Concurrent Horizontal Flame Spread: The Combined Effect of Oxidizer Flow Velocity, Turbulence and Oxygen Concentration

Abstract

ABSTRACT Experiments have been conducted to study the transport and chemical kinetics mechanisms controlling the spread of flames over the surface of a thick solid fuel in an oxidizing gas flow moving in the direction of flame propagation (concurrent or flow assisted flame spread). In the experiments, the solid fuel (PMMA) is unchanged, but the oxidizer gas flow velocity, turbulence intensity and oxygen concentration, are systematically varied to determine their effect on the flame spread process. This is done by measuring the rate of flame spread, flame length, surface heat flux, exhaust gas temperature, products of combustion and soot. The results of the experiments show that the combined effect of flow velocity, turbulence intensity, and oxygen concentration has a complex influence on the flame spread process. The observed variation of the flame spread rate with the flow parameters appears to be due to a strong influence of these parameters on the flame temperature and length, and on the heat flux from the flame to the solid fuel surface. Measurements of the combustion products provide further information about how the flow parameters affect the flame length and surface heat flux. It is shown that the flame length and heat flux data can be correlated in terms of non-dimensional expressions derived from boundary layer analyses of the problem. The best correlation of the data is obtained for the high flow velocity and oxygen concentration cases, where complete combustion occurs. The flame spread rate is the outcome of the combined effect of the flame length and the heat flux, and it is shown that the experimental data are correlated well with an expression derived from a heat transfer analysis of the problem and the nondimensional expressions for the surface heat flux and flame length. The experiments have yielded results that are potentially important not only in the modeling and prediction of flame spread in corridors, but also in other aspects of fire development and testing such as burning rates, flame lengths and combustion completeness.