Extinction and structure of methane/very lean methane-air counterflow diffusion flames

Abstract

Experimental studies of extinction and structure of methane/very lean methane-air counterflow diffusion flames were made using a porous cylinder burner. The extinction limit of diffusion flame, the onset of luminous carbon zone, thermal and chemical structure of the flame, and the limiting concentration of methane diluted with nitrogen, below which a laminar diffusion flame can never be established in the very lean methane-air mixture, were examined. The experimental results show that the critical stagnation velocity gradient for flame extinction and the critical stagnation velocity gradient for the onset of the luminous carbon zone increase almost linearly with the increase of the equivalence ratioof the very lean mixture, although the rate of increase of the latter is not so large as compared with the former. The increase of the flame strength is attributed to the extension of the high temperature region and the increase of the flame temperature. The limiting methane concentration decreases almost linearly with the increase ofin the range up to the lean flammability limit of the mixture. The results suggest that, when the diffusion flames are used in combustion apparatus, it is very attractive to use the very lean mixture as the oxidizer.