Numerical analysis of flame spread over a thin fuel inclined from vertically downward to horizontal

The behavior of flame spreading downward over a thin inclined sheet of solid fuel is analyzed numerically in this paper. Since the flame spread has both constant and unsteady flame-spread rate characteristics at different inclinations, a combustion model was developed to study the inclination effect. An unsteady solid phase is coupled with a quasi-steady gas phase via the linkage of mass and energy balance to solve the problem with progression over time. Since the computations are very time-consuming, the model was applied to the downward flame spread problem at only four inclination angles, 90°, 60°, 30° and 0°. The parametric study showed that a constant flame-spread rate flame propagation is still retained at inclination angles of 90° and 60°. But solid fuel inclined at 30° and 0° exhibit unsteady flame-spread rates with nearly periodic accelerations and decelerations. Investigating unstable mechanisms in horizontal flame spread, we find the primary one is that the fuel supply rate by mass diffusion can not match the fuel consumption rate in the flame reaction. The heat flux from the gas phase in the preheat zone is found to be a dominant heat transfer mechanism to accelerate and decelerate flame spread. A comparison made with a constant-rate horizontal flame spread model shows that the unsteady model presented here is better suited to study the problem of unstable flame spread, both on unsteady characteristics and flame-spread rate values.