Monday, 17 November 2003
Non-local chemistry implementation in HIGRAD/FIRETEC
In order to study the interactions between the important processes within a wildland fire, Los Alamos National Laboratory and the USDA Forest Service are continuing to develop the HIGRAD/FIRETEC wildfire behavior model. 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. Currently, all of the chemistry and heat production in HIGRAD/FIRETEC occurs within the same computational cell as the burning wood and a fraction of the heat produced is returned to the wood to sustain combustion. In some situations, this is very realistic, however in other situations, this treatment is not as realistic since it is possible for volitalized hydrocarbons to travel tens of meters before they combust. We are investigating the trade offs involved in a non-local burning scheme by separating the pyrolysis from the combustion of the various pyrolysis products. The initial products of pyrolysis (an endothermic process) in our scheme are char, a generic hydrocarbon mixture, and inert gas. We are allowing the overall gas/char ratio to vary with temperature and fuel type (e.g. grass, oak or pine). In a separate step the hydrocarbon mixture is allowed to mix with oxygen and combust the only product being an inert gas. The hydrocarbon combustion is exothermic and a fraction of the heat produced is returned to the wood to sustain combustion. Char is tracked explicitly and allowed to combust in a slower exothermic step. Here we report on preliminary investigations into this simple non-local chemistry scheme.