Species produced in fires burning in two-layered and homogenous vitiated environments

Citation

Morehart, James Henry (1991) Species produced in fires burning in two-layered and homogenous vitiated environments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9xja-9n92. https://resolver.caltech.edu/CaltechETD:etd-07032007-142951

Abstract

The chemical species produced in a buoyant, turbulent diffusion flame exposed initially to a supply of fresh air and extending into a reduced-oxygen environment containing products of combustion are investigated. The stably stratified, vitiated region is formed by placing a hood above a burner so that it accumulates the gases of the fire plume, while the direct injection of air into the upper portion of the hood allows conditions to be studied where the stoichiometry of the collected gases is different than that of the plume flow crossing the interface between these two regions.

Measurements of the composition show that the species produced in the flame depend primarily on the stoichiometry of the gases in the vitiated region, but are independent of the fuel-air ratio of the mass transported across the interface by the plume. A weak dependence of species concentrations on the temperature of the product gas layer was observed over the range 500 to 900 K. Using a detailed chemical kinetics model, the composition of the product gases is found to be stable for the temperatures measured in the experiments, but reactions do occur at temperatures above 700 K.

The effects of varying the fuel's molecular structure on the product gas composition are also considered. Experiments were conducted with natural gas, ethylene, and propylene fuels. The presence of a double bond between carbon atoms appears to assist the combustion process towards further completion. As expected, the behavior of the propylene fuel (which contains both single and double carbon-carbon bonds) fell between that of the other fuels.

Additionally, the minimum oxygen concentration necessary to support a diffusion flame in a homogeneous, vitiated environment is investigated. By submerging the entire flame into the interior volume of the hood, the oxygen content of the supporting atmosphere is controlled. As conditions approach the limit of flammability, radiation from soot in the reaction zone becomes imperceptible, leaving only a weakly-luminous blue flame. Even with significant reductions in both the flame height and luminosity, these fires near the limiting conditions completely consume the fuel and generate no measurable amounts of incomplete combustion products.

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