Solid fuel combustion in a forced, turbulent, flat plate flow: The effect of buoyancy

Abstract

An experimental study is conducted of the effect of flow velocity, grid-generated turbulence and buoyancy on the combustion of a solid fuel in a flat plate boundary layer flow. The effect of buoyancy is determined by comparing the surface regression rates, flame characteristics, and exhaust products composition for the solid burning in a floor and ceiling orientation. Measurements are presented for the ceiling surface regression rate of PMMA sheets burning in air, and compared with previously obtained floor data. It is shown that the ceiling and floor data can be correlated in terms of a non-dimensional mass burning rate "x/(αρB) that is approximately linearly proportional to the non-dimensional flow parameter, aRe x 0.5 [1+ b(u′/U) 0.5 ] . It is found that although the overall trends are similar for the ceiling and floor, there are differences caused by buoyancy effects on the flame structure. This is corroborated with measurements of the exhaust species concentrations that show higher CO and HC concentrations in the ceiling than in the floor burning. It is concluded that buoyancy has two major, and counteracting, effects. One is to affect the heat transfer by shifting the flame away from the surface in the floor burning and closer to the surface in the ceiling. The other is also due to the flame shifting and the instabilities associated with it that favors, in the floor case, the mixing of the reactants, and consequently the overall heat release rate. In the ceiling, the proximity of the flame to the cold surface causes quenching of the reaction, and the stability of the flame hampers reactants mixing, both of which reduce the heat generation rate. The overall result of these two counteracting mechanisms is that the surface regression rates are not that different in both geometries, although the composition of the combustion products are quite different.