Structure of downward spreading flames: a comparison of numerical simulation, experimental results and a simplified parabolic theory

Temperature and velocity fields in a downward flame spread over flat, solid fuels in a gravitational field are numerically simulated and compared with available experimental measurements and a simplified theory. The two-dimensional steady numerical model solves the mass, energy, species-mass, and momentum equations in the gaseous phase and the energy equation in the solid phase and includes gas-phase and pyrolysis kinetics, gas and surface radiation with radiation feedback. The published experimental results include measured temperature and velocity profiles, surface regression data, visible images of flames, and images taken with interferometers. A simplified parabolic solution for the temperature field in an opposed-flow configuration is extended to the downward configuration and compared with the simulation and experimental results. Flames over thin cellulosic fuels and PMMA, both in the thick and thin limits are considered. With one exception, the numerical model is found to reproduce the observed flame structure for a diverse range of fuel and ambient conditions. The simplified theory, based on a parabolic solution of the coupling functions, is found to reproduce the temperature fields in the gas and the solid reasonably well for flame spreads over thick fuels.