Burning Rates of Wood Cribs with Implications for Wildland Fires

Wood cribs are often used as ignition sources for room fire tests and the well characterized burning rates may also have applications to wildland fires. The burning rate of wildland fuel structures, whether the needle layer on the ground or trees and shrubs themselves, is not addressed in any operational fire model and no simple model exists. Several relations exist in the literature for the burning rate of wood cribs, but the cribs used to generate them were built with fairly limited geometries. This work explores the burning rate of cribs with a wide variety of geometries and aspect ratios in the loosely-packed regime to evaluate the rigor of several correlations from the literature. Specifically, stick thicknesses ranged from 0.16 cm to 1.27 cm and lengths from 6.4 cm to 61.0 cm resulting in aspect ratios (stick length/thickness) from 10 cm to 160. As wildland fuel beds occur both directly on the ground and suspended in the air, the effect of the vertical gap between the ground and crib base was also examined. The critical vertical gap was shown to be larger than previously thought (7.6 cm for all cribs) and a function of the aspect ratio. It was quite apparent that as the aspect ratio increases, a significant portion of the required oxidizer comes from the bottom of the crib. A relation is then found to adjust the predicted values for the reduction in burning rate due to insufficient vertical gap.