Flammability limits for methane-air and propane-air mixtures were measured using the counterflow type burner which consisted of a couple of converging-diverging nozzles made of quartz glass. As several types of flames other than the usual twin flames were obtained in this burner, a simple theoretical analysis on the shape and nature of the flame developing in the counterflow field was first made, and flame stability and correspondence between the theoretically obtained and experimentally observed flame shapes were discussed. Flammability limits were then discussed with respect to the flame stretch effect and the preferential diffusion effect.
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