Numerical study of piloted ignition of forest fuel layer

Abstract

A numerical study is carried out to analyze piloted ignition of forest fuel layer composed of oven dried maritime pine needles exposed to external heat fluxes up to 30 kW m −2. Two fuel volume fractions are considered. The model solves a set of time-dependent conservation equations for both phases, the gas and the vegetation elements, coupled through exchange terms relative to mass, momentum and energy transfers. The solid-phase processes include drying, pyrolysis and char oxidation. Pyrolysis products are considered to be a mixture of CH 4, CO, CO 2 and H 2O, and gas phase chemical reactions are modeled by a two-step global kinetic mechanism for the oxidation of CH 4 and CO. The radiative heat transfers are determined by solving the multiphase radiative transfer equation in which both absorption and scattering due to the solid phase and the contribution of gaseous products are considered. Ignition is determined by the onset of thermal runway in the gas phase. Predicted times to ignition and mass loss at ignition are found in satisfactory agreement with data obtained in previous bench-scales experiments carry out in Tewarson calorimeter (Fire Propagation Apparatus). Results show that the weak moisture content resulting from self-rehydration has a moderate effect on the ignition delays but influences significantly the mass losses at ignition. Char oxidation can be neglected for high external heat fluxes but affects significantly the ignition process for heat fluxes close to the critical heat flux for ignition. For a given fuel volume fraction, the mass flow rate of combustible gases at the surface of the fuel bed at ignition is found to be independent on the external heat flux. This critical mass flow rate increases as the fuel volume fraction decreases.