Separating combustion from pyrolysis in HIGRAD/FIRETEC

Abstract

HIGRAD/FIRETEC is a coupled atmosphere/wildfire behavior model based on conservation of mass, momentum, species, and energy. It combines a three-dimensional transport model that uses a compressible-gas fluid dynamics formulation with a physics-based wildfire model, to represent the coupled behavior of the local atmosphere and wildfire. 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 mixing, and a probability distribution function (PDF) for temperature in the solid. The PDF is employed to estimate the volume fraction that is hot enough to burn. This burning model is now being extended to deal with pyrolysis and combustion as separate processes. Some fire behaviors, such as flash events, crowning, and fire ‘whirls’, may depend on the ability of combustion to take place in a separate spatial location from the pyrolysis. We refer to this burning model as ‘non-local’. In the non-local burning model, pyrolysis is dealt with in roughly the same way as formerly, but now as an endothermic process. Instead of producing solely inert gasses, it now produces a mixture of inert and combustible gasses. Combustion is handled as a separate gas–gas reaction, which is highly exothermic. The basic premise of the HIGRAD/FIRETEC burning model is retained, i.e. that the rate of a reaction is limited by the rate at which the reactants can be brought together (mixing limited). In the non-local burning model, the reactants for pyrolysis can be thought of as heat and wood, for combustion: the reactive gas and oxygen. A few simple test cases that used idealised geometries were simulated with both burning models, and the results were compared. The non-local burning model was found to give results comparable to the local burning model in terms of the fire-line shape and the spread rate for these simple test cases.