An experimental study of the influence of Lewis number on turbulent flame speed at different pressures

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 ϕ=0.45) H₂/air mixture, (ii) lean (ϕ=0.45) H₂/O₂/He mixture, and (iii) the stoichiometric CH₄/air mixture. Mixtures (i) and (iii) are characterized by significantly different Lewis numbers (Le=0.35 and Le≈1, respectively), approximately equal thermal laminar flame thicknesses δ_L, 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 N₂ 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 δ_L 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 H₂/air flames and stoichiometric CH₄/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 H₂/O₂/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 H₂/O₂/He mixture.