Fire Safety Science Digital Archive

A theoretical model is developed of the effect on the spread of flames over the surface of a thick solid combustible, of the velocity and oxygen concentration of a gas flow opposing the direction of the spread. In the analysis the transient, reactive, gas phase balance equations of energy and species coupled at the interface to the solid phase energy equation are solved numerically to predict the flame spread rate and flame structure dependence on the characteristics of the flow. The calculations for PMMA agree with previous experimental results predicting a spread rate that, for a fixed oxygen concentration, first increases, reaches a maximum and then decreases as the flow velocity is increased. The analysis shows that this behavior is the result of the interaction of two controlling mechanisms: a flame to fuel heat transfer mechanism that dominates at low flow velocities and/or high oxygen concentrations, and a gas phase chemical kinetics mechanism that dominates at high velocities and/or low oxygen concentrations.