Flame quenching in turbulent flowing gaseous mixtures

Abstract

A series of tests has been carried out on flowing combustible mixtures in order to examine the influence of various flow parameters, notably pressure, velocity and turbulence, on quenching distance. A wide range of mixture compositions was employed. In some experiments the nitrogen in the air was partially or totally replaced with oxygen, while in others the nitrogen was replaced by various inert gases such as argon, helium and carbon dioxide. The purpose of these variations in chemical composition was to try and distinguish between the relative importance of thermal and diffusional mechanisms in the quenching of hydrocarbon flames. The fuels employed were methane and propane. The tests were conducted in a 9 cm. square working section through which the combustible mixture flowed at various levels of pressure and turbulence intensity and at velocities up to 30 m/sec. At each test condition the spark energy required to ignite the flowing mixture was measured for several different values of electrode gap width. By plotting a graph of ignition energy against gap width, the quenching distance was obtained as the gap width corresponding to minimum ignition energy. The results obtained generally confirmed the importance of thermal diffusion processes, even at the highest flow velocities. Quenching distance was found to increase with increases, in turbulence intensity and to diminish with increases in pressure and laminar flame speed. No appreciable effect of turbulence scale or flow velocity was observed. Analysis of the experimental data showed that for turbulent, flowing mixtures the quenching distance is obtained as the product of the laminar value and a factor containing the term ( u′/S L ) 0.5 which takes into account the additional quenching due to turbulent diffusion.