Flammability limits upward propagating lean

The effect of natural gravity on flammability limits has been known for a long time. The difference between flammability limits for downward and upward flame propagation was first observed by White [26], for hydrogen/air mixtures. Subsequently, similar effects were also found for other mixtures. For propane flames, the lean flammability limit for both downward and upward propagation was observed to be = 0.53. The rich limits were = 1.64 for downward and = 2.62 for upward propagation. Such wide gap between the flammability limits for rich mixtures is explained in... 

Jarosinski ]., Strehlow R.A., and Azarbarzin A., The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube, Proc. Combust. Inst., 19 1549-1557,1982. 

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