Departure from quasi-homogeneity during laminar flame propagation in lean, compositionally stratified methane-air mixtures

Methane-air flame propagation was studied in compositionally stratified, lean mixtures with a particular emphasis on the determination of conditions for which the propagation does not proceed in a "quasi-homogeneous" manner, i.e. with the laminar flame speed corresponding to the local equivalence ratio in the unburnt mixture. Steady mixture stratification was established in an appropriately designed burner and flame speed was measured with high-speed visualization as a function of the equivalence ratio distribution in the unburnt mixture, which was measured with PLIF of acetone fuel tracer. It was established that significant deviations from "quasi-homogeneity" can occur for mixtures in the vicinity of the lean flammability limit. Back-supported by heat released during combustion with equivalence ratios close to stoichiometric, flames in the stratified medium can reach speeds up to a factor of two higher than the laminar flame speed corresponding to the local equivalence ratio. Also, a significant extension of the lean flammability was observed, since during stratified combustion flames propagated in subflammable mixtures, reaching under certain conditions areas where the equivalence ratio in the unburnt mixture was as low as 0.35. It was established experimentally that the local equivalence ratio gradient was not sufficient to characterize stratified combustion. The results were rationalized in terms of theoretical considerations showing that the heat input into the flame is sufficient to compensate for the decrease of equivalence ratio upstream the propagating flame, thus generating a flame that propagates faster than the one propagating in a homogeneous mixture. This phenomenon is particularly important for mixtures in the vicinity of the lean flammability limit. For propagation into mixtures that are sufficiently far from the flammability limit, "quasi-homogeneity" is a valid assumption.