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Burning Rate

Understanding Burning Rate and Residence Time of Porous Fuel Beds Using Wood Cribs

Flame residence time is critical to the spread of wildland fires; if it is less than the ignition time, the fire won’t spread. Although even surface fires demonstrate spread thresholds, this is of particular concern when discussing the thresholds for crown fire spread, a currently poorly understood aspect of wildland fire. Curiously, no single theory exists for the prediction of flame residence time. Expressions in the literature vary from linear to quadratic dependence of flame residence time on fuel thickness. Better understanding of flame residence time and burning rate of fuel structures will allow for better fire spread and fire effects predictions.

Research Mechanical Engineer Sara McAllister studied the burning rate of fuel structures to better understand residence time using three-dimensional grids of sticks called cribs, commonly used in the fire protection engineering (structural fire) literature. Cribs were built with different stick thicknesses and densities to vary the burning rate of the source fire. Even though wildland fuels do not have the same predictable arrangement as cribs, wildland fuels are similar to cribs in that they are essentially individual fuel particles arranged with some spacing distance between them. Thus, the fundamental understanding of what governs the burning rate of a crib would apply to the wildland fire context.

Burning rates of cribs with a wide variety of layouts and geometries were explored to determine whether results from structural fire hold in the wildland context. Comparisons included the effect of stick dimension (length and width) ratios and the effect of spacing distance between the crib and the support platform. Cribs tested with geometries similar to those tested in literature matched predicted values well. However, the burning rate of cribs built with sticks of large length-to-thickness ratios (such as long, thin sticks) was considerably lower than predicted, indicating that there is insufficient airflow inside the crib not predicted by current models. The effect of spacing distance between the crib and the support platform was strongly dependent on the stick length-to-thickness ratio, with no difference seen for cubic cribs and a >60 percent change for cribs with large stick length-to-thickness ratios. Experiments indicated that cribs with large length-to-thickness ratios required a substantial amount of airflow through the bottom of the crib. As the crib-platform spacing increased, however, the burning rate of the large length-to-thickness ratio cribs increased to more closely match the predicted values. The effect of other environmental variables, such as the presence of wind and a chimney effect, are also being explored.

Photo: Change in burning regime
Modified: Dec 20, 2016

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