Turbulent Premixed Flame Propagation Research Articles

A consistent MMC-LES approach for turbulent premixed flames

A multiple mapping conditioning (MMC) approach is coupled with large eddy simulations (LES) for the prediction of the flame propagation, the flame structure and finite rate chemistry effects including NO formation in turbulent premixed flames. The mixing term in MMC relies on a reference field that characterizes the chemical composition within the flame and it is typically taken to be the reaction progress variable. In LES, this reference field needs to be artificially thickened to be resolved on the LES grid. A novel MMC mixing time scale model that had been developed with the aid of direct numerical simulation (DNS), has been adapted to account for the thickening of the reference field. Equally, the particle selection mechanism for particle mixing has been modified to ensure localness of mixing in composition space and to preserve the prediction of a realistic (thin) flame thickness by the stochastic particles. A set of freely propagating turbulent premixed flames with varying equivalence ratios is used to test the performance of the MMC model. Results of MMC-LES are compared with reference DNS that include full chemistry. Additional simulations were carried out where MMC is coupled with quasi-DNS where the flow field is fully resolved but the reaction progress variable field is thickened and its source term is closed with a flamelet approach. This allows to isolate the different modelling assumptions invoked in MMC-LES. Deviations from the exact solution can then be associated with the MMC approach and with thickening of the reference field separately and they are small. The MMC-LES predictions of the turbulent premixed flame propagation speed and flame structure are very good and finite rate chemistry effects can be captured well which is demonstrated by good agreement of predicted NO with DNS data.

Effects of water droplet injection on turbulent premixed flame propagation: a direct numerical simulation investigation

The influence of water droplet injection on the propagation rate of statistically planar stoichiometric n-heptane-air flames has been analysed based on three-dimensional carrier phase Direct Numerical Simulations for different turbulence intensities and different initially mono-sized droplets. It has been found that most water droplets do not completely evaporate within the flame due to their large latent heat of evaporation for the conditions considered here. Thus, the cooling effect due to the extraction of latent heat during the evaporation of water droplets dominates over the dilution of the concentration of reactants and gives rise to smaller reaction rate of reaction progress variable and thicker flame front than in the corresponding premixed turbulent flame without droplets. These effects (a) strengthen with decreasing droplet size due to higher rate of evaporation for smaller droplets, but (b) weaken with an increase in turbulence intensity. The interaction of water droplets with the flame affects the density-weighted displacement speed through its reaction and molecular diffusion components and the magnitudes of these components remain much greater than the components due to cross-scalar dissipation rate and two-phase coupling. The flame-water droplet interaction for the parameter range considered here acts to reduce the mean density-weighted displacement speed, consumption speed and turbulent flame speed, and this reduction becomes increasingly prominent with decreasing droplet diameter. However, it has been found that the presence of water droplets does not alter the qualitative nature of the strain rate and curvature dependences of both density-weighted displacement speed and consumption speed for the range of parameters considered here, but the correlation strength is altered by the presence of water droplets.

Isolating effects of Darrieus–Landau instability on the morphology and propagation of turbulent premixed flames

The objective of this work is to provide physical insight into the mechanisms governing flame–turbulence interactions and explore the impact of the ubiquitous Darrieus–Landau instability on the propagation. It is based on the hydrodynamic theory of premixed flames that considers the flame thickness much smaller than all other fluidynamical length scales. In this asymptotic limit, the flame is thus confined to a surface whilst the diffusion and reaction processes occurring inside the flame zone are accounted for by two parameters: the unburned-to-burned density ratio and the Markstein length. The robust model, which is free of phenomenology and turbulence modelling assumptions, makes transparent the mutual interactions between the flame and the fluid flow, and permits examining trends in flame and flow characteristics while varying the turbulence intensity and mixture properties. It is used in this study to examine the morphological changes of the flame surface that result from the intertwined effects of the turbulence and instability, as demonstrated by the local displacement and curvature of the flame front, the extent of wrinkling and folding of the flame surface, and the overall flame brush thickness. It also provides a direct evaluation of the turbulent flame speed and its dependence on the mean flame curvature and on the hydrodynamic strain that it experiences. Also discussed are the effects of the flame on the flow by examining the various mechanisms of enstrophy and scalar gradient production/destruction, the degree of anisotropy created in the burned gas, and the restructuring of the vortical motion beyond the flame.

Quantifying the role of Darrieus–Landau instability in turbulent premixed flame speed determination at various burner sizes

To probe the impact of Darrieus–Landau (DL) instability on turbulent premixed flame propagation at various burner sizes, methane–air premixed flames from five Bunsen-type burners with different nozzle diameters (4 mm, 6 mm, 8 mm, 10 mm, and 12 mm) were investigated at Reynolds numbers ranging from 1000 to 8500. The flame curvatures used to identify DL instability were determined using Mie scatter images captured by a particle image velocimetry system. The flame speed was further derived by applying an asymmetric hypothesis to the images. The energy-frequency spectrum of the inflow disturbance was determined using a hot-wire anemometry system, and specific wavelet transform analysis was performed to investigate the dependence of DL instability on the proportion of effective disturbances (Ped) and quantify the role of DL instability in determining the turbulent flame speed. The results showed that the burner diameter had an obvious effect on the presence of DL instability and its role in flame propagation. The ability of DL instability to enhance the flame curvature skewness and the turbulent flame speed was closely related to Ped. Ped increased when the burner diameter increased from 6 mm to 12 mm, thus enhancing the DL instability. Changing the burner diameter also affected the interplay between DL instability and turbulence. The above interactions and their effects on the flame speed during the change of inflow disturbances could be formulated by Ped. Finally, a Ped-based correlation was proposed to describe the dependence of the turbulent flame speed on the burner size.

Effect of CO2 on self-acceleration properties of syngas/air turbulent premixed flame at different equivalence ratios

For studying the effect of carbon dioxide (CO2) on the self-accelerating properties of syngas/air mixture's turbulent premixed flame, an experimental study was carried out on 90%H2/10%CO/air mixture at the turbulence intensity of 1.31 m/s. The effects of various equivalence ratios (0.6, 0.8 and 1.0) and carbon dioxide contents (10%, 20% and 30%) on the flame evolution and propagation speed were studied. The effects of the CO2 contents and equivalence ratios on the self-acceleration properties in the flame propagation transition stage were evaluated. The effects of effective Lewis number and Zel'dovich number on the fractal excess and acceleration exponent were analyzed. The results showed that, as the CO2 contents increased, the development process of the flame slowed down, and the rate of growth of the flame's radius and the turbulent premixed flame propagation speed decreased at the same radius. The fractal excess and acceleration exponent increased with the increasing CO2 contents and decreased with the increasing equivalence ratio. Under different equivalence ratios and CO2 contents, the syngas/air premixed flame acceleration exponent was less than 1.5 in the transition stage.

Characterization of Turbulence in an Optically Accessible Fan-Stirred Spherical Combustion Chamber

ABSTRACT Fundamental characteristics of fan-generated turbulence inside a constant-volume combustion chamber (CVCC) with 400 mm inner diameter under high-pressure and high-temperature conditions have been investigated to establish benchmark data for studies of turbulent premixed flame propagation. Here, the CVCC was designed to withstand up to 1.0 MPa initial pressure and 600 K initial temperature and equipped with four fans oriented in a tetrahedron configuration. A time-resolved particle imaging velocimetry (TR-PIV) is employed to characterize the turbulence under various thermodynamic conditions. Additionally, stereoscopic TR-PIV measurements were conducted to characterize the three-dimensional turbulent flow field inside the chamber. The results showed that the chamber could achieve homogenous and isotropic turbulence. A reasonably isotropic stationary turbulence with large turbulence intensities (up to ≈ 4.7 m/s) was achieved in the central region extending to almost 50 mm radius. This has been manifested by near-zero mean velocities and directional invariance of turbulence intensities. Global homogeneity and isotropy ratios in the chamber were close to unity, indicating weak anisotropy. The spectral energy spectra of the generated turbulent flow fields were in satisfactory agreement with the well-known Kolmogorov’s – 5/3 law associated with homogeneous isotropic turbulence irrespective of the intensity of the turbulence. Various spatio-temporal length scales characterizing the turbulent flow field including the integral, Taylor, and Kolmogorov length scales were reported. Turbulence statistics and inherent parameters were investigated at elevated pressure and temperature conditions as well as results from test cases to determine turbulent–flame propagation speeds were reported. Accordingly, the present canonical configuration apparatus can be conveniently adopted for several turbulent combustion studies including mainly the determination of turbulent burning velocity for gaseous and liquid premixed flames in nearly homogeneous isotropic turbulence.

General Correlations of Iso-octane Turbulent Burning Velocities Relevant to Spark Ignition Engines

A better understanding of turbulent premixed flame propagation is the key for improving the efficiency of fuel consumption and reducing the emissions of spark ignition gasoline engines. In this study, we measure turbulent burning velocities (ST) of pre-vaporized iso-octane/air mixtures over wide ranges of the equivalence ratio (φ = 0.9–1.25, Le ≈ 2.94–0.93), the root-mean-square (r.m.s.) turbulent fluctuating velocity (u′ = 0–4.2 m/s), pressure p = 1–5 atm at T = 358 K and p = 0.5–3 atm at T = 373 K, where Le is the effective Lewis number. Results show that at any fixed p, T and u′, Le < 1 flames propagate faster than Le > 1 flames, of which the normalized iso-octane ST/SL data versus u′/SL are very scattering, where SL is the laminar burning velocity. But when the effect of Le is properly considered in some scaling parameters used in previous correlations, these large scattering iso-octane ST/SL data can be collapsed onto single curves by several modified general correlations, regardless of different φ, Le, T, p, and u′, showing self-similar propagation of turbulent spherical flames. The uncertainty analysis of these modified general correlations is also discussed.

Cyclically resolved flame and flow imaging in an alcohol fuelled SI engine

The work was concerned with improving understanding of the interaction of the bulk in-cylinder flow with turbulent premixed flame propagation when using varied fuels including iso-octane, ethanol or butanol. The experiments were performed in a single cylinder research engine equipped with a modern central direct injection combustion chamber and Bowditch style optical piston. Results were obtained under typical part-load engine operating conditions. High speed cross-correlated particle image velocimetry was undertaken at 1500 rpm under motoring conditions with the plenum pressure set to 0.5 bar absolute, with the horizontal imaging plane fixed 10 mm below the combustion chamber “fireface”. Comparisons were made to CFD computations of the flow. Complementary flame images were then obtained via natural light (chemiluminescence) over multiple engine cycles. The flame images revealed the tendency of the flame to migrate towards the hotter exhaust side of the combustion chamber, with no complementary bulk air motion apparent in this area in the imaging plane. In terms of fuel effects, the addition of 16% butanol to iso-octane resulted in marginally faster combustion. Fastest combustion was observed with ethanol, in good agreement with laminar burning velocity correlations within the literature. The ethanol could be seen to offset the tendency of migration of the flame toward the exhaust walls under the fixed spark timing conditions. This exhaust migration phenomenon has been noted previously by others in optical pent-roofed engines but without both flow and flame imaging data being available. The results may imply that the spark plug should ideally be biased further towards the intake side of the chamber if the flame is to approach the intake and exhaust walls at similar times resulting in symmetrical flame propagation, reduced premature wall quenching and hence increase combustion stability and thermal efficiency. Such a layout is typically not preferred due to the priority given to the central fuel injector (and associated cooling jacket) location and maximizing the size of the inlet valves for improved volumetric efficiency.

Pressure effects and transition in the stabilization mechanism of turbulent lifted flames

This study reports novel measurements on the effects of pressure on the lift-off behavior and the stabilization mechanism of turbulent non-premixed methane jet flames. A high-pressure combustion duct (HPCD) was operated within the range of pressure P = 1–7 bar using jet velocities of 4 m.s−1 ≤Uj≤30 m.s−1 and co-flow velocities of 0.23 m.s−1 ≤Uc≤0.60 m.s−1. Lift-off heights were measured from chemiluminescence pictures while joint images of hydroxyl and velocity, performed using joint PLIF-OH/PIV, were used to extract information about the stabilization mechanism. It is shown that while the lift-off height generally increases with pressure, the impact of pressure depends on the magnitude of the co-flow velocity. For Uc = 0.30 m.s−1, the flame's base remains near the nozzle over the entire pressure range and the measured flame speeds indicate that edge-flame stabilization is dominant. The slope of the lift-off height vs. jet velocity curves is positive. For Uc = 0.60 m.s−1 and P > 2 bar, the flame stabilizes further downstream and a transition to turbulent premixed flame propagation appears to have occurred. At these conditions, the slope of the lift-off height vs. jet velocity curves becomes negative. This reversal at high pressure is a new result for methane. More importantly, the transition in the stabilization mechanism with increasing Uc is consistent with results reported earlier for ethylene and appears to be independent of the fuel.

Topology of pocket formation in turbulent premixed flames

In turbulent premixed flame propagation, the formation of isolated pockets of reactants or products is associated with flame pinch–off events which cause rapid changes in the flame surface area. Previous topological analysis of these phenomena has been carried out based on Morse theory and Direct Numerical Simulation (DNS) in two spatial dimensions. The present work extends the topological analysis to three dimensions with emphasis on the formation and subsequent burnout of reactant pockets. Singular behaviour observed previously for terms of the Surface Density Function (SDF) transport equation in the two–dimensional case is shown to occur also in three dimensions. Further singular behaviour is observed in the displacement speed close to reactant pocket burnout. The theory is compared against DNS data from hydrogen–air flames and good agreement is obtained.

Optimized single-step (OSS) chemistry models for the simulation of turbulent premixed flame propagation

A simple and efficient triple-optimization procedure (TOP) is introduced to determine single-step chemistry model parameters. The cornerstone of the proposed approach lies in the introduction of a fictive (or virtual) chemical species, the physical characteristics of which are set to recover some essential properties of combustion and flames. Special emphasis is presently placed on the capability of such a global kinetic scheme to recover three parameters that are recognized as the most influential in terms of turbulence–chemistry interactions and turbulent premixed combustion regimes. These parameters are (i) the burnt gases temperature Tb, which settles the value of the thermal expansion factor τ=(Tb−Tu)/Tu, (ii) the propagation velocity SL0, which is mandatory to reproduce the flame dynamics, and (iii) the laminar premixed flame thickness δL0. In practice, the thermochemical properties of the fictive species (hereafter denoted by AΦ) are set to account for the impact of dissociation effects and partial oxidation of the fuel, i.e., presence of species other than H2O and CO2 in the burnt gases, which allows to recover a satisfactory estimate of Tb. The value of the pre-exponential factor associated to the single-step Arrhenius law is also optimized to reproduce the laminar flame propagation velocity. Finally, the transport characteristics are determined to recover a satisfactory estimate of the thermal flame thickness. The method is general in its principles and quite easy to implement. It is applicable to any couple of fuel and oxidizer. Attention is focused on the application of the method to any stoichiometry but it is also shown that the influence of both pressure and fresh reactants temperature can be recovered. The performance of the resulting optimized single step (OSS) chemistry models are assessed through a direct comparison with detailed chemistry results. Computations of one-dimensional laminar flames are performed with the OSS model using the Cantera software for a wide range of pressure levels, fresh reactant temperatures, and equivalence ratios. Obtained results do show that the flame propagation velocity is correctly reproduced for the whole range of parameters, with a maximum value recovered in the vicinity of stoichiometry, a decrease towards rich conditions, and a satisfactory pressure dependence. Burnt gases temperature as well as thermal flame thickness values are also in excellent agreement with those issued from the reference detailed kinetics models. The OSS model is then used to perform direct numerical simulation (DNS) computations of flame kernel growths in both laminar and turbulent conditions. The comparison of obtained OSS results with detailed chemistry computations further confirms the relevance and performance of the proposed methodology.

LES/PDF Modeling of Turbulent Premixed Flames with Locally Enhanced Mixing by Reaction

Large-eddy simulations (LES) combined with the transported probability density function (PDF) method are carried out for two turbulent piloted premixed methane-air jet flames (flame F1 and flame F3) to assess the capability of LES/PDF for turbulent premixed combustion. The conventionally used model for the sub-filter scale mixing time-scale (or the mixing frequency) fails to capture the premixed flames correctly. This failure is expected to be caused by the lack of the sub-filter scale premixed flame propagation property in the sub-filter scale mixing process when the local flame front is under-resolved. It leads to slower turbulent premixed flame propagation and wider flame front. A new model for specifying the sub-filter scale mixing frequency is developed to account for the effect of sub-filter scale chemical reaction on mixing, based on past development of models for the sub-filter scale scalar dissipation rate in premixed combustion. The new model is assessed in the two turbulent premixed jet flames F1 and F3. Parametric studies are performed to examine the new model and its sensitivity when combined with the different mixing models. Significantly improved performance of the new mixing frequency model is observed to capture the premixed flame propagation reasonably, when compared with the conventional model. The sensitivity of the flame predictions is found be relatively weak to the different mixing models in conjunction with the new mixing frequency model.

Effects of pressure on cellular flame structure of high hydrogen content lean premixed syngas spherical flames: A DNS study

The structure and propagation of turbulent lean premixed high hydrogen content syngas flames at pressure values of 1 bar, 2 bar and 4 bar are studied using three-dimensional direct numerical simulations. Simulations for each pressure considered were performed for three different initial turbulence levels at the turbulent Reynolds numbers of 50, 100 and 150. The DNS of expanding spherical flames has taken into account detailed chemical kinetics and preferential diffusion effects. The mixture-averaged transport model has been employed. Effects of pressure on flame structures, heat release rate and radical species distributions under low to high initial turbulent conditions are examined. The results show that elevated pressures greatly influence the cellular flame structure of syngas spherical flames under different initial turbulent conditions. At the atmospheric pressure, the flame shows weakly wrinkling structures due to the initial turbulence level imposed in the simulation. At elevated pressures under turbulence conditions, the flame develops cellular burning structures, superimposed by flame wrinkles due to turbulence. It is shown that highly wrinkled cells that develop over the surface of a spherical flame increase its area and thereby the global propagation speed at elevated pressures. Results show noticeable increases in the local heat release rate with increasing pressure. The analysis also reveals the formation of cusped structures with low heat release rate values in areas of negative curvatures at elevated pressures, in agreement with previous studies. The joint probability density functions of OH radical mass fraction and temperature show broadening OH values in the high temperature zone at elevated pressures compared to its distribution at the atmospheric pressure. Comparisons of mean species distributions of OH and HO2 over temperature between laminar and turbulent flames show that pressure elevation has a major influence on the flame structure in the composition space.

Leading points and heat release effects in turbulent premixed flames

This paper presents an investigation of the role of leading points in the propagation of turbulent premixed flames. The long time behaviors of turbulent flame speeds, brush thickness, and flame surface density are analyzed using direct numerical simulation (DNS) data for statistically stationary planar flames in homogeneous isotropic turbulence. Results suggest the existence of a regime where the leading points dynamics controls premixed flame propagation in homogeneous isotropic turbulence, which supports the viewpoint that under certain conditions, the processes near the leading edge determine the propagation speed of turbulent premixed flames. It is found that the enhanced apparent propagation speed due to heat release effects contributes to the dominant role of the leading points in the unity Lewis number flames investigated here.

Direct numerical simulation of the turbulent premixed flame propagation with radiation effects

This paper investigates the effects of turbulence on radiative heat transfer in premixed flame propagation using a combination of direct numerical simulation (DNS) and the discrete ordinates method (DOM). The DNS code has been validated and established in previous research, and the newly written DOM code is validated by solving a sample radiative heat transfer problem. The DOM code is explicitly combined with the DNS code using the 3rd-order Runge–Kutta scheme. In the numerical experiments, an initially flat laminar premixed flame interacts with imposed turbulent fluctuations and evolves over time. A remarkable increase in the temperature self-correlation factor <T4>/<T>4 is observed in the middle of the reaction. Higher u′ conditions induced faster and more intensive flame wrinkle evolution. However, flow turbulence reduces the radiative heat loss in planar premixed flame due to the flame curvature effects.

Influence of flame geometry on turbulent premixed flame propagation: a DNS investigation

AbstractThe sensitivity of the turbulent flame speed to the geometry of the flame is investigated using direct numerical simulations of turbulent premixed flames in three canonical configurations: freely propagating statistically planar flames, planar flames stabilized in stagnating flows, and rod-stabilized V-flames. We consider both the consumption speed, which measures the integrated rate of burning, and the propagation speed, which measures the speed of an isosurface within the flame brush. An algebraic model for the propagation speed of the leading edge of the flame brush, which is blind to flame geometry, is also applied to the data for the purposes of establishing its range of validity and the causes of its failure. The turbulent consumption speed is found to be strongly geometry dependent, primarily due to the continuous growth of the flame brush thickness. Changes in the structure and consumption speed of instantaneous flame fronts are found to be only weakly sensitive to flame geometry. The turbulent propagation speed is analysed in terms of its reactive, diffusive and turbulent flux components. All three terms are shown to be significant, both through the flame brush and along the leading edge. The leading-edge propagation speed is found to be sensitive to flame geometry only in the V-flames under certain conditions. It is suggested that this apparent geometry dependence, which the model cannot capture, results from the relation between the turbulence and mean flow time scales in these particular cases, and is not intrinsic to the flame geometry itself.

Measurements and LES calculations of turbulent premixed flame propagation past repeated obstacles

Measurements and Large Eddy Simulations (LES) have been carried out for a turbulent premixed flame propagating past solid obstacles in a laboratory scale combustion chamber. The mixture used is a stoichiometric propane/air mixture, ignited from rest. A wide range of flow configurations are studied. The configurations vary in terms of the number and position of the built-in solid obstructions. The main aim of the present study is two folded. First, to validate a newly developed dynamic flame surface density (DFSD) model over a wide range of flow conditions. Second, to provide repeatable measurements of the flow and combustion in a well-controlled combustion chamber. A total of four groups are derived for qualitative and quantitative comparisons between predicted results and experimental measurements. The concept of groups offers better understanding of the flame–flow interactions and the impact of number and position of the solid baffle plates with respect to the ignition source. Results are presented and discussed for the flame structure, position, speed and accelerations at different times after ignitions. The pressure–time histories are also presented together with the regimes of combustion for all flow configurations during the course of flame propagation.

Influence of Turbulent Scalar Mixing Physics on Premixed Flame Propagation

The influence of reactive scalar mixing physics on turbulent premixed flame propagation is studied, within the framework of turbulent flame speed modelling, by comparing predictive ability of two algebraic flame speed models: one that includes all relevant physics and the other ignoring dilatation effects on reactive scalar mixing. This study is an extension of a previous work analysing and validating the former model. The latter is obtained by neglecting modelling terms that include dilatation effects: a direct effect because of density change across the flame front and an indirect effect due to dilatation on turbulence‐scalar interaction. An analysis of the limiting behaviour shows that neglecting the indirect effect alters the flame speed scaling considerably when is small and the scaling remains unaffected when is large. This is evident from comparisons of the two models with experimental data which show that the quantitative difference between the two models is as high as 66% at but only 4% at . Furthermore, neglecting the direct effect results in a poor prediction of turbulent flame speed for all values of , and both effects are important for practically relevant values of this velocity ratio.

Dynamics of Lean-Premixed Turbulent Combustion at High Turbulence Intensities

Premixed turbulent flames of methane-air stabilized on a Bunsen-type burner were studied to investigate the structure of the flame front at a wide range of turbulence intensities. The nondimensional turbulence rms velocity, rms velocity divided by the laminar flame speed, covered the range from about 3 to 24. The equivalence ratio was varied from 0.6 to stoichiometric. The flame front data were obtained using planar Rayleigh imaging, and particle image velocimetry was used to measure instantaneous velocity field for the experimental conditions studied. Flame front thickness increased slightly with increasing nondimensional turbulence rms velocity. There was no significant difference in flame thickening whether the flame thickness was evaluated at progress variable 0.5, corresponding to the reaction zone, or 0.3, corresponding to the preheat zone. Flame front curvature decreased with increasing turbulence rms velocity. Flame front curvature statistics displayed a Gaussian-like distribution, which centered about zero for all the flame conditions studied during the investigation. Flame surface densities evaluated from flame front images showed almost no dependence on the nondimensional turbulence intensity. Flame surface densities integrated over the flame brush volume also did not show any sensitivity to the nondimensional turbulence rms velocity. This was discussed in the framework of a flame surface density-based turbulent premixed flame propagation closure model. The implication is that the conceptual increase in flame surface density with turbulence may not be the dominant mechanism for flame velocity enhancement in turbulent combustion in the region specified as the flamelet combustion regime by the current turbulent premixed combustion diagrams. Further, the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.

CMC simulations of lifted turbulent jet flame in a vitiated coflow

Lifted turbulent jet diffusion flame is simulated using Conditional Moment Closure (CMC). Specifically, the burner configuration of Cabra et al. [R. Cabra, T. Myhrvold, J.Y. Chen, R.W. Dibble, A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29 (2002) 1881–1887] is chosen to investigate H 2 / N 2 jet flame supported by a vitiated coflow of products of lean H 2/air combustion. A 2D, axisymmetric flow-model fully coupled with the scalar fields, is employed. A detailed chemical kinetic scheme is included, and first order CMC is applied. Simulations are carried out for different jet velocities and coflow temperatures ( T c ). The predicted liftoff generally agrees with experimental data, as well as joint-PDF results. Profiles of mean scalar fluxes in the mixture fraction space, for T c = 1025 and 1080 K reveal that (1) Inside the flame zone, the chemical term balances the molecular diffusion term, and hence the structure is of a diffusion flamelet for both cases. (2) In the pre-flame zone, the structure depends on the coflow temperature: for the 1025 K case, the chemical term being small, the advective term balances the axial turbulent diffusion term. However, for the 1080 K case, the chemical term is large and balances the advective term, the axial turbulent diffusion term being small. It is concluded that, lift-off is controlled (a) by turbulent premixed flame propagation for low coflow temperature while (b) by autoignition for high coflow temperature.