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, currently a 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 is studying 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 should apply to wildland fires.
Using a wide variety of layouts and geometries, wood cribs were burned to determine whether results from structural fire hold in the wildland. Comparisons included the effect of stick dimension (length and width) ratios and the effect of the vertical gap between the crib and the ground. McAllister found a correlation that predicts the burning rate for even the most unconventional crib designs as well as shapes more likely to resemble wildland fuel beds.
A correction factor was also developed that adjusts this predicted burning rate for the changes that occur as the vertical gap between the ground and the fuel bed varies. Since this vertical gap changes the burning rate by as much as 90 percent, the correction factor will be critical when comparing surface fuels to crown fuels.
As wildland fires rarely occur on calm days, the effect of wind is also being considered. Cribs of a wide variety of designs are being burned in the Fire Lab’s wind tunnel. Preliminary results indicate that the effect of wind depends on the fuel element thickness. Cribs built with thicker fuels show an increase in burning rate with wind, while cribs built with thinner fuels show a decrease. Work is ongoing to confirm and explain these trends.