Chemical and physical effects of water vapor addition on diffusion flames

Abstract

The use of water mist to extinguish fires is a problem of particular interest since the banning of halogen-based agents for environmental reasons. This interest is reflected in the large number of studies performed on the main mechanisms of extinguishment: heat extraction, oxygen displacement and attenuation of radiant heat fluxes. In contrast, little is still known about the chemical and some other aspects of water vapor addition in hydrocarbon diffusion flames. In this paper, a phenomenological study is conducted of the effect of water vapor addition, through the base of a small-scale heptane pool fire. Heptane, supported on a pool of liquid water, is burned as a pool flame while the water underneath is heated to boiling. This is a unique and original method in that boiling the water under the fuel is used for adding water vapor at the base of a diffusion flame where chemical reactions and air entrainment take place. This region is the most interesting regarding flame reactivity and soot formation and, in spite of the perturbing influence of the injected water vapor, the major characteristics of the flame are conserved. The procedure permits measurements of exhaust gas composition by means of a flow through system. Measurements of temperature, CO and CO 2 species concentrations, and monochromatic absorption coefficient which can be associated with soot concentration, are performed. It is seen that adding water vapor in such a way affects both physical phenomena and chemical reactions. That water vapor inhibits the soot formation and shifts CO to CO 2 is further confirmed by injection of an inert gas instead of water vapor. However, the resulting heat release is not efficient enough to counteract the cooling effect of water vapor and the temperature is significantly decreased.