On extinction mechanism of lean limit methane-air flame in a standard flammability tube

Gas-dynamic and qualitative thermal structure of lean methane-air flames propagating upward in a standard flammability tube has been experimentally investigated at near-limit concentrations. Local burning velocity related to the cold and hot flame boundaries, and stretch rates have been measured along studied flames. The measured stretch rate attains its maximum value at the flame tip. It was experimentally found that flame temperature is minimum at the flame tip, which agrees with the observation that flame extinction starts from the leading point, but cannot be explained by the combined effect of stretch rate and preferential diffusion. In the coordinate system moving with the flame, a zone of stagnation of combustion products was observed in limit flame near its tip, while at higher methane concentrations stagnation zone does not exist. This observation suggests that limit flame extinction behavior is connected with the formation of the stagnation zone: leading edge of the flame can be cooled by heat conduction to the stagnation zone, which rises upward together with flame and is effectively cooled due to radiation heat loss. Simplified analysis of this effect in a single stretched flame is carried out in terms of coupling radiation losses with local stretch rate. It was found that flame is accompanied by a rising upward combustion products flow, resembling a hot bubble and located about 35 cm downstream of the flame.