Analysis of Experimental Simulation of Ground Surface Heating During a Prescribed Burn

Abstract

Ground surface heating during a prescribed bum is modeled. Primarily, the ground surface heating occurs by radiant exchange with the flames. Additional surface heating occurs when hot ash is deposited upon the ground. Heat losses from the ground surface occur by radiant transfer to the sky and surroundings, by conductive transfer of energy into the ground, and by convective transfer to the ambient air. Important flame input characteristics used in the model are: the flame residence time, flame geometry, and flame temperature. Important ash parameters are: inif al ash temperature, location of initial contact of ash with the ground surface, and the thickness of the ash layer. The analysis indicates that in the absence of an ash layer the location of the highest surface temperature is dependent only upon the flame geometry pararneters. However, the magnitude of the ground surface temperature is dependent upon flame geometry, the radiant properties of the flame, and the thermophysical properties of the ground. Results from the model are compared with experimental data. The data were obtained by igniting a prepared fuel bed of chamise and manzanita branches loaded over a table top in which sand filled canisters, instrumented with thermocouples, were set flush with the table surface. The temperature distributions within the sand are found to compare well with the model predictions. Exceptions to the model are found to occur in burns featuring extensive smoldering ember beds.