Planar Flame Research Articles

A Priori Analysis on Deep Learning of Filtered Reaction Rate

A filtered reaction rate model driven by deep learning is proposed and analyzed a priori in the context of large eddy simulation (LES). A deep artificial neural network (ANN) is trained on the explicitly filtered reaction rate source term extracted from a database comprised of turbulent premixed planar flame direct numerical simulations (DNSes) employing single-step chemistry. The filtered DNS database to be used for the training of the ANN covers a wide range of turbulence intensities and LES filter widths. An interpretation technique of deep learning is employed to search the principal input parameters in the high dimensional database to alleviate the model complexity. The deep learning filtered reaction rate model is then tested on the unseen filtered planar flames featuring untrained turbulence intensities and LES filter widths, in conjunction with another canonical type of flame configuration that it has not been trained on. The deep learning filtered reaction rate model achieves good agreement with the filtered DNS results and also provides a quantitatively accurate surrogate model when compared to existing algebraic models and other combustion models from the literature.

Assessment of Damköhler's hypotheses in the thin reaction zone regime using multi-step chemistry direct numerical simulations of statistically planar turbulent premixed flames

The effects of the definition of the reaction progress variable and equivalence ratio on the validity of Damköhler's hypotheses for turbulent premixed flames belonging to the thin reaction zone regime have been studied using multi-step chemistry direct numerical simulations of statistically planar CH4–air premixed flames with equivalence ratios of 0.8 and 1.0. Although CH4–air premixed flames with equivalence ratios of 0.8 and 1.0 have effective Lewis numbers close to unity, local differential diffusion effects can play a non-negligible role in determining the turbulent burning velocity and flame surface area in all cases. However, the augmentations of burning rate and flame surface area under turbulence do not occur in equal proportion, but their ratio remains of the order of unity. This conclusion holds irrespective of the definition of the reaction progress variable for the cases considered here. Damköhler's second hypothesis, which relates the ratio of turbulent burning velocity and the unstretched laminar burning velocity to the ratio of turbulent diffusivity and molecular diffusivity, has been found not to hold in the sense of equality, but it is valid in an order of magnitude sense for all choices of reaction progress variable definition. The findings of the current analysis indicate that Damköhler's first and second hypotheses should only be interpreted in an order of magnitude sense in the thin reaction zone regime even when the effective Lewis number remains close to unity.

A multidimensional combustion model for oblique, wrinkled premixed flames

A new premixed turbulent combustion model is proposed, which is based on one-dimensional (1D) filtering of density times progress variable and reaction source term of laminar premixed flame profiles using a filter kernel that reflects the variation of the area of planar flame fronts moving through filter volumes. It is shown that these genuinely multidimensional effects qualitatively change the relation between the filtered reaction source term and the Favre-filtered reaction progress variable compared to strictly 1D models, particularly at large filter widths. Analytical results for the filtered quantities are obtained by approximating the laminar flame profiles by suitable ansatz functions. Filtered data from Direct Numerical Simulations (DNS) of statistically planar turbulent premixed flames at a range of turbulence levels of the unburnt mixture and for two different heat release parameters is used to optimize and validate the model. For filter widths smaller than the laminar flame thickness, minor effects from subgrid flame folding are observed. For larger filters, the effect of subgrid flame folding can be represented well through a wrinkling factor, which reduces the effective filter width used in the 1D filtering operation. Two wrinkling factor models are proposed, using fractal dimensions patterned on literature models. The modelled reaction source term written as function of Favre averaged progress variable and filter width shows excellent agreement with filtered DNS data conditioned on the Favre averaged progress variable for all investigated turbulence levels, filter widths and both heat release parameters.

Effects of buoyancy on turbulent scalar flux closure for turbulent premixed flames in the context of Reynolds Averaged Navier–Stokes simulations

The effects of body force on the statistical behaviour of turbulent scalar flux and its closure in the context of Reynolds Averaged Navier–Stokes simulations have been studied using Direct Numerical Simulations (DNS) of statistically planar turbulent premixed flames under different turbulence intensities and Froude numbers. An increase in body force magnitude in the case of unstable density stratification has been found to augment flame wrinkling, burning rate and gradient transport in comparison to a case without body force but with statistically similar unburned gas turbulence. By contrast, an increase in body force magnitude in the case of stable stratification reduces the flame wrinkling, burning rate and gradient transport in comparison to the flame without body force subjected to statistically similar unburned gas turbulence. Based on a-priori DNS analysis, an algebraic closure for turbulent scalar flux has been identified where the Froude number effects are explicitly accounted for. The body force has been found to have significant influence on the statistical behaviours and magnitudes of the terms of the scalar flux transport equation and this effect is particularly strong for the mean pressure gradient term in the scalar flux transport equation. Based on a detailed a priori DNS analysis, suitable model expressions have been identified for the turbulent transport, pressure gradient, dissipation and reaction rate-velocity correlation terms of the scalar flux transport equation by incorporating the effects of body force (e.g. Froude number effects) for improved model performance.

Assessing diffusion model impacts on enstrophy and flame structure in turbulent lean premixed flames

Diffusive transport of mass occurs at small scales in turbulent premixed flames. As a result, multicomponent mass diffusion, which is often neglected in direct numerical simulations (DNS) of premixed combustion, has the potential to impact both turbulence and flame characteristics at small scales. In this study, we evaluate these impacts by examining enstrophy dynamics and the internal structure of the flame for lean premixed hydrogen-air combustion, neglecting secondary Soret and Dufour effects. We performed three-dimensional DNS of these flames by implementing the Stefan–Maxwell equations in the code NGA to represent multicomponent mass transport, and we simulated statistically planar lean premixed hydrogen-air flames using both mixture-averaged and multicomponent models. The mixture-averaged model underpredicts the peak enstrophy in the multicomponent simulation by up to 13% in the flame front. Comparing the enstrophy budgets of these flames, the multicomponent simulation yields larger peak magnitudes compared to the mixture-averaged simulation in the reaction zone, showing differences of 17% and 14% in the normalised vortex stretching and viscous effects terms. In the super-adiabatic regions of the flame, the mixture-averaged model overpredicts the viscous effects by up to 13%. To assess the effect of these differences on flame structure, we reconstructed the average local internal structure of the turbulent flame through statistical analysis of the scalar gradient field. Based on this analysis, we show that large differences in viscous effects contribute to significant differences in the average local flame structure between the two models.

On the evolution of the scalar flux through a planar premixed turbulent flame brush

ABSTRACT The thermal expansion induced by the exothermicity of chemical reactions taking place in a turbulent flame affects the flow dynamics so deeply that the velocity field can be imposed by chemistry rather than turbulence. Moreover, thermal expansion is known to be responsible for flame-generated turbulence (FGT) as well as non-gradient or counter-gradient diffusion (CGD) phenomena. In the present study, a specific description of the joint probability-density function (PDF) of the progress variable and velocity is introduced. The corresponding PDF accounts for the finite thickness of the local flame. On the basis of this theoretical framework, the evolution of the scalar fluxes is analyzed across a planar premixed turbulent flame brush described as a boundary layer. The corresponding analysis recovers a CGD region in the planar flame brush as well as a region controlled by gradient diffusion (GD) transport at its leading edge. This region, which corresponds to small values of the mean progress variable, is dominated by finite Damköhler number effects. Finally, the dependency of the normalized turbulent scalar flux to classical nondimensional numbers – i.e., the Bray, Karlovitz and Reynolds numbers – is put into evidence. The obtained results provide a relatively simple basis for the development of closure models for the turbulent flux of the progress variable.

Effects of Body Forces on Turbulent Kinetic Energy Transport in Premixed Flames

The effects of buoyancy on turbulent premixed flames are expected to be strong due to the large changes in density between the unburned and fully burned gases. The present work utilises three-dimensional direct numerical simulations of statistically planar turbulent premixed flames under decaying turbulence to study the influence of buoyancy on the evolution of turbulent kinetic energy within the flame brush. Four sets of turbulence parameters have been studied, with four different values of Froude number for each set. It is found that for a given set of turbulence parameters, flame wrinkling increases with an increase in body force magnitude in the case of unstable stratification, which is also reflected in the increased values of turbulent burning velocity and flame surface area. An increase in body force magnitude in the case of stable stratification acts to reduce the extent of flame wrinkling. Turbulent kinetic energy and its dissipation rate are found to be affected by both the magnitude and direction of the body force. For low turbulence intensities considered here, turbulent kinetic energy increases from the leading edge of the flame brush before decaying eventually towards the product side of the flame brush. For high turbulence intensities, the turbulent kinetic energy is found to decay across the flame brush, and it is also found that the effect of body force on the evolution of turbulent kinetic energy is marginal in the case of high turbulence intensities. The effects of body force magnitude and direction on the statistical behaviours and closures of the various terms of the turbulent kinetic energy transport equation are analysed, and existing models have been modified to account for Froude number effects, where necessary.

Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames

The characterisation of small-scale turbulence has been an active area of research for decades and this includes, particularly, the analysis of small-scale isotropy, as postulated by Kolmogorov. In particular, the question if the dissipation tensor is isotropic or not, and how it is related to the anisotropy of the Reynolds stresses is of particular interest for modelling purposes. While this subject has been extensively studied in the context of isothermal flows, the situation is more complicated in turbulent reacting flows because of heat release. Furthermore, the landscape of Computational Fluid Dynamics is characterised by a multitude of methods ranging from Reynolds-averaged to Large Eddy Simulation techniques, and they address different ranges of scales of the turbulence kinetic energy spectrum. Therefore, a multiscale analysis of the anisotropies of Reynolds stress, dissipation and sub-grid scale tensor has been performed by using a DNS database of statistically planar turbulent premixed flames. Results show that the coupling between dissipation tensor and Reynolds stress tensor is weaker compared to isothermal turbulent boundary layer flows. In particular, for low and moderate turbulence intensities, heat release induces pronounced anisotropies which affect not only fluctuation strengths but also the characteristic size of structures associated with different velocity components.

A Lattice-Boltzmann study of premixed flames thermo-acoustic instabilities

We present possibly for the first time Lattice-Boltzmann numerical simulations of thermo-acoustic instabilities of premixed flames. We study flames interacting with an imposed acoustic field where flames submitted to a parametric instability can be observed, as well as plane flames re-stabilized by the acoustic forcing. Self-induced thermo-acoustic oscillations of flames propagating in narrow channels are also studied, indicating an unexpected dependency with the channel width. For both excited and self-excited flames, results confirm that Lattice-Boltzmann method can capture the complex coupling between flame dynamics and acoustics.

Modeling of the turbulent burning velocity for planar and Bunsen flames over a wide range of conditions

Modeling of the turbulent burning velocity for planar and Bunsen flames over a wide range of conditions

Intrinsic instabilities in premixed hydrogen flames: parametric variation of pressure, equivalence ratio, and temperature. Part 2 – Non‐linear regime and flame speed enhancement

The propensity of lean premixed hydrogen flames to develop intrinsic instabilities is studied by means of a series of simulations at different equivalence ratios [0.4–1.0], unburned temperatures [298 K–700 K], and pressures [1 bar–20 bar]. In addition to the Darrieus-Landau or hydrodynamic instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which lead to significant flame front wrinkling and a chaotic process of formation and destruction of cellular structures along the flame front. In part 1 of this work (L.Berger et al., Combust. Flame, 2022), the stability of planar flames that are exposed to weak harmonic perturbations has been studied. In this part, the long-term dynamics of these flames, which become strongly corrugated and show strong flame speed enhancements and variations of the local reaction rates, are studied. In particular, local extinction events and strong peaks of the reaction rates, sub- and super-adiabatic temperatures in the burned gas, and variations of the local flame thickness are observed such that the flame reactivity significantly deviates from an unstretched laminar flamelet. Consistent with the stability analysis in part 1, the impact of instabilities increases if the equivalence ratio or unburned temperature are decreased or the pressure is increased. In particular, the propensity of intrinsic instabilities to increase at high pressure is relevant to several combustion applications operating at elevated pressures. Further, the effect of the global flame parameters such as the expansion ratio, Zeldovich number, and Lewis number on the flame speed enhancement is quantitatively assessed. Finally, the flame speed enhancement due to instabilities is found to correlate well with the maximum growth rates of the perturbed flames that have been measured in part 1 indicating a strong link between the two different flame regimes.

Intrinsic instabilities in premixed hydrogen flames: Parametric variation of pressure, equivalence ratio, and temperature. Part 1 - Dispersion relations in the linear regime

The impact of intrinsic combustion instabilities is studied for lean premixed hydrogen flames by means of a series of simulations at different equivalence ratios [0.4-1.0], unburned temperatures [298K-700K], and pressures [1bar-20bar]. In addition to the Darrieus-Landau, or hydrodynamic, instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which lead to significant flame front wrinkling and a chaotic process of formation and destruction of cellular structures along the flame front. Theoretical models are not yet capable of accurately describing the evolution of such flames, so the propensity of lean hydrogen flames to develop instabilities is studied numerically in a parametric variation in this work. A stability analysis is conducted, in which planar flames are initially exposed to weak harmonic perturbations and the response of the flame is studied. In the initial phase referred to as linear phase, a growth or decrease of the initially imposed perturbation amplitude is observed, while for long times, chaotic cellular structures are formed along the flame front, which are studied in part 2 of this work (L.Berger et al., Combust. Flame, 2022). The growth rates of the perturbation amplitude that are obtained from the initial phase are a measure of the strength of the intrinsic instability mechanisms and vary with respect to the wave number of the harmonic perturbation yielding characteristic dispersion relations. A decrease of equivalence ratio and unburned temperature and an increase of pressure are found to enhance the growth rates and hence intrinsic instabilities. The variation of dispersion relations is analyzed with respect to variations of the expansion ratio, the effective Lewis number of the mixture, and the Zeldovich number. For the lean hydrogen flames, with increasing pressure a decrease of the cut-off wave number, which represents the change of the sign of the dispersion relation at high wave numbers, is observed. This is the opposite trend compared to flames that are not affected by thermodiffusive instabilities. Further, numerical growth rates are compared to theoretical models. The results show that lean hydrogen flames are prone to develop instabilities at conditions that are relevant to several combustion devices such as gas turbines that operate at lean equivalence ratios, elevated pressures and temperatures or domestic and industrial heaters that operate at low temperatures and ambient pressure.

Scalar Forcing Methodology for Direct Numerical Simulations of Turbulent Stratified Mixture Combustion

Scalar forcing in the context of turbulent stratified flame simulations aims to maintain the fuel-air inhomogeneity in the unburned gas. With scalar forcing, stratified flame simulations have the potential to reach a statistically stationary state with a prescribed mixture fraction distribution and root-mean-square value in the unburned gas, irrespective of the turbulence intensity. The applicability of scalar forcing for Direct Numerical Simulations of stratified mixture combustion is assessed by considering a recently developed scalar forcing scheme, known as the reaction analogy method, applied to both passive scalar mixing and the imperfectly mixed unburned reactants of statistically planar stratified flames under low Mach number conditions. The newly developed method enables statistically symmetric scalar distributions between bell-shaped and bimodal to be maintained without any significant departure from the specified bounds of the scalar. Moreover, the performance of the newly proposed scalar forcing methodology has been assessed for a range of different velocity forcing schemes (Lundgren forcing and modified bandwidth forcing) and also without any velocity forcing. It has been found that the scalar forcing scheme has no adverse impact on flame-turbulence interaction and it only maintains the prescribed root-mean-square value of the scalar fluctuation, and its distribution. The scalar integral length scale evolution is shown to be unaffected by the scalar forcing scheme studied in this paper. Thus, the scalar forcing scheme has a high potential to provide a valuable computational tool to enable analysis of the effects of unburned mixture stratification on turbulent flame dynamics.

Assessment of planar laminar flame speed of Hythane generated in-situ from non-thermal plasma reforming of Methane : Flame tube based experiments and thermo-chemical analysis

The article addresses the characterization of laminar flame speed of Hythane generated from non thermal plasma reforming of Methane using a dielectric barrier discharge reactor. Hythane with 5%, 10% and 15% Hydrogen fraction is generated by controlling the Methane residence time in the plasma zone. Planar laminar flame speed for the Hythane so generated is estimated using a flame tube and compared with flame speed of bottled Hythane over a range of operating stoichiometry. It is observed that Hythane from plasma reforming of Methane has substantially higher laminar flame speed as compared to bottled Hythane. At the peak flame speed equivalence ratio of ϕ=1.1, Hythane generated from plasma activation of Methane has laminar flame speed higher than corresponding bottled Hythane to the extent of 13.3%, 34.1% and 38.9% for 5%, 10% and 15% Hydrogen in Hythane respectively. It is also observed that flame speed difference is muted for lean mixtures as compared to rich mixtures. Thermo-chemical analysis indicates that the enhanced flame speed can neither be addressed based on thermal effects nor due to the presence of neutral intermediate species and non thermal reforming of Methane to generate Hythane potentially results in additional reaction kinetics influences which accelerate the combustion process. Such influences could arise from presence of long lived species formed (charged or otherwise) not detected in conventional analysis and changes to the energy levels of the reaction species, principally Methane and Hydrogen. In terms of terminal utility, non thermal plasma reforming of Methane can have path breaking influence considering that enhanced flame speeds can significantly improve the thermodynamic efficiency of a spark ignited engine with lower than pure thermo-chemical Hydrogen requirement in Hythane.

A study of thermal effect depending on temperature field adjacent to turbulent premixed flames using DNS

Turbulent premixed flames are heat release phenomena in the turbulent flow with premixed fuel and oxidiser. Turbulent premixed flames form the temperature field and are affected by the interaction between flow and heat. This study focused the temperature field adjacent to the flames and considered the effect of the interaction between flow and heat on the flames using DNS (Direct numerical simulation). In the development of planar flame in the turbulence field, the results show that spread of the flame zone leads to enhancement of the effect of turbulence field on the isosurface of temperature upstream of flames, and that the distribution of heat conduction does not deviate from the isosurface of temperature in the case of small wrinkles on the isosurface of temperature.

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.

Morphology and structure of spherically propagating premixed turbulent hydrogen - air flames

Three-dimensional direct numerical simulations of spherically propagating premixed turbulent stoichiometric hydrogen-air flames with detailed chemistry and detailed diffusion are employed to clarify the influence of turbulence-flame interactions with respect to flame structure and morphology. Four cases are considered within the corrugated flamelets and the thin reaction zone regimes. The most significant fuel consumption and heat release rates occur at the negatively-curved flamelets. Furthermore, the increments in burning velocity are lower than the increments in flame surface area, which is due to the reduction in the local burning intensity at the positively-curved flamelets. The morphology of intense reaction zones is quantified using Minkowski functionals and their shapes include ”tubes”, ”pancakes” and more complex shapes, which are compared to their counterparts in planar flames. As turbulence level increases, the number of locally defined intense reaction zones increases, and their boundaries expand to cover more extensive parts of the flame front. However, intense reaction zones’ geometrical dimensions do not significantly differ for each flame as it propagates. Local turbulence properties are obtained for each intense reaction zone. The conditional averages of local Taylor microscale and local Kolmogorov scale, conditioned based on the shapefinders, are investigated. The conditional averages of the local Taylor microscale scale correlate with the planarity and filamentarity of intense reaction zones. Therefore, turbulent motions at Taylor microscale size have a significant role in characterizing turbulence-flame interactions relevant to flame morphology and relevant to the local flame thickness and reaction layers of developing-flames. On the other hand, local Kolmogorov scales’ turbulent motions show weaker or no such correlations. There is a dissidence between global turbulence statistics and local ones representing the interactions at the flame front. Local turbulence-flame interactions of Taylor microscale sizes occur at specific length scales, depending on the flame size and irrelevant of eddies with other length scales.

Evolutions of strain rate and dissipation rate of kinetic energy in turbulent premixed flames

The statistical behaviors of the evolutions of the components of the strain rate tensor and Favre-averaged dissipation rate of kinetic energy are analyzed using direct numerical simulations of statistically planar turbulent premixed flames propagating into forced unburned gas turbulence for different turbulence intensities spanning a range of different Karlovitz numbers. The pressure Hessian contribution and the combined molecular diffusion and dissipation terms are found to play dominant roles in the transport equations of diagonal strain rate components and the Favre-averaged dissipation rate of kinetic energy for flames with small Karlovitz numbers. By contrast, the leading order balance is maintained between the strain rate, vorticity, and molecular dissipation contributions for flames with large Karlovitz numbers, similar to non-reacting turbulent flows. The contributions of the terms arising from the correlation between pressure and density gradients and pressure Hessian in the strain rate and dissipation rate of kinetic energy transport equations weaken in comparison to the magnitude of the molecular dissipation contribution with an increase in Karlovitz number. These behaviors have been explained in terms of the alignments of vorticity, pressure gradient, and pressure Hessian eigenvectors with strain rate eigendirections. The magnitudes of the terms in the transport equation of the Favre-averaged dissipation rate of kinetic energy are also found to increase with increasing Karlovitz number, which is explained with the help of a detailed scaling analysis. This scaling analysis also explains the leading order contributions to the dissipation rate of kinetic energy for different combustion regimes.

Influence of thermal wall boundary condition on scalar statistics during flame-wall interaction of premixed combustion in turbulent boundary layers

The influence of thermal wall boundary conditions on the behaviour of scalar statistics during flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Three-dimensional direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal as well as adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is representative of head-on interaction (HOI) of a planar flame in a turbulent boundary layer. The turbulence in the non-reacting conditions for these simulations is representative of a friction velocity based Reynolds number of Reτ=110 while the combustion characteristics are representative of stoichiometric methane–air mixture under atmospheric conditions. Differences in the mean behaviours of the progress variable and non-dimensional temperature have been observed for the two configurations in response to different wall boundary conditions. Mean scalar variance of the progress variable and temperature and their respective scalar dissipation rates have been investigated and it is found that the variances of the progress variable and temperature remain coupled in the case of adiabatic walls, while significant differences exist between these quantities in the vicinity of the wall under isothermal wall conditions for both flow configurations. The mean scalar dissipation rates of progress variable and temperature during FWI decrease towards the wall in both flow configurations under different thermal wall boundary conditions, but in the case of isothermal wall conditions a higher scalar dissipation rate for the non-dimensional temperature is observed when compared with the scalar dissipation rate for the progress variable. The behaviours of the mean turbulent scalar fluxes of progress variable and the non-dimensional temperature suggest that the scalar fluxes can show both gradient and countergradient type behaviour depending on the flow configuration and also on the proximity of the flame to the wall. In the case of V-flame OWI, the choice of the wall boundary condition has a significant influence on the behaviour of the scalar fluxes, whereas in the case of turbulent boundary layer HOI, no significant differences in the scalar fluxes are observed for different wall boundary conditions.

Assessment of local and non–local turbulent flow components on turbulence–flame interaction

In the framework of turbulence-flame interaction, the flame is characterized by the gradient of a reactive scalar such as the progress variable, whereas the turbulence is represented by the vorticity and the strain rate. Quantitative assessment of this interaction is performed trough the study of the coupled transport between these quantities that are subject to the effects of heat release and chemical reactions. The present analysis aims at improving the understanding of the small scale turbulence – flame interaction properties, through the introduction of an additive decomposition of the strain rate and vorticity fields into their local and non-local components. The respective role of the local and non-local effects is studied for a broad range of Karlovitz numbers, by virtue of direct numerical simulations (DNS) of turbulent, premixed, lean, and statistically planar flames of methane-air. In the conditions of the present study, the alignment between flame front normals and the strain rate is found to be dominated by the local contribution from the strain rate tensor.