Laser Extinguishment of CH_4-N_2/Air Counterflow Diffusion Flame

We have examined the laser extinguishment of a CH_4-N_2/Air counterflow diffusion flame. Laser extinguishment is to blow off flames by a laser-induced breakdown. A Q-switched Nd:YAG laser (energy; 200mJ/pulse) and a 100-mm focal-length lens were used. The laser-extinguishment probabilities were examined by varying the fuel concentration and the distance from the breakdown point to the flame. As a result, the two concentration limits were clarified. One was the laser extinguishable limit, below which the flame could be extinguished perfectly by laser. The other was the laser inextinguishable limit, over which any flame could not be extinguished. Moreover, it was considered that determination of the laser extinguishable limit was mainly influenced by the laser-induced blast wave. On the other hand, it could be considered that not only the blast wave but also hot gas and turbulence due to the optical breakdown influenced the determination of the laser inextinguishable limit.