Experimental study of the influence of Lewis number, laminar flame thickness, temperature, and pressure on turbulent flame speed using hydrogen and methane fuels

To experimentally explore the influence of Lewis number Le, laminar flame thickness δ_L, pressure P, and unburned gas temperature T_u on turbulent flame speed S_T, a set of conditions is designed by adjusting nitrogen mole fraction in lean H₂/O₂/N₂ (Le=0.35) and stoichiometric CH₄/O₂/N₂ (Le≈1) mixtures. The adjustment is performed by simulating complex-chemistry laminar flames to obtain the same laminar flame speeds S_L not only for different fuels, but also for different pressures (1, 2, and 5 atm). The mixtures are characterized by significantly different S_L at T_u=300 K and 400 K, whereas variations in δ_L with the temperature are sufficiently weak. Moreover, laminar flame thicknesses are approximately equal for H₂-based and CH₄-based mixtures at the same P, but are significantly decreased with increasing pressure. For this set of conditions, S_T is measured by applying schlieren imaging techniques to film expansion of centrally ignited, statistically spherical flames in homogeneous isotropic turbulence generated by a dual-chamber, constant-pressure, fan-stirred explosion facility. Analyses of the measured data show the following trends. First, turbulent flame speed is increased by both P and T_u, whereas S_T/S_L is decreased with increasing T_u. Second, turbulent flame speed measured at different P and T_u can be predicted by allowing for S_L(P,T_u) and δ_L(P,T_u). Thus, the present data do not call for explicitly substituting normalized pressure or temperature into a turbulent flame speed approximation. Third, S_T is increased with decreasing laminar flame thickness. Fourth, speeds of the lean H₂/O₂/N₂ flames are higher when compared to the stoichiometric CH₄/O₂/N₂ flames, with this difference is increased (reduced) by P (T_u, respectively). Fifth, all measured data on S_T can quantitatively be described by substituting S_L and δ_L with the counterpart characteristics of highly strained twin laminar flames. The latter finding supports leading point concept of premixed turbulent combustion.