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 & 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.