Power of the fire—a thermodynamic analysis

Abstract

G. M. Byram (1959) proposed a method for computing the ratio of the vertical flux of buoyancy generated by a wildfire (power of the fire P_f ) to the horizontal flux of kinetic energy due to the atmospheric winds (power of the wind P_w). Utilizing this power ratio in his case studies of wildfires, Byram concluded that when the ratio exceeds unity for at least 0.3 km above the fire, phenomena associated with blowup behavior such as firewhirls, spotting, and strong indrafts occur. Though Byram made this claim over 40 years ago, there has been little effort on the part of fire researchers to test it. Byram's calculation of P_f describes an ideal case—a fire that burns in a neutrally stable atmosphere and does not entrain atmospheric air into its convection column. In the present paper, these restrictions are relaxed and Byram's calculations are repeated for a fire whose column entrains air from a non-neutral atmosphere. The new model yields a P_f that is smaller than Byram's P_f by a factor of about 2, is weakly dependent on stability, and is independent of entrainment above a height of 0.5 km. Both models are moderately restricted in that column processes involving liquid water are not considered.