Effects of Karlovitz number on super-adiabatic temperature and burning rate in lean hydrogen-air turbulent flames

Analyzed in the paper are previously published and newly generated three-dimensional direct numerical simulation data obtained from statistically planar and one-dimensional, lean complex-chemistry H2-air flames propagating in a box with forced turbulence. Under the study conditions, root-mean-square turbulent velocity is varied from 2.2 to 54 laminar flame speeds SL, integral length scale of turbulence is varied from 0.5 to 2.25 laminar flame thicknesses, Damköhler and Karlovitz numbers are varied from 0.01 to 0.53 and from 10 to 1315, respectively. Two equivalence ratios, 0.5 and 0.35, are explored. Turbulent burning velocities are evaluated not only for these eight low Lewis number flames, but also for equidiffusive counterparts to them. Analyses of these burning velocities and conditioned profiles of temperature, fuel consumption, and heat release rates show significant influence of differential diffusion effects on the local structure of flame reaction zones and burning rate in all eight cases, even at Karlovitz number Ka as high as 1315. On the contrary, both magnitude of superadiabatic temperature and probability of finding it are decreased with increasing Ka. The latter trend is attributed to intensification of turbulent mixing in flame oxidation zones where fuel consumption rate is low. Therefore, the two phenomena (increase in burning rate due to differential diffusion effects and appearance of superadiabatic hot spots) are not inextricably linked. Differential diffusion effects can increase burning rate even if turbulence is intense enough to erase superadiabatic hot spots.