Extinction of nonpremixed flames with halogenated fire suppressants

Abstract

An experimental, analytical, and numerical study was performed to elucidate the influence of eleven gaseous agents, considered to be substitutes for CF 3Br, on the structure and critical conditions of extinction of diffusion flames burning liquid hydrocarbon fuels. The effectiveness of these agents in quenching flames was compared to those of CF 3Br and an inert diluent such as nitrogen. Experiments were performed on diffusion flames stabilized in the counterflowing as well as in the coflowing configuration. The fuels tested were heptane in the counterflowing configuration, and heptane, the jet fuels JP-8, and JP-5, and hydraulic fluids (military specifications 5606 and 83282) in the coflowing configuration. The oxidizing gas was a mixture of air and the agent. On a mass and mole basis CF 3Br was found to be most effective in quenching the flames and the mass-based effectiveness of the other eleven agents was found to be nearly the same as that of nitrogen. Experimental results were interpreted using one-step, activation-energy asymptotic theories and the results were used to provide a rough indication of the thermal and chemical influence of these agents on the flame structure. To understand in some detail the influence of CF 3Br on the structure and mechanisms of extinction of the flame, numerical calculations using detailed chemistry were performed. The calculated structure of counterflow heptane-air diffusion flames inhibited with CF 3Br was found to consist of three distinct zones including a CF 3Br consumption zone which appears to act as a sink for radicals. The calculated values of the critical conditions of extinction of counterflow heptane-air diffusion flames inhibited with CF 3Br were found to agree fairly well with measurements. The study suggests the need for refinement of the inhibition chemistry.