WIND-AIDED FIRESPREAD ACROSS ARRAYS OF DISCRETE FUEL-ELEMENTS .1. THEORY

The propagation for spread rate of wind-aided firespread across an array of very-small-diame- ter, discrete fuel elements is sought, as a step toward the objective of predicting the advance in time of a firefront through either strewn debris in a heavily-blast-damaged scenario or through brush-and-grass-type wildlands. Here, the quasisteady rate (if one exists) is sought for conditions under which there is both (I) a wind whose mean speed U is constant and whose direction is constant, and (2) a horizontal bed with a macroscopically uniform fuel distribution m. More precisely, m is the mass of fuel (per unit planform area of the bed) consumed with firefront passage; here the fuel elements are thin, so that, for conditions under which fire propagates at all, the fuel consumed is identical with the fuel loading initially present. In accord with laboratory-scale experiments in a specially dedicated firetunnel, to be reported separately, analysis suggests that vfvaries with (U/m)l/2over a wide (but obviously finite) range of the parameters U and m. Also in accord with observations, the model adopted postulates a steadily translating firefront of finite streamwise expanse sandwiched between a thoroughly burned-out upwind expanse of the bed and a downwind expanse which is undergoing preheating from the ambient temperature to the onset-of-pyrolysis temperature. The analytic modeling further suggests the inference that fresh fuel is preheated to sustain the firespread by convection/diffusion. Only if the thin-fuel loading m approaches the rather large value of very roughly 2g/cm2is radiative preheating likely to play a role comparable to that of convective/diffusive preheating. © 1991, Taylor & Francis Group, LLC. All rights reserved.