Extinction condition of counterflow diffusion flame with polydisperse water sprays

Abstract

We experimentally investigated the effect of the mean droplet diameter of polydisperse water spray on the oxygen concentration at the extinction of a counterflow diffusion flame. Water spray produced by a twin fluid atomizer and oxidizer was introduced from the lower duct while methane as a fuel gas flowed from the upper duct. The burning behavior of the counterflow flame with water spray was observed and the extinction condition was gained by decreasing the oxygen concentration of the oxidizer stream for different water droplet size distribution. The results show that the optimum mean droplet diameter of polydisperse water spray for suppressing fire exists in an area where the droplets have large penetration to the counterflow flame and short time vaporization. The optimum droplet size shifted to a smaller droplet size when increasing the strain rate of the counterflow. These results can be explained based on the coupled effect of the droplet behavior in the vicinity of the counterflow flame such as the Stokes number effect and the vaporization Damköler number effect. A new parameter for characterizing the extinction condition for a relatively small vaporization Damköler number is also presented.