and the rate of heat release. These turbulent jet flames may serve as interesting test cases for models aimed at predicting the performance of practical burners that operate with mixed combustion modes.

Comparison of one-dimensional turbulence and direct numerical simulations of soot formation and transport in a nonpremixed ethylene jet flame

Accurate models of soot formation in turbulent flames are important for correctly predicting and simulating flames and fires. Modeling soot formation and transport is challenging due to the complex chemical formation processes, and differential diffusion of soot relative to a flame. Direct numerical simulations (DNS) have highlighted the importance of such transport on soot concentrations, however DNS is computationally expensive. The one-dimensional turbulence (ODT) model is able resolve a full range of length and timescales and solves the evolution of diffusive and reactive scalars in the natural physical coordinate. We present results of soot formation in ODT and compare the model to simulation results from DNS in a temporally-evolving planar ethylene jet flame where the same transport, thermodynamic, and kinetic models are applied. Good agreement is found for the jet evolution in terms of the mixture fraction profiles. Conditional soot statistics (mean and fluctuations) are presented, along with joint soot-mixture fraction PDFs that illustrate the location and motion of soot in the mixture fraction coordinate. Good qualitative agreement between the models is found and the soot behavior is similar. While the ODT cannot capture three-dimensional flow structures, the ODT simulations are less computationally expensive than the DNS suggesting its use in conjunction with DNS for parametric study, model validation, and investigation at parameter ranges not currently available to DNS.

A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

A modification of the well-known jet piloted burner is introduced to enable the stabilization of partially premixed flames with varying degrees of inhomogeneity in mixture fraction or equivalence ratio. A second tube is added within the pilot annulus which now surrounds two concentric pipes, one carrying fuel and the other air. The central pipe can also be recessed within the annulus upstream of the burner’s exit plane. Two flow configurations are tested: FJ which refers to fuel issuing from the central pipe while air issues from the annulus, and FA where the reverse is true. A key feature of the FJ configuration is that when the central tube is slightly recessed, the fuel partially premixes with air from the annulus inducing inhomogeneity, the extent of which depends on the recession distance.It is found that flame stability is significantly improved due to this inhomogeneity such that, for intermediate recession distances in the range 50–100mm, and for the same air/fuel ratio, the blow-off limits for the FJ cases are more than 50% higher than those of the FA counterparts where fuel is injected in the annulus. Detailed stability limits for both the FJ and FA configurations are presented here along with measurements of velocity and mixing fields at the jet exit plane. Rayleigh scattering is used to image mixture fraction in non-reacting jets while measurements of velocity and turbulence fields are made using standard Laser Doppler Velocimetry. It is shown that, at intermediate recession distances, significant differences in the mean and rms fluctuations of the velocity and mixture fraction profiles exist between the FA and FJ cases. An indicator of stratification, extracted from the mixture fraction images at the exit plane of non-reacting jets, confirms that a high degree of inhomogeneity correlates well with improved flame stability.

Investigation of conditional source-term estimation applied to a non-premixed turbulent flame

A turbulent combustion model, Conditional Source-term Estimation (CSE) is applied to a non-premixed turbulent jet methane flame. The conditional chemical source terms are determined on the basis of first order closure and the conditional averaged species concentrations are obtained by inverting an integral equation. The Tikhonov method is implemented for regularisation. Detailed chemistry is tabulated using the trajectory generated low-dimensional manifold method. Radiation due to the gaseous species is included. Reynolds Averaged Navier–Stokes calculations are performed using two different turbulence models. The objectives of the paper are (i) assessment of the impact of the main numerical parameters in CSE and (ii) comparison of the CSE numerical predictions with available experimental data and results from previous simulations for the selected flame. The number of CSE domains and the number of points in each CSE domain are shown to have a significant impact on the results if not selected appropriately. The present CSE calculations always converge to unique and stable predictions. The corrected k–ε model yields mixture fraction profiles in good agreement with the experimental data values for axial locations in the first half of the flame. Farther downstream, the RNG k–ε model performs better. Overall, the current predictions for the mixture fraction are in good agreement with the experimental data. The predicted temperatures using CSE and the k–ε turbulence model with a modified value of Cε1 = 1.47 are found to be in very good agreement with the experimental data. Further, the current CSE results are of comparable quality with previous simulations using the flamelet model and conditional moment closure. Future work may include further investigation on optimal determination of the regularisation parameter and alternative regularisation techniques, soot modelling within the CSE formulation, and improved formulation of radiation.

Experimental and Numerical Study of the Scalar Turbulent/Non-Turbulent Interface Layer in a Jet Flow

Based on two large-eddy simulations (LES) of a non-reacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/non-turbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial self-similar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.

DIESEL SPRAY CFD SIMULATIONS BASED ON THE Σ-Υ EULERIAN ATOMIZATION MODEL

This work presents an implementation and evaluation of the §-Y atomization model for Diesel spray CFD simulations. The §-Y model is based on an Eulerian representation of the spray atomization and dispersion by means of a single-fluid variable density turbulent flow within a RANS framework. The locally homogeneous flow approach has been applied in order to develop a spray vaporization model based on state relationships. A finite-volume solver for model equations has been created using the OpenFOAM CFD open-source C++ library. Model predictions have been compared to experimental data from free Diesel sprays under nonvaporizing and vaporizing conditions. High-speed imaging, PDPA, and Rayleigh-scattering measurements have been used in order to assess the CFD model. Accurate predictions of liquid and vapor spray penetration, as well as axial velocity and mixture fraction profiles, can be simultaneously achieved for a wide range of injection pressure and ambient conditions, despite only having qualitatively correct predictions of droplet size. The success of these predictions supports the mixing-limited vaporization hypothesis. Model accuracy is better for high ambient density and injection pressure conditions. It is proposed that under low ambient density and injection pressure conditions, interfacial dynamics become more important and the single velocity field assumption is less appropriate.

LES-CMS simulations of a turbulent lifted hydrogen flame in vitiated co-flow

Results from a study of turbulent lifted jet flame with Conditional Moment Closure (CMC) turbulent combustion model are reported. First, a qualitative description of the simulation results is given, by examining the instantaneous resolved species mass fraction fields. The structure of the lifted flame is also assessed by comparing the measured temperature and species mass fraction profiles and the simulation results. The model is able to capture the axial and radial profiles of mixture fraction, temperature and major species. The sensitivity of prediction to boundary conditions is also explored and the lift-off height is found to be very sensitive to the co-flow temperature. Finally, the stabilisation mechanisms, auto-ignition or premixed flame propagation, are addressed. No evidence of premixed flame propagation is found: the diffusion in physical space is negligible for all studied conditions.

Mixing properties of coaxial jets with large velocity ratios and large inverse density ratios

An experimental study was conducted to better understand the mixing properties of coaxial jets as several parameters were systematically varied, including the velocity ratio, density ratio, and the Reynolds number. Diameters of the inner and outer jet were also varied. Coaxial jets are commonly used to mix fluids due to the simplicity of their geometry and the rapid mixing that they provide. A measure of the overall mixing efficiency is the stoichiometric mixing length (Ls), which is the distance along the jet centerline where the two fluids have mixed to some desired concentration, which was selected to be the stoichiometric concentration for H2/O2 and CH4/O2 in this case. For 56 cases, the profiles of mean mixture fraction, rms mixture fraction fluctuations (unmixedness), and Ls were measured using acetone planar laser induced fluorescence diagnostics. Results were compared to three mixing models. The entrainment model of Villermaux and Rehab showed good agreement with the data, indicating that the proper non-dimensional scaling parameter is the momentum flux ratio M. The work extends the existing database of coaxial jet scalar mixing properties because it considers the specific regime of large values of both the velocity ratio and the inverse density ratio, which is the regime in which rocket injectors operate. Also the work focuses on the mixing up to Ls where previous work focused on the mixing up to the end of the inner core. The Reynolds numbers achieved for a number of cases were considerably larger than previous gas mixing studies, which insures that the jet exit boundary conditions are fully turbulent.

High-order Implicit Large Eddy Simulation of gaseous fuel injection and mixing of a bluff body burner

Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling was applied to a turbulent mixing fuel injector flow. In the ILES calculation, the governing equations for three dimensional, non-reactive, multi-species compressible flows were solved using a finite volume Godunov-type method. Up to ninth-order spatial accurate reconstruction methods were examined with a second order explicit Runge–Kutta time integration. Mean and root mean square velocity and mixture fraction profiles showed good agreement with experimental data, which demonstrated that ILES using high-order methods successfully captured complex turbulent flow structure without using an explicit subgrid scale model. The effects of grid resolution and the influence of order of spatial accuracy on the resolution of the kinetic energy spectrum were investigated. An k −5/3 decay of energy could be seen in a certain range and the cut-off wavenumbers increased with grid resolution or order of spatial accuracy. The effective cut-off wavenumbers are shown to be larger than the maximum wavenumbers appearing on the given grid for all test cases, implying that the numerical dissipation represents sufficiently the energy transport between resolved and unresolved eddies. The fifth-order limiter with a 0.6 million grid points was found to be optimal in terms of the resolution of kinetic energy and reasonable computational time.

Mixture fraction imaging in turbulent non-premixed flames with two-photon LIF of krypton

The use of a noble gas as an inert tracer for mixing studies in combustion systems is investigated. Simultaneous two-photon laser-induced fluorescence (LIF) of krypton and Rayleigh scattering are used for imaging measurements of mixture fraction and temperature in turbulent non-premixed jet flames. The turbulent flames investigated in this study include a piloted CH4/air flame (Sandia flame D) and a CH4/H2/N2 flame (DLR-B flame). These flames are well-documented in the literature and enable an evaluation of krypton as a tracer in different fuel mixtures with varying degrees of differential diffusion. Krypton is excited from the ground state to the 5p[3/2]2 state using 215nm laser radiation, and the fluorescence decay to the metastable state, 5s[3/2]2, is detected at 760nm. Single-shot krypton LIF and Rayleigh scattering images are analyzed in an iterative routine to determine mixture fraction and temperature. Measurements of the temperature- and species-dependent quenching rates for Kr-LIF are incorporated into this routine. The resulting average radial profiles of mixture fraction and temperature for both flames agree well with previously published measurements. The use of a noble gas as a chemically inert tracer has potential applications for mixing studies in a broad range of combustion environments.

Large eddy simulation of bluff-body stabilized flames using a multi-environment filtered density function model

A multi-environment turbulent combustion model, originally developed for Reynolds averaged Navier–Stokes equations (RANS), was extended to large eddy simulation (LES) and was used to model a bluff-body stabilized flame series (HM1 and HM3) that ranges from near equilibrium to near global extinction. The multi-environment filtered density function model (MEFDF) depicts the filtered density function (FDF) as a weighted summation of a small number of multi-dimensional Dirac delta functions in composition space. It is derived from the transport FDF equation using the direct quadrature method of moments (DQMOM). The MEFDF method with multiple reactive scalars retains the unique property of the joint FDF model of treating the chemical source term exactly. It was coupled with a 19-species reduced mechanism based on quasi-steady-state assumption and an In Situ adaptive tabulation (ISAT) algorithm for chemical source term integration. The model successfully captures the level of local extinction of the two flames exhibited in experiment. The predicted radial profiles of mixture fraction, temperature and CO mass fraction are in good agreement with measurements. The current work highlights the potential of LES/MEFDF approach as a tool for modeling of turbulent flame stability and combustion dynamics as well as pollutant emissions.

Demonstration of high-speed 1D Raman scattering line imaging

In this manuscript, we demonstrate high-speed (10-kHz acquisition rate) 1D Raman scattering line imaging under turbulent flow conditions. Because of the inherently weak scattering process, Raman scattering measurements typically require high laser pulse energies that are not available from commercial high-repetition-rate laser systems. Using a custom pulse-burst laser system at OSU, we demonstrate the ability to generate a series of high-energy laser pulses with 100 μs pulse separations, which can be used for high-speed Raman scattering imaging. The ultimate goal is a capability of quantitatively measuring all major combustion species and deducing time-varying mixture fraction profiles in turbulent combustion environments. Toward this goal, we report initial results depicting the Raman scattering imaging of O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2 jet issuing into air. To the authors’ knowledge, this represents the first reported temporally-sequential 1D image sequences of major species measured via high-speed Raman scattering.

Large-eddy simulation of a bluff-body-stabilized non-premixed flame using a recursive filter-refinement procedure

A large-eddy simulation (LES) of a bluff-body-stabilized flame has been carried out using a new strategy for LES grid generation. The recursive filter-refinement procedure (RFRP) has been used to generate optimized clustering for variable density combustion simulations. A methane–hydrogen fuel-based bluff-body-stabilized experimental configuration has been simulated using state-of-the-art LES algorithms and subfilter models. The combustion chemistry is described using a precomputed, laminar flamelet model-based look-up table. The GRI-2.11 mechanism is used to build the look-up table parameterized by mixture fraction and scalar dissipation rate. A beta function is used for the subfilter mixture fraction filtered density function (FDF). The simulations show good agreement with experimental data for the velocity field. Time-averaged profiles of major species and temperature are very well reproduced by the simulation. The mixture fraction profiles show excellent agreement at all locations, which helps in understanding the validity of flamelet assumption for this flame. The results indicate that LES computations are able to quantitatively predict the flame structure quite accurately using the laminar flamelet model. Simulations tend to corroborate experimental evidence that local extinction is not significant for this flame.

Steady flamelet modelling of a turbulent non-premixed flame considering scalar dissipation rate fluctuations

A piloted turbulent methane/air jet flame (Sandia Flame D) is calculated by the combination of a multiple-time scale (MTS) k – ε turbulence model, the steady laminar flamlet model (SLFM), and the probability density function (PDF) transport equation approach. A joint PDF transport equation of mixture fraction and turbulent kinetic energy dissipation rate is computed by the node-based Monte Carlo particle method. The fluctuations of scalar dissipation rate at stoichiometric condition are deduced, in an ad hoc manner, from the distribution of turbulent kinetic energy dissipation rate based on their similar PDF distribution, viz. approximately lognormal. The SLFM is employed to decouple the interaction between turbulent fluctuations and chemistry through the instantaneous mixture fraction and the instantaneous stoichiometric scalar dissipation rate. The MTS k – ε turbulence model produces better prediction of the centerline profiles of mean velocity and mixture fraction than the standard k – ε model. Within the current modelling scope, the value of 1.5 for the ratio of mechanical to scalar timescales is better than the usual value of 2.0 since it gives better profiles of the root-means-square (rms) mixture fraction and mean CO, H 2 mass fractions. Compared to the presumed-PDF for scalar dissipation rate, the present ad hoc model improves the predictions of flame structure to some extent, especially the conditional means of CO and H 2 mass fractions in the fuel-rich region of the flame.

LIFTED LAMINAR JET DIFFUSION FLAMES

ABSTRACT This paper addresses the numerical description of lifted flames in axisymmetric laminar coflow jets. The analysis considers moderately large values of the Reynolds number, when the boundary-layer approximation can be used to describe the slender mixing region that extends between the jet exit and the flame, providing the profiles of velocity and mixture fraction that exist immediately upstream from the flame front region. The description of the nonslender flame front region, which provides the front propagation velocity as an eigenvalue, requires integration of the Navier-Stokes Equations with account taken of the reaction and thermal expansion effects. The limiting formulations corresponding to cases of practical interest, including large values of the air-to-fuel stoichiometric ratio, are briefly discussed. Illustrative numerical results are given for flames lifted or propagating at distances small compared with the jet development length, where the mixing layer is nearly planar.

Finite-rate chemistry effects in turbulent opposed flows: comparison of Raman/Rayleigh measurements and Monte Carlo PDF simulations

This study reports results from experimental and numerical investigations of a partially premixed turbulent opposed methane/air jet flame. Experimentally determined properties of the scalar and the flow field are compared to the results from a Monte Carlo simulation. One-dimensional spatially resolved Raman/Rayleigh scattering serves to quantify the mean species concentrations and temperature, whereas laser Doppler velocimetry is used to measure axial and radial velocity components. The simulation is simplified by using a one-dimensional formulation. It includes a Reynolds-stress turbulence model and a Monte Carlo simulation of the joint scalar probability density function (PDF). A non-uniform Monte Carlo particle distribution is used to minimize stochastic errors. The flame is operated close to extinction with strong interactions between turbulence and chemistry. Comparisons between experimental and numerical results reveal a good agreement of mixture fraction profiles along the centreline. However, species scatter plots and mixture fraction PDFs show discrepancies between experiment and simulation. Numerical simulations over-predict the extinction limits and therefore under-predict the intermittent nature of turbulence and mixing of the scalars.

Measurements of Scalar Variance, Scalar Dissipation, and Length Scales in Turbulent Piloted Methane/Air Jet Flames

One-dimensional (line) measurements of mixture fraction, temperature, and scalar dissipation in piloted turbulent partially premixed methane/air jet flames (Sandia flames C, D, and E) are presented. The experimental facility combines line imaging of Raman scattering, Rayleigh scattering, and laser-induced CO fluorescence. Simultaneous single-shot measurements of temperature and the mass fractions of all the major species (N2, O2, CH4, CO2, H2O, CO, and H2) are obtained along 7 mm segments with a nominal spatial resolution of 0.2 mm. Mixture fraction, ξ, is then calculated from the measured mass fractions. Ensembles of instantaneous mixture fraction profiles at several streamwise locations are analyzed to quantify the effect of spatial averaging on the Favre average scalar variance, which is an important term in the modeling of turbulent nonpremixed flames. Results suggest that the fully resolved scalar variance may be estimated by simple extrapolation of spatially filtered measurements. Differentiation of the instantaneous mixture fraction profiles yields the radial contribution to the scalar dissipation, χr= 2Dξ(∂ξ/∂r)2, and radial profiles of the Favre mean and rms scalar dissipation are reported. Scalar length scales, based on autocorrelation of the spatial profiles of ξ and χr, are also reported. These new data on this already well-documented series of flames should be useful in the context of validating models for sub-grid scalar variance and for scalar dissipation in turbulent flames.

Scalar dissipation rate measurements in flames: A method to improve spatial resolution by using nitric oxide PLIF

Scalar dissipation rate (χ) previously has been identified as one of the most important parameters inturbulent non-premixed flames. A new method to image χ is demonstrated which is a variation of the method of Strner, Bilger, Frank, and Long; nitric oxide (NO) is added to the jet fluid to mark the fuel, replacing previous fuel markers such as acetone. Planar laser-induced fluorescence of NO is combined with temperature images from Rayleigh scattering to measure mixture fraction and scalar dissipation rate with greatly improved signal-to-noise ratio (SNR) and spatial resolution. Calibration measurements of mixture fraction (f) and χ in a laminar flame and qualitative images of a turbulent flame are presented. The method has three advantages. (1) Differential diffusion problems (that have limited the use of acetone) are eliminated, since the diffusivity of NO is equivalent to that of the methane/nitrogen fuel mixture. (2) NO is an excellent marker of the fuel (unlike acetone) since it properly disappears from centerline to the flame boundary due to rapid reactions with H and CH radicals. A small correction (typically 5% of the centerline value) must be applied to account for slight leakage of NO through the CH layer, but this correction can be made entirely from the experimental data. (3) NO can be added in large amounts to achieve ecceptional SNR values exceeding 150 when the spatial resolution equals a typical Taylor scale of 400 μm. Radial profiles of mixture fraction and scalar dissipation rate in the calibration flame compare favorably with previous measurements.

Passive scalar measurements in a planar mixing layer by PLIF of acetone

Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re δ up to 104 using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer. This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer. The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re δ ≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal mixing before the transition point.

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.