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In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a premixed flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local flame thickness in a turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇ c |, i.e., the absolute value of 3D gradient of the combustion progress variable c , or (ii) 1/|∇ c |, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and terms are extracted from DNS data obtained from (i) a highly turbulent, constant-density, dynamically passive, single-reaction wave, (ii) moderately and highly turbulent, single-step-chemistry flames, and (iii) moderately and highly turbulent, complex-chemistry lean methane-air flames. In all those cases, all studied flame thickness measures are reduced during an early stage of premixed turbulent flame development, followed by local flame re-broadening at later stages. Analysis of various terms in the aforementioned evolution equations shows that the initial local flame thinning is controlled by turbulent strain rates. The subsequent local flame re-broadening is controlled by (i) curvature contribution to the stretch rate, which counter-balances the strain rate, (ii) spatial non-uniformities of the normal diffusion contribution to the local displacement-speed vector S d n , and (iii) dilatation, which plays an important role in moderately turbulent flames, but a minor role in highly turbulent flames. Moreover, the present study shows that differently defined measures of a local flame thickness can be substantially different. This difference should also be borne in mind when comparing data that indicate local flame thinning with data that indicate local flame broadening. [ABSTRACT FROM AUTHOR]

This paper reports on Large Eddy Simulation (LES) of turbulent premixed methane/air flames approaching blow-off. The study focuses on a stable flame, and on a flame just prior to blow-off, both stabilized by the Cambridge bluff-body burner. For turbulence-chemistry interaction, a model based on transported probability density function (TPDF) in conjunction with Eulerian stochastic fields is used. Velocity, species-concentration and heat release fields were first compared against experimental data showing good agreement. The results demonstrate that simulations of such complex combustion phenomena are possible and that the model is capable of reproducing the flame and the flow characteristics under both stable and close to blow-off conditions. A blow-off sequence was then examined and the results were used to evaluate some of the theories and mechanisms responsible for flame blow-off. It was found that the local extinction in the shear-layers had only minor impact on the flame blowing off and that the blow-off is a result of a series of events starting with the flame migrating into the recirculation zone. In the end, a mechanistic explanation is proposed for this series of events leading to full extinction of the flame. [ABSTRACT FROM AUTHOR]

Simultaneous planar laser-induced fluorescence (PLIF) measurements of a series of reactive scalars and Rayleigh scattering measurements of temperature, i.e. CH/CH 2 O/OH, HCO/CH 2 O/OH and T /CH 2 O/OH, and laser Doppler anemometry (LDA) measurements are carried out to characterize the flame/turbulence interaction in various regimes of turbulent combustion, including the laminar flamelet regime, the thin reaction zone (TRZ) regime, and the distributed reaction zone (DRZ) regime. A series of turbulent premixed methane/air jet flames with different jet speeds and equivalence ratios are studied. The jet Reynolds number ranges from 6000 to 40,000 and the Karlovitz number ( Ka ) of the studied flames varies from 25 to 1470. It is shown that in the TRZ regime CH/HCO layer remain thin but the layer of CH 2 O and temperature gradient are broadened owing to the rapid turbulence transport. In the DRZ regime the CH and HCO layers are also broadened owing to the rapid transport of reactive species such as OH radicals from the high temperature regions where these radicals are formed to the low temperature region. In the DRZ regime CH and HCO are found to coexist with OH or CH 2 O owing to the rapid turbulence eddy interaction, which differs fundamentally from that in the TRZ regime and the laminar flamelet regime. For the present investigated flames, the temperature range for the distributed reaction to occur is found to be between 1100 K and 1500 K. It is shown that the structures of flames in different regimes can affect the turbulence field differently. In the DRZ regime the temperature gradient is lower than that in the laminar flamelet and the TRZ regimes, which results in a lower peak of turbulence intensity owing to the retarded velocity gradient across the flames and thereby a lower rate of turbulence production. [ABSTRACT FROM AUTHOR]

Simultaneous two-species imaging using single-shot planar laser-induced fluorescence have been performed to record high quality image pairs of CH/OH, CH/CH 2 O and OH/CH 2 O to visualize the flame front structures in swirl-stabilized lean premixed methane/air flames. The results show that the investigated flames exhibit various flame front structures distinctly in space, which covers: (1) the corrugated flamelet at the leading front; (2) the thin reaction-zone layer with distorted preheat zone in the shear-layer downstream; and (3) quenching, re-ignition and distributed reactions further downstream. The large variation of the flame characteristics in space stems from the entrainment of ambient cold air to the flame that results in flame quenching at the trailing edge of the flame. Thereafter, the unburned fuel/air mixture in the downstream region mixes with the entrained air and the hot combustion products from the upstream leading flame front, leading to reignition with distributed reactions. The current results provide a direct experimental evidence that distributed reactions can be a common combustion mode along with the results (Ref. [1], Zhou et al., 2015) recently reported in the highly turbulent premixed jet flames. [ABSTRACT FROM AUTHOR]