Propagation speeds of expanding turbulent flames of C₄ to C₈ n-alkanes at elevated pressures: Experimental determination, fuel similarity, and stretch-affected local extinction

In this study we experimentally investigated the propagation and the associated propagation speeds of expanding C₄ to C₈ n-alkane flames in near-isotropic turbulence, from 1 to 5 atm, using a constant-pressure, dual-chamber, fan-stirred vessel. The motivation of the work is to explore whether the previously observed fuel similarity for C₄-C₈ n-alkanes for laminar flames also holds for turbulent flames, and to investigate the possible influence of mixture nonequidiffusion on the stretch-affected flame structure especially the occurrence of local extinction. Extensive results show that within the present parametric range of investigation, relevant for the flamelet and thin-reaction zones in the conventional turbulent combustion regime diagram, the turbulent flame speeds of stoichiometric and rich mixtures of these fuels, whose Lewis numbers (Le) are close to or smaller than unity, indeed assume similar values at various pressures, equivalence ratios and turbulence intensities. However, the corresponding lean flames, whose Le is greater than unity, exhibit strong propensity of local extinction, ostensibly caused by local stretch through the Le > 1 mixture nonequidiffusion.