Blowoff characteristics of edge flames in the stagnant region of an axisymmetric impinging jet

Blowoff characteristics of methane-air and propane-air edge flames are experimentally investigated by using the newly proposed burner that utilizes the stagnation region of an axisymmetric impinging jet. In the burner, "edge flame without premixed flame" can be formed as a hole in the diffusion flame, and, by changing the hole diameter as a parameter, the diffusion flame can be gradually turned from a pure diffusion flame to the diffusion flame with a hole. In the present study, the observation of flame shapes and the blowoff processes, and the measurements of the blowoff and separation limits have been performed. As a results, it is found that the blowoff and separation limits of the diffusion flames with a hole show qualitatively same tendencies with a pure diffusion flame, that is, the flames with a hole have critical stagnation velocity gradients for blowoff and separation phenomena, beyond which the flame can never be stabilized or existed. Especially, the critical stagnation velocity gradient for blowoff of "edge flame without premixed flame" decreases as the hole diameter is increased, and always shows lower value than that of a pure diffusion flame. Finally, it is clarified that Peclet number which is a ratio of the characteristic diffusion time to the characteristic convection time, is an important parameter for edge flame's blowoff process, and the transition of the blowoff process from blowoff without separation to blowoff with separation occurs at a constant I'eclet number.