DETERMINATION OF FLAMMABILITY LIMITS USING A TUBULAR FLAME GEOMETRY

Using a porous cylinder burner through which a combustible mixture is injected, the flammability limits in a tubular flame geometry have been investigated experimentally. Results show that flame behaviour in a tubular flame geometry is strongly dependent on the Lewis number (Le) of the deficient species in the mixtures. For mixtures with Le < 1, such as lean methane, rich propane or lean hydrogen mixtures, the flame is small at extinction, whereas for mixtures with Le > 1, the flame does not become small at extinction. Therefore, in the flame of Le > 1 mixtures the heat loss to the burner surface is increased, resulting in narrowing of the flammable range. Results also show that the flame is significantly deformed due to buoyancy when the burner is horizontally mounted. For mixtures with Le < 1, a splitting of a tubular flame occurs in rich propane and lean hydrogen mixtures, resulting in a narrowing of the flammable range. At dilution limits, the flames are well folded in the burner, resulting in widening of the flammable range respectively. Regardless of any deformations, the flammability limits determined by this method give somewhat wider flammable ranges than the standard method. The lean and rich limits for methane are 4.7% and 15.1% and those for propane are 2.0% and 9.8%, respectively. The lean limit for hydrogen is 4.2%. The dilution limit is 45% for methane and 50% for propane. The extension of the flammable limits seems to be mainly due to a reduction of the downstream heat loss in the present system.