Description

A model was developed to predict the ignition of forest crown fuels above a surface fire based on heat transfer theory. The crown fuel ignition model (CFIM) integrates fluid dynamics and heat transfer principles with empirical formulations where our knowledge of the physical processes is still incomplete or unsatisfactory. The CFIM uses surface fire flame front properties to define the heat source, determines the buoyant plume dynamics and heat transfer (gain and losses) to the crown fuels. Fuel particle temperature increase is determined through an energy balance relating heat absorption to fuel particle temperature. The final CFIM output is the temperature of the crown fuel particles which upon reaching ignition temperature are assumed to ignite. The performance of the CFIM was evaluated through the analysis of its behaviour and comparison against other crown fire initiation models. Results indicate that the primary factors influencing crown fuel ignition are those determining the depth of the surface fire burning zone and the vertical distance between the ground/surface fuel strata and the lower boundary of the crown fuel layer. A comparative analysis of the relative role of surface fuelbed structure and wind and fuel moisture conditions on the likelihood of crowning did not identify any superior role of these variables over surface fuelbed structure, or vice-versa, with respect to inducing crowning. The results suggest that the relative role of these variables are not independent and that their effect varies with the fuel complex characteristics and burning conditions. The simulations indicate that some fuel types showed higher sensitivity to changes in burning conditions, implying a dominance of the role of climate/weather variables, while for other fuel types, changes in the severity of burning conditions on the likelihood of crowning were inconsequential. Comparison of CFIM predictions against predictions from empirical based models gave encouraging results relative to the validity of the model system.