An experimental study of effect of inert gases on extinction of laminar diffusion flames

The limiting fuel concentration and the limiting oxygen concentration required to maintain the diffusion flame and the limit flame temperatures were measured using a counterflow diffusion flame established in the forward stagnation region of a porous cylinder. The fuels used were methane and hydrogen, and three kinds of inert gas (nitrogen, argon, and helium) were used as the diluent. The flame temperature at the limiting fuel concentration coincides with that at the limiting oxygen concentration, and therefore, the controlling factor with diffusion flames under limiting conditions is the limit flame temperature. The limit flame temperatures for methane and for hydrogen, diluted with nitrogen, are 1,200°C and 740°C, respectively. The limit flame temperature is considerably higher with helium than with nitrogen or argon. The limit flame temperature of a diffusion flame diluted with an inert gas is closely related to the flame temperature at the lean flammability limit of the premixed combustible gas of the fuel and “air”, including the same inert gas. The lean flammability limit of a premixed flame and the limiting concentrations of the reactants for a diffusion flame are primarily controlled by the same factor. In the special case of the hydrogen diffusion flame where the flame lies on the fuel side of the stagnation point, the nonuniform extinction of the flame caused by the preferential diffusion of hydrogen occurs before the flame temperature is reduced to the limit temperature.