Separating Combustion From Pyrolysis in HIGRAD/FIRETEC

HIGRAD/FIRETEC is a coupled atmosphere/wildfire behavior model and is based on conservation of mass, momentum, species and energy. HIGRAD/FIRETEC is a three-dimensional transport model that uses a compressible-gas formulation to couple its physics based wildfire model with the motions of the local atmosphere. In its current formulation combustion and pyrolysis are treated as a single process, which depends on the local densities of wood and oxygen, the levels of turbulent diffusion, and a probability distribution function (pdf) for temperature in the solid. The pdf is employed to give an ‘ignited volume fraction, i.e. the fraction of wood within a resolved volume that is actively burning. We are extending this fuel model to deal with pyrolysis and combustion as separate processes. Some fire behaviors, such as flash events, crowning, and fire ‘whorls', may depend on the ability of combustion to take place in a separate spatial location from the pyrolysis. In this new fuel model pyrolysis will be dealt with in roughly the same way as formerly, but it will now be an endothermic process. Instead of producing solely inert gasses, it will now produce a mixture of inert and combustible gasses. Combustion will now be dealt with as a separate process, which is highly exothermic. We will define a separate pdf for the temperature in the gas phase. The basic premise of the HIGRAD/FIRETEC fuel model will be retained, that is that the rate of a reaction is limited by the rate at which the reactants can be brought together (mixing limited). In this fuel model the reactants for pyrolysis can be thought of as heat and wood, for combustion the combustible gas and oxygen. A few simple test cases utilizing idealized geometries will be simulated with both fuel models and the results compared.