Comparison of characteristics of unstable laminar and turbulent lean hydrogen–air flames

Analyzed in the paper are three-dimensional complex-chemistry direct numerical simulation data obtained from lean hydrogen–air flames under various conditions (25 cases; equivalence ratios ϕ=0.35 and 0.5; the normalized root mean square velocity 0.25≤u′/SL≤54.1; the normalized turbulence length scale 0.5≤L/δLT≤2.6; and Karlovitz number 1.0≤Ka≤1315, where SL and δLT are laminar flame speed and thickness, respectively). These data are supplemented with data obtained in two-dimensional and three-dimensional complex-chemistry simulations of unstable laminar flames in the same computational domains. The reported results show that diffusional-thermal instability (DTIns) dominates turbulence in three weakly turbulent flames (Ka≤1.5). In more intense turbulence, burning rate is still significantly increased by differential diffusion effects, but manifestations of DTIns appear to be less pronounced. Specifically, (i) contribution of highly curved reaction zones (curvature radius is smaller than the instability neutral wavelength) to the bulk burning rate is significantly increased with increasing u′/SL and Ka, (ii) amplitudes of local perturbations in the equivalence ratio, temperature, fuel consumption, or heat release rate are significantly different in unstable laminar and turbulent flames, and (iii) bulk burning rates are also significantly different. The emphasized differences are increased with increasing u′/SL and Ka. These trends imply a decreasing role played by DTIns in turbulent flames at higher Karlovitz numbers, whereas differential diffusion effects result in significantly increasing bulk burning rate at Ka as large as 1315.