Phenomenology of methane flame propagation into compositionally stratified, gradually richer mixtures

Flame propagation in compositionally stratified, rich methane air mixtures was investigated experimentally. Compositional stratification was established in a counterflow, tubular burner employing very mild strain and the equivalence ratio in the unburnt mixture was measured with line-Raman imaging of methane. Particular emphasis was placed in the determination of differences with stratified combustion in lean mixtures that was considered in earlier work. It was established that departure from ″quasi-homogeneity″, i.e. from flame propagation that can be viewed as a series of flamelets corresponding to the local equivalence ratio, was more pronounced in most cases of rich mixtures than in lean combustion. In certain cases, we were able to record flame speeds in stratified media that were smaller than the speeds of ″quasi- homogeneous″ propagation, something that was never observed in lean stratified combustion. A significant extension of the rich flammability limit was observed in stratified mixtures and flames propagated all the way into mixtures of equivalence ratio equal to 1.7, which may point to a possibility of hydrogen production through flame propagation into ″super″-flammable mixtures. Also, in the rich mixtures, the propagating flame fronts did not develop the instabilities that were observed in lean mixtures in the vicinity of the flammability limit. This was rationalized based on well-established theoretical results on flame stability.