Experimental and Numerical Modeling of Shrub Crown Fire Initiation

The transition of fire from dry surface fuels to wet shrub crown fuels was studied using laboratory experiments and a simple physical model to gain a better understanding of the transition process. In the experiments, we investigated the effects of varying vertical distances between surface and crown fuels (crown base height), and of the wind speed on crown fire initiation. The experimental setup was designed to model an isolated clump of crown fuel such as a single tree or group of shrubs. Three wind velocities (0, 1.5, and 1.8ms-1) and three crown base heights (0.20, 0.30, and 0.40m) were used. Crown fuel (solid) and the air temperature within the elevated fuel bed were measured. Crown bulk density and fuel moisture content were held constant in all the experiments. As crown base height increased, crown fire initiation success decreased. Non-zero wind speeds reduced crown fire initiation success because of reduced heating. A simple physical model based on convective and radiative heat exchanges was developed to predict crown fire initiation above a surface fire. The predicted results for different wind speeds and crown base heights were in good agreement with the experimental measurements. Because of its relative simplicity and inclusion of basic physics, it is anticipated that the model can be readily applied and/or adapted to model diverse fuel configurations.