Experimental and theoretical investigation of non-quasi-homogeneity of flame propagation in stratified media

A combined experimental and theoretical study is presented of the conditions under which flame propagation in stratified media can differ from "quasi-homogeneity", i.e. propagation with the flame speed corresponding to the local equivalence ratio in the unburnt mixture. To this extent, OH chemiluminescence intensity measurements were performed in flames propagating through step-changes in equivalence ratio (from Φ=1 to 0.9, 0.6 and 0.5). The results showed a heat "back-support" into the flames, a phenomenon that was stronger with increasing equivalence ratio steps. In order to theoretically model flame propagation in fuel concentration gradients, heat flux from burnt gases and fuel mass fraction gradient across the reaction sheet were incorporated into the conventional asymptotic solution for flame propagation in perfectly premixed mixtures. Theoretical results provided an explanation for the experimental finding that departure from quasi-homogeneity is more pronounced in leaner mixtures and greater equivalence ratio gradients.