Effect of layout and below-bed ventilation on burning rate of porous fuel beds

Abstract

Wood cribs are often used as ignition sources for room fire tests. A fundamental understanding of the mechanisms that govern the burning rate of a wood crib may also have applications to wildland fires. The burning rate of unconfined cribs has long been identified to occur in two regimes: the densely-packed regime where the burning rate is proportional to the crib porosity and the loosely-packed regime where the burning rate is independent of porosity. Though the cribs used to define these burning regimes were primarily cubic in dimension, there are seemingly endless possible ways to build a crib with a given porosity. This work explores the burning rate of cribs with a wide variety of geometries in the loosely-packed regime to determine whether the porosity-burning rate relation in the literature holds. The porosity was kept approximately constant while the number of sticks per layer, number of layers and the length to thickness ratios (l/b) were varied. For l/b less than 36, the burning rate of all cribs matched the porosity-burning rate relation from the literature. For larger l/b, the burning rate was considerably reduced, implying that the crib porosity is a function of l/b above some critical threshold. A second set of experiments was performed to examine the effect of the spacing distance between the crib and the support platform. The critical spacing distance was shown to be larger than previously thought and a function of the length to thickness ratio. It was quite apparent that as the l/b ratio increases, a significant portion of the required oxidizer comes from the bottom of the crib, even for supposedly loosely-packed cribs. For the cribs with l/b = 48, a larger crib-platform spacing the increased the burning rate of these cribs so that the burning rate was closer to the cribs with smaller l/b. For cribs with l/b = 96, the burning rate was still well below the predicted value, even for large crib-platform spacing. Future work will focus on exploring the burning rate and the effect of the crib-platform spacing for cribs with large l/b.