Numerical simulations of grass fires using a coupled atmosphere-fire model: Dynamics of fire spread

摘要

Numerical simulations using a coupled atmosphere-fire model (called HIGRAD/FIRETEC) are examined to investigate the dynamics of fire behavior in grasslands, focusing specifically on the relative roles and contributions of radiative and convective heat transfer and the relationships of these processes to the evolution of the solid fuel temperature; the three-dimensional velocity fields in the vicinity of the fire; and the depletion of fuel, fuel moisture, and oxygen. The progression of the fire past a given point in these simulations is divided into a preheating period and an active burning period. The preheating period is characterized by a slowly increasing radiative heating of the fuel that evaporates fuel moisture and raises the temperature of the fuel slightly and by weak convective cooling because the gases flowing over the heated solid fuel are still cooler than the fuel itself. The active burning period is characterized by the presence of a strong pulse of convective heating and continued radiative heating, accompanied by the development of large vertical velocities and a rapid increase in fuel temperature that causes the reaction rates to increase and the fuel to begin to burn, producing heat and increasing the rates of depletion of fuel and oxygen. In all simulations, the magnitude of the convective heat transfer is greater than that of the radiative heat transfer; however, these processes and their relationships to the three-dimensional structure and evolution of the fire are shown to depend both on the ambient wind speed and on the specific location along the fire front (e.g., at the head of the fire where the fire is spreading in the direction of the ambient wind, or on the flank of the fire where the fire is spreading in the direction almost perpendicular to the ambient wind).