Basic Studies on Flame Stabilization

Abstract

This is a report of initial studies to determine the mechanism of anchoring flame in high-velocity air-fuel streams. Simple bluff shapes were used as flame stabilizers in a constant area 1X 3-in. rectangular duct carrying homogeneous mixtures of air and gaseous fuel. The limiting gas velocity above which flames blew off the stabilizers were studied as it was affected by : (1) The approach gas stream variables, (a) Fuel-air ratio (over the whole range of inflammability), (b) Turbulence (intensities from 0.3 to 8 per cent of the approach stream velocity). (c) Fuel type (commercial propane and city gas), (2) Stabilizer variables, (a) Size (Characteristic dimension from 0.02 to 0.5 in.) (b) Shape (simple, faired, and shortened rods, flat plates, gutters, spheres). (c) Addition or extraction of heat from stabilizer. On the basis of experimental evidence, the following conclusions were reached: (1) Flame stabilization behind bluff obstacles depends on the presence of a recirculating eddy region tha t is filled with hot combustion products and furnishes continuous ignition to the approaching unburned gases. (2) Over the range of stabilizer sizes studied, the blowout velocity under steady flow conditions was found to be affected primarily by the properties of the approach gas stream and the stabilizer thickness. (a) For span-thickness ratios greater than 2, the blowout velocity at a given air-fuel ratio in the absence of approach stream turbulence was independent of the stabilizer shape and proportional to the thickness to the 0.45 power. (b) For span-thickness ratios less than 2, the corresponding blowout velocity was roughly proportional to the first power of the thickness. (c) The stability limits for a stabilizer of given characteristic dimension are unaffected by variation (simulated) of ratio of chamber width to stabilizer thickness over the range 10 to 80. (3) Increasing turbulence in the approach gas stream decreased both the velocity and air-fuel ratio ranges over which flame could be maintained. The effect of imposing turbulence of a given scale and intensity diminishes with increasing ratio of stabilizer thickness to turbulence scale. (4) Heating the stabilizer other than by the flame markedly increases the stability limits, and externally cooling the stabilizer markedly decreases the stability limits. (5) The maximum blowout velocities occur with slightly leaner than stoichiometric mixtures of city gas (26 per cent H2) and richer than stoichiometric for propane. This is attributed to the different ratios of molecular diffusivity to tha t of air for the two fuels. (6) Flame blowoff usually proceeds stepwise, flame propagation throughout the main stream ceasing before the eddy zone behind the stabilizer is quenched.