An LBM-FDM coupled numerical model for ignition and transient downward flame spread over solid fuel

The ignition and subsequent transition to steady-state flame spread behaviors over a vertical solid fuel in normal gravity and static environment were numerically studied in this work. Unlike the general approach based on the NS equation for buoyancy flow, the lattice Boltzmann method (LBM) was employed to solve the velocity field in the gas phase. Elliptic equations of temperature and concentration in gas and solid phases were solved by the finite difference method (FDM). An LBM-FDM coupling model for investigating the downward flame spread over solid fuel was developed. The ignition and the flame propagation behavior can be predicted quantitatively. The results show that a point flame with the shape of a disk appears first at the sheet surface when the vertical solid fuel is ignited. The instantaneous temperature response of ignition lags behind the chemical reaction. The point flame gradually develops into an asymmetric arch-shaped flame with two flame fronts near the sample surface. And then, the flame in the downstream zone is separated from the upstream flame source under the effect of the induced buoyant flow, and eventually disappears at the downstream boundary. Finally, the flame on the upstream gradually develops into a steady downward flame spread. The developed model in this work is validated by comparing the gas-phase temperature and flame spread rate between the predicted results and previous experimental data. This paper provides another method to predict the development of solid flame spread.