Numerical Simulations of Fires in Grasslands: Roles of Convective and Radiative Heat Transfer

Numerical simulations using a coupled atmosphere-fire model (HIGRAD/FIRETEC) are examined to investigate several fundamental aspects of fire behavior in grasslands, and specifically the dependence of this behavior on the ambient atmospheric winds and on the initial length of the fire line. Results from the model show that the forward spread of the simulated fires increases with increasing ambient wind speed, and the spread rates are consistent with those observed in field experiments of grass fires; however, the forward spread also depends significantly on the initial length of the fire line, and for a given ambient wind speed the spread rate for long lines is greater than that for short lines. Moreover, for weak ambient winds, the shape of the fire perimeter is dramatically different from that seen with higher wind speeds.

To explore further the dependence of fire behavior on wind speed, two simulations of long line fires in the presence of different ambient atmospheric wind speeds are examined in greater detail. Specifically, for each of these simulations the respective contributions of convective and radiative heat transfer to grass fire behavior are illustrated in terms of their evolution at specific points on the heading and flanking portions of the fires. These contributions are also described in relation to the simultaneous evolution of mass loss of fuel and water, solid fuel temperature, and local velocities in the simulations. The balances in these heat transfer processes that result in the simulated fire behavior will be described, as will the dependence of these processes on the ambient wind.