The number of batteries in various applications at end-of-life and production waste from battery gigafactories increase significantly. At the same time, new EU regulations are introduced to promote battery recycling, which is a new and rapidly growing business. Large amounts of combustible dust are generated in battery recycling. Managing combustible dust hazards at the battery recycling plants is one of the key factors to minimize the incidents and down time and, therefore, to improve the work environment, and to increase the profitability of the business. Accordingly, the present work aims at exploring the risk of explosion of black mass dusts associated with battery recycling. Specifically, four black mass samples from different battery recycling plants are experimentally investigated. Microscope images, particle size distribution, water content and organic carbonates are analyzed. Dust explosion experiments are performed in a 20-L vessel. Parameters including dust concentration, ignition energy, ignition delay, dust injection pressure are varied. Results show that a 10 kJ ignition energy cannot generate high explosion overpressure, whereas an ignition energy of 20 kJ yields an explosion overpressure above 6 bar for black mass sample C at a concentration of 300 g/m3. The obtained experimental results are compared with published data on various explosion-related characteristics of other dusts relevant to battery recycling, in particular, aluminum and graphite dusts.

Inter-scale kinetic energy transfer in turbulent flows is accompanied by very intense and intermittent spatial-temporal fluctuations. Such intermittency is expected to be particularly prominent in premixed flames, where heat release, density variations, dilatation, and chemical reactions are localized to spatial scales that are substantially smaller than scales of large turbulent eddies but are often larger than or comparable with Kolmogorov length scale. Nevertheless, intermittency of inter-scale energy transfer and of heat exchange between internal and kinetic energy in premixed flames has not yet been given due attention. The present work aims at bridging this knowledge gap. For this purpose, three-dimensional direct numerical simulation data obtained earlier from a statistically stationary, planar, one-dimensional lean hydrogen-air flame propagating in moderately intense turbulence are filtered using cubes of various widths. Subsequently, Probability Density Functions (PDFs) of various instantaneous flow characteristics are sampled by processing the filtered fields of velocity, pressure, density, and their spatial gradients. The sampled PDFs exhibit long tails, are highly skewed, and are characterized by a large kurtosis, thus, evidencing significant intermittency of inter-scale energy transfer and heat exchange between internal and kinetic energy in the flame.

Ammonia-hydrogen (NH3-H2) blends stand as a promising carbon-neutral fuel alternative. This study delves into the variation in the internal structure of NH3-H2 flames with varying Lewis numbers (Le) and turbulence intensities. Simultaneous multi-scalar imaging of NH3/NH/OH was performed to characterize the structures of turbulent jet flames with the same unstretched laminar flame speed (30.8 cm/s) but different H2 contents. Internal flame structures were characterized by isocontours of NH3, NH (both inner and outer edges, NHi and NHo), and OH to delimit the reaction zone. The parallelism of the selected isocontours (i.e., NH3, NHi, and NHo) with the OH baseline was quantified by calculating the probabilities, P(δp), of the selected isocontours overlapping with the OH baseline shifted by a distance of δp along its normal direction. On the one hand, results have revealed that increasing turbulence disrupts the parallelism of the selected isocontours with the OH baseline, leading to a larger and FWHM, as well as a reduced skewness of the P(δp) profiles. On the other hand, flames with higher H2 content and lower Le exhibit great resistance to turbulence but experience enhanced wrinkling due to the differential diffusion effect, which can also cause deviations in scalar parallelism at low turbulence intensity. For flames with reduced Le numbers, the NH3 isocontours are observed to be located even downstream of the NHi isocontours, which belong to the reaction zone. The extent of parallelism for the selected isocontours within the reaction zone is found to respond to turbulence and Le differently, deviating from an idealized picture of the flamelet manifold.

Two groups of both lean and rich NH3/H-2/O-2/N-2 turbulent premixed piloted jet flames with the same laminar flame speed are investigated using simultaneous multi-scalar laser diagnostics techniques including NH3/NH/OH planar laser induced fluorescence (PLIF) and Rayleigh scattering. One group uses air as the oxidizer and the other uses an adapted mixture of atomic N-to-H ratio of 1:3 associated with in-situ hydrogen production from ammonia cracking. Turbulent rms velocity u' is varied in a wide range by changing the bulk flow velocity. Global consumption speeds S-T,S- G are evaluated by measuring the inlet bulk flow rates of reactants and areas of time-averaged flame fronts extracted using different flame marker contours, i.e., c(NH3) = 0.3, c(NH) = 0.3 (inner contour), and c(OH) = 0.4, where c designates species number density normalized using its maximum value. The obtained results show the following trends to be emphasized. First, an increase in S-T,S- G,S-NH3 by u' is more pronounced when compared to two other global consumption speeds (S-T,S- G,S-NHi, where i indicates the inner contour, and S-T,S- G,S-OH) and is close to a linear dependence. The adapted mixture shows only a moderate influence on S-T,S- G,S-NH3 but not on the other two. Second, S-T,S- G,S-NH3 is weakly affected by variations in Lewis number Le. On the contrary, third, S-T,S- G,S-NHi is significantly higher in lean mixtures characterized by Le < 1 when compared to rich mixtures characterized by Le > 1. Fourth, these variations in S-T,S- G,S-NHi are reasonably well approximated introducing an empirical factor of Le(-0.9) into fitting S-T,S- G,S-NHi/S-L onto u'/S-L. The qualitative difference between sensitivities of S-T,S- G,S-NH3 and S-T,S- G,S-NHi to variations in Le is attributed to the fact that the NH3 and NH isosurfaces are localized to preheat and reaction zones, respectively, of the laminar flames. The reported experimental data imply different influence of differential diffusion on flame preheat and reaction zones, thus, calling for further research into the issue.

Simulations of unperturbed lean hydrogen-air flames were performed using three state-of-the-art chemical mechanisms under various conditions: pressure 1≤P≤50 atm, unburned gas temperature 300≤Tu≤900 K, and the equivalence ratio 0.3≤Φ≤0.5. Multicomponent diffusion and Soret effect were considered. The computed results show that, under certain conditions, (i) differently defined Zel'dovich numbers decrease with increasing P and (ii) sensitivity coefficients of Ze to the rates of the most important chain-branching reaction (R1) H + O2OH+O and chain-terminating reaction (R9) H + O2+M=HO2+M change their signs from negative and positive, respectively, to positive and negative, respectively, at high P. Analysis of the computed data shows that this transition occurs when the rates of the chain-terminating reaction (R14) 2HO2H2O2+O2 and the chain-branching reaction (R15) H2O2+M = 2OH+M are almost equal. Under such conditions, these two rates are much higher than a rate of another reaction that involves H2O2 in the largest parts of flame reaction zones. Moreover, in the vicinity of the upstream boundaries of the reaction zones, these two rates are significantly higher than a rate of another bimolecular reaction that involves HO2. Accordingly, in such a flame zone, whose location controls Zel'dovich number, almost all H2O2 formed in reaction (R14) is immediately converted to two radicals OH via reaction (R15). Therefore, the entire root of reactions (R9)→(R14)→(R15) becomes chain-propagating root, with its rate being significantly increased by P, because reactions (R9) and (R15) are termolecular ones. The emphasized effects are mitigated by both Tu and Φ, i.e., they are observed at higher pressure if Tu or Φ is increased. The explored negative pressure-dependence of Zel'dovich number should be considered when analyzing experimental or numerical data obtained from lean hydrogen-air turbulent flames under elevated pressures or when modeling such flames.

This paper aims at assessing a hypothesis that resolution required to evaluate fuel consumption and heat release rates by directly (i.e., without a subgrid model of unresolved influence of small-scale turbulent eddies on the local flame) processing filtered fields of density, temperature, and species mass fractions should be significantly finer than resolution required to directly compute flame surface density by processing the same filtered fields. For this purpose, box filters of various widths Delta are applied to three-dimensional direct numerical simulation data obtained earlier from a statistically one-dimensional and planar, moderately lean H-2/air complex-chemistry flame propagating in a box under conditions of sufficiently intense small-scale turbulence (Karlovitz number is larger than unity, and a ratio of laminar flame thickness delta(L) to Kolmogorov length scale is about 20). Results confirm this hypothesis and show that the mean flame surface density and area can be predicted with acceptable accuracy by processing filtered combustion progress variable fields computed using a sufficiently wide filter, e.g., Delta/delta(L)=4/3. Such an approach does not require a model of the influence of subgrid turbulent eddies on flame surface density provided that Delta and delta(L) are of the same order of magnitude. Good performance of this approach is attributed to inability of small-scale (when compared to delta(L)) turbulent eddies to substantially change the local flame structure, which, nevertheless, is significantly perturbed by larger turbulent eddies that strain the local flame.

To explore the importance of thermodiffusive and hydrodynamic instabilities of laminar flames in turbulent flows, previously generated direct numerical simulations of statistically one-dimensional complex-chemistry lean hydrogen-air flames in forced turbulence were continued by switching-off turbulence forcing. Three sets of flames characterized by different ratios of initial root-mean-square velocity to laminar flame speed, i.e., (A) u′/SL=2.2, (B) u′/SL=4.0, and (C) u′/SL=8.3, were addressed. Moreover, new complementary simulations of unstable laminar flames were performed. Analyses from the obtained numerical results indicate (but do not prove) that laminar flame instabilities play a minor role at sufficiently high Karlovitz numbers Ka. This supposition is supported, first, in cases B and C, where the initial value of turbulent burning velocity UT is significantly higher than burning velocity evaluated during non-linear stage of laminar flame instability development in the same computational domain. Second, regular large-scale perturbations of instantaneous flame surface are prominent in the unstable laminar flame but are not observed at high Ka. Third, Karlovitz numbers associated with appearance of such perturbations as the turbulence decays are scattered between 4 and 10 and are consistent within an order of magnitude to a recently proposed criterion (Chomiak and Lipatnikov, Phys. Rev. E 107: 015102, 2023) of importance for laminar flame instabilities in turbulent flows. Fourth, at such transition instants, a ratio of potential and solenoidal turbulent kinetic energies, averaged over the flame-brush leading edge, is close to 2.0 in 11 studied cases and a ratio of UT/SL varies between 3.0 and 3.5. Fifth, the maximum fuel consumption rate (over the computational domain) decreases as the turbulence decays. Thus, the initial maximum rates are significantly higher than the counterpart rate in the unstable laminar flame. Together these results show that the hypothesis that laminar flame instabilities play only a minor role at high Ka deserves further study.

Ammonia-hydrogen (NH 3-H 2 ) blends stand as a promising carbon-neutral fuel alternative. This study delves into the variation in the internal structure of NH 3-H 2 flames with varying Lewis numbers (Le) and turbulence intensities. Simultaneous multi-scalar imaging of NH 3 /NH/OH was performed to characterize the structures of turbulent jet flames with the same unstretched laminar flame speed (30.8 cm/s) but different H 2 contents. Internal flame structures were characterized by isocontours of NH 3 , NH (both inner and outer edges, NH i and NH o ), and OH to delimit the reaction zone. The parallelism of the selected isocontours (i.e., NH 3 , NH i , and NH o ) with the OH baseline was quantified by calculating the probabilities, P ( delta p ), of the selected isocontours overlapping with the OH baseline shifted by a distance of delta p along its normal direction. On the one hand, results have revealed that increasing turbulence disrupts the parallelism of the selected isocontours with the OH baseline, leading to a larger and FWHM, as well as a reduced skewness of the P ( delta p ) profiles. On the other hand, flames with higher H 2 content and lower Le exhibit great resistance to turbulence but experience enhanced wrinkling due to the differential diffusion effect, which can also cause deviations in scalar parallelism at low turbulence intensity. For flames with reduced Le numbers, the NH 3 isocontours are observed to be located even downstream of the NH i isocontours, which belong to the reaction zone. The extent of parallelism for the selected isocontours within the reaction zone is found to respond to turbulence and Le differently, deviating from an idealized picture of the flamelet manifold.

The paper aims at assessing a hypothesis that resolution required to evaluate fuel consumption and heat release rates by directly (i.e., without a subgrid model of unresolved influence of small-scale turbulent eddies on the local flame) processing filtered fields of density, temperature, and species mass fractions should be significantly finer than resolution required to directly compute flame surface density by processing the same filtered fields. For this purpose, box filters of various widths are applied to three-dimensional Direct Numerical Simulation data obtained earlier by Dave et al. (Combust. Flame 196 (2018) 386-399) from a statistically one-dimensional and planar, moderately lean H2/air complex-chemistry flame propagating in a box under conditions of sufficiently intense small-scale turbulence (Karlovitz number is larger than unity and a ratio of laminar flame thickness to Kolmogorov length scale is about 20). Results confirm this hypothesis and show that the mean flame surface density and area can be predicted with acceptable accuracy by processing filtered combustion progress variable fields computed using a sufficiently wide filter. Such an approach does not require a model of the influence of subgrid turbulent eddies on flame surface density provided that filter width and laminar flame thickness are of the same order of magnitude. Good performance of this approach is attributed to inability of small-scale (when compared to the thickness) turbulent eddies to substantially change the local flame structure, which, nevertheless, is significantly perturbed by larger turbulent eddies that strain the local flame.

A ratio of turbulent burning velocity to laminar flame speed is well known to be abnormally high in lean hydrogen-air mixtures, with this phenomenon being commonly attributed to differential diffusion effects. Magnitude of such effects is known to be increased by pressure, but a few recent studies have indicated that the effects are mitigated when reactants are preheated. It is not yet known, however, which of these two counteracting trends is of more importance under elevated pressures and temperatures associated with combustion in engines. Accordingly, it is not yet clear whether or not models of differential diffusion effects are required for research and development of future ultra-clean and highly efficient engines that burn hydrogen (the emphasized effects are typically ignored in engineering computations). To clarify the issue, numerical simulations of lean complex-chemistry hydrogen-air strained laminar flames are performed by varying strain rate, pressure 1≤P≤50 bar, temperature 300≤Tu≤900 K, and equivalence ratio (Φ=0.4, 0.55, and 0.7). This simple problem is selected because maximal consumption speeds reached in critically strained (close to extinction) lean hydrogen-air laminar flames are considered to characterize the influence of differential diffusion on turbulent burning rates within the framework of Zel'dovich's leading point concept, which was well supported in recent studies. Computed results show that, even at Tu=900 K, the aforementioned consumption speeds are significantly larger than the unperturbed laminar flame speeds if the mixture is sufficiently lean (the equivalence ratio Φ=0.55 or lower) and pressure is sufficiently high (P≥30 bar). Therefore, differential diffusion effects are expected to be of importance when burning so lean hydrogen-air mixtures in engines and should be properly modeled.

The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced.

To experimentally explore the influence of Lewis number Le, laminar flame thickness delta L, pressure P, and unburned gas temperature Tu on turbulent flame speed ST, a set of conditions is designed by adjusting nitrogen mole fraction in lean H-2/O-2/N-2 (Le=0.35) and stoichiometric CH4/O-2/N-2 (Le approximate to 1) mixtures. The adjustment is performed by simulating complex-chemistry laminar flames to obtain the same laminar flame speeds SL not only for different fuels, but also for different pressures (1, 2, and 5 atm). The mixtures are characterized by significantly different SL at Tu=300 K and 400 K, whereas variations in delta L with the temperature are sufficiently weak. Moreover, laminar flame thicknesses are approximately equal for H-2-based and CH4-based mixtures at the same P, but are significantly decreased with increasing pressure. For this set of conditions, ST is measured by applying schlieren imaging techniques to film expansion of centrally ignited, statistically spherical flames in homogeneous isotropic turbulence generated by a dual-chamber, constant-pressure, fan-stirred explosion facility. Analyses of the measured data show the following trends. First, turbulent flame speed is increased by both P and Tu, whereas ST/SL is decreased with increasing Tu. Second, turbulent flame speed measured at different P and Tu can be predicted by allowing for SL(P,Tu) and delta L(P,Tu). Thus, the present data do not call for explicitly substituting normalized pressure or temperature into a turbulent flame speed approximation. Third, ST is increased with decreasing laminar flame thickness. Fourth, speeds of the lean H-2/O-2/N-2 flames are higher when compared to the stoichiometric CH4/O-2/N-2 flames, with this difference is increased (reduced) by P (Tu, respectively). Fifth, all measured data on ST can quantitatively be described by substituting SL and delta L with the counterpart characteristics of highly strained twin laminar flames. The latter finding supports leading point concept of premixed turbulent combustion.

Analyzed in this paper are three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by the present authors from (i) 16 statistically planar and one-dimensional, lean complex-chemistry hydrogen-air turbulent flames propagating in forced turbulence in a box and (ii) nine counterpart equidiffusive flames. The simulation conditions cover a wide range of non-dimensional turbulent combustion characteristics. Specifically, root-mean-square turbulent velocity is varied from 0.3 to 55 laminar flame speeds, integral length scale of turbulence is varied from 0.5 to 2.6 laminar flame thicknesses, Damkohler and Karlovitz number are varied from 0.01 to 5.2 and from 0.7 to 1300, respectively. Three equivalence ratios, 0.7, 0.5, and 0.35, are addressed. Results of complementary two- and three-dimensional simulations of thermodiffusively unstable laminar flames are also reported. They show that neutral wavelength of laminar flame instabilities (both hydrodynamic and thermodiffusive ones are enabled in the simulations) is smaller (larger) than computational domain wide in ten (three, respectively) cases characterized by a low Lewis number. Accordingly, in three of these cases, the instabilities are suppressed. The focus of the study is placed (i) on contributions of flame surface area increase and stretch factor to turbulent burning velocity and (ii) on the influence of differential diffusion effects on these contributions. The computed results show that the flame surface area (i) is substantially increased by both turbulent rms velocity and length scale, (ii) is mainly controlled by turbulence, but (iii) is weakly affected by an increase in Lewis number. On the contrary, the stretch factor (i) is much larger than unity in low Lewis number flames, (ii) is significantly increased with decreasing equivalence ratio, and (iii) is weakly increased by rms turbulent velocity, but (iv) a notable influence of turbulence length scale on the stretch factor is not observed.

To explore the influence of dilatational and rotational motions on iso-scalar surface area within a premixed turbulent flame, three-dimensional compressible direct numerical simulation data obtained by Dave et al. (2018) from a lean, complex -chemistry, hydrogen -air flame propagating in intense small-scale turbulence (a ratio of laminar flame thickness to Kolmogorov length scale is about 20) in a box are analyzed using Helmholtz-Hodge decomposition of velocity field into solenoidal and potential components. The obtained results indicate that the flame surface area is created by potential velocity fluctuations generated due to combustion -induced thermal expansion, whereas the rotational motion acts to smooth wrinkles of iso-scalar surfaces within local preheat and reaction zones and, consequently, to reduce the flame surface area. The latter finding challenges the classical concept of flame -generated turbulence and is attributed to flame -generated solenoidal velocity fluctuations. Specifically, the incoming tangential (to the flame) vorticity is suppressed by baroclinic torque, which also generates vorticity in another tangential direction, with the latter (flamegenerated) solenoidal velocity fluctuations working to smooth the flame surface. Thus, even under conditions of moderately intense turbulence, flame surface area can primarily be created by potential velocity perturbations caused by combustion -induced thermal expansion.

Direct numerical simulation data obtained from a highly turbulent (Kolmogorov length scale is less than a laminar flame thickness by a factor of about 20) lean hydrogen–air complex chemistry flame are processed, with the focus of the study being placed on flame and flow characteristics conditioned to instantaneous local values cFx,t of the fuel-based combustion progress variable. By analyzing such conditioned quantities, the following two trends are documented. On the one hand, magnitudes of fluctuations of various local flame characteristics decrease with increasing the combustion progress variable, thus implying that the influence of small-scale (when compared to the laminar flame thickness) turbulence on internal flamelet structure is reduced as the flow advance from unburned reactants to combustion products. On the other hand, neither local turbulence characteristics (conditioned rms velocities, total strain, and enstrophy) nor local characteristics of flame–turbulence interaction (flame strain rate) decrease substantially from the reactant side to the product side. To reconcile these two apparently inconsistent trends, the former is hypothesized to be caused by the following purely kinematic mechanism: residence time of turbulence within a large part of a local flamelet is significantly shortened due to combustion-induced acceleration of the local flow in the direction normal to the flamelet. This residence-time reduction with increasing cF is especially strong in the preheat zone (cF<0.3) and the residence time is very short for 0.3<cF<0.8. Therefore, small-scale turbulence penetrating the latter zone is unable to significantly perturb its local structure. Finally, numerical results that indirectly support this hypothesis are discussed.

To explore the direction of inter-scale transfer of scalar variance between subgrid scale (SGS) and resolved scalar fields, direct numerical simulation data obtained earlier from two complex-chemistry lean hydrogen–air flames are analysed by applying Helmholtz–Hodge decomposition (HHD) to the simulated velocity fields. Computed results show backscatter of scalar (combustion progress variable $c$ ) variance, i.e. its transfer from SGS to resolved scales, even in a highly turbulent flame characterized by a unity-order Damköhler number and a ratio of Kolmogorov length scale to thermal laminar flame thickness as low as 0.05. Analysis of scalar fluxes associated with the solenoidal and potential velocity fields yielded by HHD shows that the documented backscatter stems primarily from the potential velocity perturbations generated due to dilatation in instantaneous local flames, with the backscatter being substantially promoted by a close alignment of the spatial gradient of mean scalar progress variable and the potential-velocity contribution to the local SGS scalar flux. The alignment is associated with the fact that combustion-induced thermal expansion increases local velocity in the direction of $\boldsymbol {\nabla } c$ . These results call for development of SGS models capable of predicting backscatter of scalar variance in turbulent flames in large eddy simulations.

To explore effects of pressure on the magnitude of the influence of differences in molecular transport coefficients on turbulent flame speed S T , experiments with statistically spherical flame kernels expanding in homogeneous isotropic turbulence in a fan-stirred bomb were performed. Flame speeds were evaluated by analyzing flame images obtained using a high-speed Schlieren technique. To reach the study goals, the measurements were done at three different pressures ( P = 1, 3, and 5 atm) with three different mixtures: (i) lean (the equivalence ratio & phi;= 0.45) H 2 /air mixture, (ii) lean ( & phi;= 0.45) H 2 /O 2 /He mixture, and (iii) the stoichiometric CH 4 /air mixture. Mixtures (i) and (iii) are characterized by significantly different Lewis numbers ( Le = 0.35 and Le & AP;1, respectively), approximately equal thermal laminar flame thicknesses 8L , and close values of the laminar flame speed S L at the three pressures. Combustion chemistry is expected to be weakly affected by substitution of N 2 in mixture (i) with He in mixture (ii), with this substitution increasing Le to 0.91, but the two mixtures are characterized by significantly different 8L at the three pressures and different S L at P = 3 and 5 atm. Comparison of the normalized turbulent flame speeds S T / S L obtained from the lean H 2 /air flames and stoichiometric CH 4 /air flames shows that S T / S L is significantly higher in the former flames, with the difference being significantly increased by P . These experimental data indicate a substantial increase in the magnitude of the influence of differential diffusion effects on S T with pressure. An analysis of data obtained from the lean H 2 /O 2 /He flames further supports this conclusion. Based on results of numerical simulations of complex-chemistry laminar premixed flames, the pressure-dependence of the magnitude of the influence of differential diffusion effects on S T / S L is attributed to an increase in the Zel'dovich number by pressure, with the latter effect being most pronounced for the lean H 2 /O 2 /He mixture.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Direct numerical simulation data obtained from four pairs of turbulent, lean hydrogen–air, complex-chemistry flames are analysed to explore the influence of molecular diffusion on flame surface density, displacement speed $S_d$ and the flame surface density transport equation terms. Each pair involves (i) a flame where mixture-averaged molecular diffusivities are adopted and Lewis number $Le$ is significantly less than unity and (ii) an equidiffusive flame where all molecular diffusivities are set equal to molecular heat diffusivity of the mixture and $Le=1$ , with other things being equal. Reported results show that significantly higher turbulent burning rates simulated in the former flames result mainly from an increase in the local fuel consumption rate, whereas an increase in flame surface area plays a secondary role, especially in more intense turbulence. The rate increase stems from (i) an increase in the peak local fuel consumption rate and (ii) an increase in a width of a zone where the rate is significant. The latter phenomenon is of more importance in richer flames and both phenomena are most pronounced in the vicinity of the flame leading edges, thus indicating a crucial role played by the leading edge of a premixed turbulent flame in its propagation. Moreover, mean displacement speed differs significantly from the laminar flame speed even in the equidiffusive flames, varies substantially across flame brush and may be negative at the leading edges of highly turbulent flames.

By (i) highlighting the mitigation effect of strain rates on laminar flame instabilities and (ii) comparing peak growth rates of laminar flame instabilities with strain rates generated by small-scale turbulent eddies, a simple criterion of importance of the influence of the instabilities on an increase in premixed flame surface area in turbulent flows is suggested. The criterion implies that, even in lean hydrogen-air mixtures, laminar flame instabilities can significantly affect the flame area only in weak or moderate turbulence (the Karlovitz number defined using laminar flame speed, thermal flame thickness, and Kolmogorov time scale is on the order of 10 or less under room conditions).

Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by Dave et al. ( J Fluid Mech 2020; 884: A46) from a statistically planar and one-dimensional, highly turbulent, moderately lean hydrogen-air flame propagating in a box are processed to perform a priori test of perfectly stirred reactor model. The test aims in particularly at estimating mesh resolution (or filter width within large eddy simulation framework) required to neglect variations in the temperature and mixture composition within a computational cell when evaluating mean (or filtered) fuel consumption and heat release rates. For this purpose, fuel consumption and heat release rates sampled directly from the DNS data and averaged over a cube of width [Formula: see text] are compared with fuel consumption and heat release rates calculated using the temperature and species concentrations averaged over the same cube. Moreover, turbulent burning velocities computed by integrating the former and latter rates are compared with one another. A ratio of [Formula: see text] to a laminar flame thickness [Formula: see text] is varied from 0.44 to 1.8. The obtained results indicate that the tested simple approach performs reasonably well (poor) if [Formula: see text] ([Formula: see text], respectively). This result is further supported by directly filtering fuel consumption rate in a laminar premixed flame. The values of the thickness [Formula: see text], calculated using detail chemical mechanisms for different fuels under elevated temperatures and pressures associated with combustion in piston engines, indicate that it is difficult to satisfy the constraint of [Formula: see text] in contemporary unsteady multidimensional numerical simulations of turbulent burning in such engines.