Effect of crib dimensions on burning rate

Abstract

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 to determine whether the porosity-burning rate relation in the literature holds. One set of experiments was performed to validate the testing apparatus against the known data in the literature. A second set of experiments explored the effect of crib layout in the loosely-packed crib regime. 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 third set of experiments was performed to examine the effect of the spacing distance between the crib and the support platform. The effect of spacing distance is strongly dependent on l/b, with no difference seen for l/b = 10 and a 60% change for l/b =96. Future work will focus on exploring the burning rate and the effect of the crib-platform spacing for cribs with large l/b.