Laser Extinguishment of CH4-N2/Air Counterflow Diffusion Flame

Abstract

We have examined laser extinguishment of CH4-N2/Air counterflow diffusion flame. Laser extinguishment is to blow off flames with a laser-induced breakdown. Q-switched Nd : YAG laser [wavelength ; 532 nm, energy ; 200 mJ/pulse, pulse duration (FWHM); 6 ns] and focal lens (focal length ; 100 mm) were used for the laser-induced air breakdown. Probabilities of the laser extinguishment were examined by varying the fuel concentration and the distance from the breakdown point to the counterflow diffusion flame. As a result, the two concentration limits of the laser extinguishment were clarified. One was the laser extinguishable limit, below which the counterflow diffusion flame could be extinguished perfectly by the laser-induced breakdown. The other was the laser inextinguishable limit, over which any flame could not be extinguished by the laser-induced break-down. Moreover, we could consider that determination of the laser extinguishable limit was mainly influenced by the laser-induced blast wave. On the other hand, it could be thought that not only blast wave but also hot gas and turbulent mixing produced by the laser-induced breakdown influenced the determination of the laser inextinguishable limit.