Reacting Shear Layer Research Articles

Mode shape-dependent thermoacoustic interactions between a lean-premixed primary flame and an axially-staged transverse reacting jet

Axial fuel staging enables elevation of the turbine inlet temperature in advanced heavy-duty gas turbine combustors to ∼2000 K, ultimately to achieve more than 65% combined cycle efficiency, while limiting excessive nitrogen oxide production. While previous investigations have identified transverse jet-flow-dependent nitrogen oxides emission trends and the topological properties of reacting shear layers in vitiated crossflow environments, there exists a critical gap in the understanding of axial-fuel-staging-induced combustion instabilities. The present study therefore investigates the link between first- and second-stage flame dynamics in a lean-premixed combustor. We demonstrate that the second-stage jet-in-vitiated-crossflow flame is preferentially coupled to higher order acoustic modes than the upstream primary flame with the same characteristic nozzle dimension. The first- and second-stage-driven acoustic modes (two different timescales) are mutually incompatible, and the ensuing selective excitation is critically dependent on the position of the secondary injector relative to the mode shape. When the second-stage flame is located near a pressure node, or a velocity antinode, the transverse jet-stabilized flame is influenced by the primary flame-driven crossflow velocity oscillations, and exhibits conspicuous flapping motions in the crossflow direction accompanied by periodic partial extinction at the windward and leeward-side flame elements. If the second-stage flame is situated near a pressure antinode, on the other hand, the secondary jet flame's dynamics are governed by a combination of jet merging-induced flame surface destruction and the growth of non-axisymmetric coherent structures. In this case, high-intensity pressure oscillations are sustained by the local heat release fluctuations generated solely from the transverse reacting jets, whereas the upstream primary flame remains nearly unperturbed, or decoupled from the underlying feedback processes. This experimental observation is remarkable, and reflects previously unknown properties originating from the complex interactions between the two axially-staged lean-premixed flames.

Experimental investigation of OH*/CH* ratio variations in turbulent, disk stabilized, lean propane-air flames with inlet mixture preheat and stratification

An experimental investigation of OH* and CH* chemiluminescence emissions from bluff-body stabilized, stratified, lean propane-air flames, operated under elevated inlet temperatures has been performed in this work. Simultaneous band-pass filtered emissions, at inlet mixture temperatures ranging from 300 to 573 K, have been acquired to evaluate the OH*/CH* ratio in an effort to identify a possible correlation between the chemiluminescence ratio and the equivalence ratio, for these flame configurations. Moreover, the impact of preheat on the disposition of the radially stratified equivalence ratio gradient, issuing from an upstream premixing section has been investigated. The employed premixer/burner set up consists of three concentric disks that form two consecutive premixing cavities and lead to a radially stratified equivalence ratio gradient, characterized by its peak profile value, at the inlet to the burner zone. Band-passed filtered chemiluminescence measurements and Fourier Transform Infrared Spectroscopy (FTIR) analysis have been performed to assess the OH*/CH* ratio and the mixing fields supporting the flames. The results revealed an altered premixedness behavior of the employed arrangement with elevated inlet preheat, when compared to a non-preheated case. Additionally, both maximum and spatially averaged OH*/CH* ratio values have been studied in an effort to assess the equivalence ratio by using line of sight chemiluminscence measurements. The maximum chemiluminscence ratio values have been proven to better compensate for the spatial inhomogenieties encountered in the turbulent, stratified set ups studied, thus better estimating the effective equivalence ratio value that is burned in the reacting shear layer, closely corroborating counterpart laminar flame investigations.

Fast reactive flow simulations using analytical Jacobian and dynamic load balancing in OpenFOAM

Detailed chemistry-based computational fluid dynamics (CFD) simulations are computationally expensive due to the solution of the underlying chemical kinetics system of ordinary differential equations (ODEs). Here, we introduce a novel open-source library aiming at speeding up such reactive flow simulations using OpenFOAM, an open-source software for CFD. First, our dynamic load balancing model by Tekgül et al. [“DLBFoam: An open-source dynamic load balancing model for fast reacting flow simulations in OpenFOAM,” Comput. Phys. Commun. 267, 108073 (2021)] is utilized to mitigate the computational imbalance due to chemistry solution in multiprocessor reactive flow simulations. Then, the individual (cell-based) chemistry solutions are optimized by implementing an analytical Jacobian formulation using the open-source library pyJac, and by increasing the efficiency of the ODE solvers by utilizing the standard linear algebra package. We demonstrate the speed-up capabilities of this new library on various combustion problems. These test problems include a two-dimensional (2D) turbulent reacting shear layer and three-dimensional (3D) stratified combustion to highlight the favorable scaling aspects of the library on ignition and flame front initiation setups for dual-fuel combustion. Furthermore, two fundamental 3D demonstrations are provided on non-premixed and partially premixed flames, viz., the Engine Combustion Network Spray A and the Sandia flame D experimental configurations, which were previously considered unfeasible using OpenFOAM. The novel model offers up to two orders of magnitude speed-up for most of the investigated cases. The openly shared code along with the test case setups represent a radically new enabler for reactive flow simulations in the OpenFOAM framework.

Mixing and combustion at low heat release in large eddy simulations of a reacting shear layer

In this paper, the mixing and combustion at low-heat release in a turbulent mixing layer are studied numerically using large eddy simulation. The primary aim of this paper is to successfully replicate the flow physics observed in experiments of low-heat release reacting mixing layers, where a duty cycle of hot structures and cool braid regions was observed. The nature of the imposed inflow condition shows a dramatic influence on the mechanisms governing entrainment, and mixing, in the shear layer. An inflow condition perturbed by Gaussian white noise produces a shear layer which entrains fluid through a nibbling mechanism, which has a marching scalar probability density function where the most probable scalar value varies across the layer, and where the mean-temperature rise is substantially over-predicted. A more sophisticated inflow condition produced by a recycling and rescaling method results in a shear layer which entrains fluid through an engulfment mechanism, which has a non-marching scalar probability density function where a preferred scalar concentration is present across the thickness of the layer, and where the mean-temperature rise is predicted to a good degree of accuracy. The latter simulation type replicates all of the flow physics observed in the experiment. Extensive testing of subgrid-scale models, and simple combustion models, shows that the WALE model coupled with the Steady Laminar Flamelet model produces reliable predictions of mixing layer diffusion flames undergoing with fast chemistry.

Experimental study on the rocket-ejector system with a throat in the secondary stream

This study concentrated on hot-fire tests to demonstrate the flow characteristics that influenced by the throttle of the secondary stream in the rocket-ejector system. The experiment was conducted under the sea level static (SLS) condition. A small rocket used gaseous oxygen / liquid kerosene propellant as a primary stream supply system, and the high-pressure air bottles were employed as the secondary stream supply system. The equivalence ratio of the rocket, the air mass flow rate and the mixing length were varied to study the performance. The results show that there is a mass flow rate mismatch between the air flow entering through the inlet and the mass flow permitted due to aerodynamic choking. If the air mass flow rate through the throat is more, the inlet may unstart. If the air mass flow rate through the throat is less, the total pressure loss of the air due to the pseudo-shock would be caused under the effect of the suction performance. The unsuitable air mass flow rate is detrimental to RBCC engine operation in the ejector mode. With the increasing equivalence ratio of the rocket, the pressurization performance is improved accordingly, but the suction performance is obviously decreased. When the equivalence ratio is more than 1.1, the reacting shear layer would affect the suction performance.

Large eddy simulations of premixed CH$$_{{\mathbf {4}}}$$ bluff-body flames operating close to the lean limit using quasi-global chemistry and an algebraic chemiluminescence model

The present work describes the numerical study of unconfined turbulent premixed methane/air flames stabilized on an axisymmetric conical baffle under lean and ultra-lean, close to blow-off conditions. A finite-volume-based LES method, using the dynamic Smagorinsky subgrid model in combination with two turbulent combustion methodologies, the thickened flame model and the implicit LES (ILES) laminar reaction rate approach were employed in the investigation. Methane–air oxidation was modeled with a 14-species reduced scheme. OH* chemiluminescent species levels were also evaluated by post-processing quasi-steady-state-derived algebraic expressions, exploiting directly simulated species thus enabling direct comparisons with experimental images. The quality of the simulations was appraised against experimental data for velocity, turbulence, temperature, species mass fractions, heat release profiles as well as chemiluminescence images for conditions far and close to blow-off. Both turbulent combustion models followed closely several intrinsic trends and variations of the flame front anchoring and disposition close to the burner rim shear layers and along the reverse flow region as the fuel level was reduced toward the lean limit. The interaction of the bluff-body recirculation with the adjacent toroidal reacting shear layer and the impact of combustion and heat release on the development of the turbulent velocity and species fields in the near-wake recovery zone were adequately reproduced for both mixtures. Quantitative deviations between simulations and measurements, regarding heat release and OH species, increased for the near-LBO flame with an attendant extrapolated underprediction of the blow-off event by about 8% in terms of equivalence ratio.

A coupled LES-ODT model for spatially-developing turbulent reacting shear layers

Large-eddy simulation (LES) for momentum transport combined with the one-dimensional turbulence (ODT) model for momentum and reactive scalars’ transport is designed to capture subgrid scale (SGS) turbulence-chemistry interactions. An extension of the original LES-ODT formulation is developed to capture these interactions in turbulent spatially-developing reacting shear layers. The LES-ODT results are compared to results from direct numerical simulations (DNS). Lewis number parametric variations for the variable-density simulations are carried out. The validation with DNS shows that the LES-ODT approach can qualitatively and quantitatively capture important salient features of turbulent shear layers statistics, including large-scale flow patterns, shear layer growth and mean and RMS statistics of velocity and reactive scalars.

Corrigendum to “Influence of the aligned flow obstacles on critical heat flux in two-phase boundary layer flow under a low flow condition” [Int. J. Heat Mass Transfer 121 (2018) 1234–1250

Large-eddy simulation (LES) for momentum transport combined with the one-dimensional turbulence (ODT) model for momentum and reactive scalars’ transport is designed to capture subgrid scale (SGS) turbulence-chemistry interactions. An extension of the original LES-ODT formulation is developed to capture these interactions in turbulent spatially-developing reacting shear layers. The LES-ODT results are compared to results from direct numerical simulations (DNS). Lewis number parametric variations for the variable-density simulations are carried out. The validation with DNS shows that the LES-ODT approach can qualitatively and quantitatively capture important salient features of turbulent shear layers statistics, including large-scale flow patterns, shear layer growth and mean and RMS statistics of velocity and reactive scalars.

Numerical simulation of high speed reacting shear layers using AUSM+- up scheme-based unstructured finite volume method solver

Development of an Advection Upstream Splitting Method (AUSM[Formula: see text]-up) scheme-based Unstructured Finite Volume (UFVM) solver for the simulation of two-dimensional axisymmetric/planar high speed compressible turbulent reacting shear layers is presented. The inviscid numerical flux is evaluated using AUSM[Formula: see text]-up upwind scheme. An eight-step hydrogen–oxygen finite rate chemistry model is used to model the development of chemical species in a supersonic reacting flow field. The chemical species terms are alone solved implicitly in this explicit flow solver by rescaling the equation in time. The turbulence modeling has been done using RNG-based [Formula: see text]–[Formula: see text] model. Three-stage Runge–Kutta method has been used for explicit time integration. The nonreacting two-dimensional Cartesian version of the same solver has been successfully validated against experimental and numerical results reported for the wall static pressure data in sonic slot injection to supersonic stream. Detailed validation studies for reacting flow solver has been done using experimental results reported for a coaxial supersonic combustor, in which species profile at various axial locations has been compared. Present numerical solver is useful in simulating combustors of high speed air-breathing propulsion devices.

Evolution of flame kernel in one eddy turnover of high-speed droplet laden shear layers

A high-speed droplet-laden reacting shear layer is modeled numerically to analyze the dynamics of flame kernel for better understanding ignition and extinction processes in spray combustion. The droplet-laden shear layer, separating a hot-air stream from a monodisperse spray-laden air stream, is modeled by the Eulerian-Lagrangian approach, in which the continuous phase is governed by the compressible Navier-Stokes equations together with species transport equations and the discrete droplets are tracked by the Lagrangian method. The convective flux terms of the conservative equations are solved by an adaptive central-upwind WENO scheme. Two-way coupling interactions consider exchanges of mass, momentum, and energy between the carrier-gas fluid and the liquid-fuel spray. The shear layer convective Mach number is specified as 0.4 in the present study, and hence, large scales of motion dominate both the fluid mixing and the droplet dispersion. The preferential concentration of droplets is observed. Auto-ignition kernels occur in the high-strain regions where sufficient fuel vapors distribute near the hot stream-boundary. The incipient chemical reaction consumes the available fuel vapors through a diffusion flame combined with a thin premixed flame, as a lean deflagration propagates across the mixing layer towards the spray. The present results unveil two different types of extinction behavior depending on the local strain rate and the available fuel for the reaction kernel. The latter type of extinction originates when the separated flame kernel is entrained into the core of an adjacent vortex but droplets cannot be entrained in the eddies to produce gaseous fuels for the chemical reaction due to the preferential segregation effect. The dynamics of ignition and extinction in the shear flows depend on the droplet dispersion, the droplet evaporation rate and the diffusion of fuel vapor.

A priori analysis of subgrid scale pressure and heat flux in high pressure mixing and reacting shear layers

Direct Numerical Simulation (DNS) data on high pressure H2/O2 and H2/air flames using the compressible flow formulation, detailed kinetics, a real fluid equation of state, and generalised diffusion are analysed. The DNS is filtered over a range of filter widths to provide exact terms in the Large Eddy Simulation (LES) governing equations, including unclosed terms. The filtered pressure and the filtered heat flux vector are extensively compared with the pressure and the heat flux vector calculated as a function of the filtered primitive variables (i.e. the exact LES term is compared with its form available within an actual LES). The difference between these forms defines the subgrid pressure and the subgrid heat flux vector. The analyses are done both globally across the entire flame, as well as by conditionally averaging over specific regions of the flame; including regions of large subgrid kinetic energy, subgrid scalar dissipation, subgrid temperature variance, flame temperature, etc. In this work, although negligible for purely mixing cases, the gradient of the subgrid pressure is shown to be of the same order as, and larger than, the corresponding divergence of the turbulent subgrid stresses for reacting cases. This is despite the fact that all species behave essentially as ideal gases for this flame and holds true even when the ideal gas law is used to calculate the pressure. The ratio of the subgrid pressure gradient to the subgrid stress tensor divergence is shown to increase with increasing Reynolds number. Both the subgrid heat flux vector and its divergence are found to be substantially larger in reacting flows in comparison with mixing due to the associated larger temperature gradients. However, the divergence of the subgrid heat flux vector tends to be significantly smaller than other unclosed terms in the energy equation with decreasing significance with increasing Reynolds number.

Ignition, flame structure and near-wall burning in transverse hydrogen jets in supersonic crossflow

We have investigated the properties of transverse sonic hydrogen jets in high-temperature supersonic crossflow at jet-to-crossflow momentum flux ratios$J$between 0.3 and 5.0. The crossflow was held fixed at a Mach number of 2.4, 1400 K and 40 kPa. Schlieren and$\text{OH}^{\ast }$chemiluminescence imaging were used to investigate the global flame structure, penetration and ignition points;$\text{OH}$planar laser-induced fluorescence imaging over several planes was used to investigate the instantaneous reaction zone. It is found that$J$indirectly controls many of the combustion processes. Two regimes for low (${<}1$) and high (${>}3$)$J$are identified. At low$J$, the flame is lifted and stabilizes in the wake close to the wall possibly by autoignition after some partial premixing occurs; most of the heat release occurs at the wall in regions where$\text{OH}$occurs over broad regions. At high$J$, the flame is anchored at the upstream recirculation region and remains attached to the wall within the boundary layer where$\text{OH}$remains distributed over broad regions; a strong reacting shear layer exists where the flame is organized in thin layers. Stabilization occurs in the upstream recirculation region that forms as a consequence of the strong interaction between the bow shock, the jet and the boundary layer. In general, this interaction – which indirectly depends on$J$because it controls the jet penetration – dominates the fluid dynamic processes and thus stabilization. As a result, the flow field may be characterized by a flame structure characteristic of multiple interacting combustion regimes, from (non-premixed) flamelets to (partially premixed) distributed reaction zones, thus requiring a description based on a multi-regime combustion formulation.

A computational approach to flame hole dynamics using an embedded manifold approach

We present a new numerical algorithm for tracking the evolution of flame holes in diffusion flames. The key element is the solution of an evolution equation for a flame state field defined on a complex moving surface. The surface itself can evolve in time and is defined implicitly as a level set of an associated Cartesian scalar field. The surface coordinates, or parameterization, do not need to be determined explicitly. Instead, the numerical method employs an embedding technique where the evolution equation is extended to the Cartesian space. In our application, the flame state field represents the chemical activity state of a diffusion flame; i.e. quenched and burning regions of the flame surface. We present a formulation that describes the formation, propagation, and growth of flame holes using edge-flame modeling in laminar and turbulent diffusion flames. The evolution equation is solved using a high-order finite-volume WENO method and a new extension algorithm defined in terms of propagation PDEs. The complete algorithm is demonstrated by tracking the dynamics of flame holes in a turbulent reacting shear layer and its applicability is also demonstrated in a turbulent reacting lifted jet simulation.

Large eddy simulation of turbulent premixed combustion flows over backward facing step

Large eddy simulation (LES) of turbulent premixed combustion flows over backward facing step has been performed using a dynamic sub-grid G-equation flamelet model. A flamelet model for the premixed flame is combined with a dynamic sub-grid combustion model for the filtered propagation of flame speed. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model in a complex turbulent premixed combustion flow. For the purpose of validating the LES combustion model, the LES of isothermal and reacting shear layer formed at a backward facing step is carried out. The calculated results are compared with the experimental results, and a good agreement is obtained.

Direct Numerical Simulation of Inert Droplet Effects on Scalar Dissipation Rate in Turbulent Reacting and Non-Reacting Shear Layers

Three-dimensional direct numerical simulation has been performed to investigate the effects of inert evaporating droplets on scalar dissipation rate χ in temporally-developing turbulent reacting and non-reacting mixing layers with the Reynolds number based on the vorticity thickness up to 8000 and the number of traced Lagrangian droplets up to 107. The detailed instantaneous field analysis and ensemble-averaged statistics reveal complex interactions among combustion, droplet dynamics and evaporation, all of which have a considerable influence on χ. The presence of inert evaporating droplets promotes χ in both non-reacting and reacting mixing layers. In the latter, combustion reduces χ, so when combustion is suppressed by evaporating droplets, χ is enhanced. The transport equation of χ has been analyzed to investigate the various effects on χ in detail. The terms in the equation contain explicitly the evaporation rate and its spatial derivative, acting as a sink and a source for χ, respectively. On the whole, the net effect of the evaporation-rate terms is to promote χ. However, the production and dissipation terms are the dominant source and sink terms, respectively.

On the effect of heat release in turbulence spectra of non-premixed reacting shear layers

Velocity, mixture fraction and temperature spectra obtained from five direct numerical simulations of non-reacting and reacting shear layers, using the infinitely fast chemistry approximation, are analysed. Two different global chemical reactions corresponding to methane and hydrogen combustion with air, respectively, are considered. The effect of heat release, i.e. density variation, on the inertial and dissipation turbulence subrange of the spectra is investigated. Analysis of the database supports the experimentally available measurements of spectra in turbulent reacting flows showing that heat release effects can be scaled out by utilizing Favre-averaged (density-weighted) large-scale turbulence quantities. This is supported by the simulation results for velocity and mixture fraction in our moderate-Reynolds-number flows but it appears to be less supported in the dissipation subrange of the temperature spectra. The departure from universal scaling using Favre-averaged quantities in the temperature spectrum, which is evident in the dissipation subrange, appears to be caused by the strong nonlinearity of the state relationship relating the mixture fraction to the temperature, as has been suggested previously. These effects are less pronounced at intermediate wavenumbers. Analysis suggests that the nonlinear state relationship and the spectra of mixture fraction moments can be used to reconstruct the temperature spectrum across the flow. Moreover, the governing equation for the temperature variance is analysed to identify a possible surrogate for the overall rate of dissipation of temperature fluctuations and their corresponding dissipation length scale. This scaling analysis is then used to separate planes across the shear layer where the temperature dissipation length scale is alike that of the mixture fraction from regions where smaller length scales are present, and are evidenced in the dissipation subrange using Kolmogorov scaling. In our simulations, these regions correspond to the centre of the shear layer and the mean flame location. The new estimate for the temperature dissipation length scale is able to collapse the compensated spectra profiles at all planes across the shear layer for all simulations.

Large Eddy Simulation of Turbulent Reacting Shear Layers Including Finite-Rate Chemistry and Detailed Diffusion Processes

Large eddy simulations (LES) of turbulent temporal shear layers with hydrogen chemistry are performed. In these simulations, approximate deconvolution is applied as an implicit subgrid-scale modeling approach to a reacting flow in combination with a steady flamelet model for the filtered heat release term. No additional heuristical or physical subgrid models are used. The formulation of the flamelet equations in physical space does not only allow to consider a detailed reaction scheme and the extinguished phase but also to take into account detailed diffusion mechanisms (Soret and Dufour effects, multicomponent diffusion coefficients). Two different levels of diffusion approximations are investigated in this work, the aim of which is twofold: Firstly, to verify approximate deconvolution as a tool for convective transport of mass, momentum and energy in gas flow, by comparing the LES results with those of a direct numerical simulation and secondly, to investigate the influence of detailed diffusion on the laminar flamelets and the LES results.

Modeling of filtered heat release for large eddy simulation of compressible infinitely fast reacting flows

A priori and a posteriori studies for large eddy simulation of the compressible turbulent infinitely fast reacting shear layer are presented. The filtered heat release appearing in the energy equation is unclosed and the accuracy of different models for the filtered scalar dissipation rate and the conditional filtered scalar dissipation rate of the mixture fraction in closing this term is analyzed. The effect of different closures of the subgrid transport of momentum, energy and scalars on the modeling of the filtered heat release via the resolved fields is also considered. Three explicit models of these subgrid fluxes are explored, each with an increasing level of reconstruction and all of them regularized by a Smagorinsky-type term. It is observed that a major part of the error in the prediction of the conditional filtered scalar dissipation comes from the unsatisfactory modeling of the filtered dissipation itself. The error can be substantial in the turbulent fluctuation (rms) of the dissipation fields. It is encouraging that all models give good predictions of the mean and rms density in a posteriori LES of this flow with realistic heat release corresponding to large density change. Although a posteriori results show a small sensitivity to subgrid modeling errors in the current problem, extinction–reignition phenomena involving finite-rate chemistry would demand more accurate modeling of the dissipation rates. A posteriori results also show that the resolved fields obtained with the approximate reconstruction using moments (ARM) agree better with the filtered direct numerical simulation since the level of reconstruction in the modeled subfilter fluxes is increased.

Investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations

Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37–55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier–Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH ∗ chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to inlet velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to velocity fluctuations.

Mixing of a conserved scalar in a turbulent reacting shear layer

Mixing of a conserved scalar representing the mixture fraction, of primary importance in modelling non-premixed turbulent combustion, is studied by direct numerical simulation (DNS) in strongly turbulent planar shear layers both with and without heat release at a reaction sheet. For high heat release, typical of hydrocarbon combustion, the mixing is found to be substantially different than without heat release. The probability density function of the scalar and the conditional rate of scalar dissipation are affected by the heat release in such a way that the heat release substantially decreases the overall reaction rate. To help clarify implications of the assumptions underlying popular models for interaction between turbulence and chemistry, the local structure of the scalar dissipation rate at the reaction sheet is extracted from the DNS database. The applicability of flamelet models for the rate of scalar dissipation is examined. To assist in modelling, a characteristic length scale is defined, representing the distance around the reaction sheet over which the scalar field is locally linear, and statistical properties of this length scale are investigated. This length scale can be used in studying values of the rate of scalar dissipation that mark the boundary between flames that feel a constant scalar dissipation field and those that do not.