Fuel preheating in free-burning fires

The propagation of a line fire front in a brush fire and a ground fire burning through combustible debris was studied. The complex integral equation for the propagation of the ignition zone in a brush fire was solved by the zoning technique used in gas-radiation calculations, with absorption coefficient or mean free path evaluated for the randomly arranged elements of the fuel bed. The solution yielded the shape of the ignition zone or locus of the burning front, the velocity of propagation of the zone, and fuel-bed temperatures ahead of the ignition front as a function of parameters describing the bed compactness, the ambient air, the flame plume, and the ember temperature. Experimental modeling studies were made with shredded newsprint and wood excelsior to simulate, respectively, fuel beds in ground and brush fires. Results from the ground-fire studies suggested a means of resolving a disagreement between theory and experiment encountered in an earlier investigation of ground-fire behavior. Although the application of the zoning technique for the brush-fire studies demonstrated its utility for predicting the effects of action-at-a-distance phenomena in free-burning fires, the treatment stopped short of provision for adequate ground-level boundary conditions. A method of allowing for this is suggested.