Modeling of marginal burning state of fire spread in live chaparral shrub fuel bed

Prescribed burning in chaparral, currently used to manage wildland fuels and reduce wildfire hazard, is often conducted under marginal burning conditions. The relative importance of the fuel and environmental variables that determine fire spread success in chaparral fuels is not quantitatively understood. Based on extensive experimental study, a two-dimensional numerical model for vegetation fire spread was developed to simulate laboratory-scale fires. This model is based on a detailed description of the complex heat transfer processes and a simple combustion mechanism contributing to the ignition of solid fuel and fire spread. The fuel bed is described as a porous medium, and the heterogeneous nature of foliage and branch is considered via specific physical properties such as surface area-to-volume ratio, density, and volume fraction. The burning of solid fuel is computed by solving mass and energy equations, including the effects of drying, pyrolysis, and char combustion and the exchanges of mass, momentum, and energy with the surrounding gas. The effects of wind, slope, fuel moisture content, fuel bed arrangement, environmental temperature, and humidity are considered in the numerical model. Computations were performed to compare successful and unsuccessful fire spread cases to highlight the effects of various factors. Numerical results were consistent with the experimental observations of the transition between no fire spread and spread under different fuel and environmental conditions. The simulated heat transfer processes and combustion mechanism in the fuel bed are helpful in identifying factors that determine fire spread success. It was found that the relative importance of modeled convective and radiative heat transfer processes to ignition of solid fuel differed with particle location, and could be switched depending on the wind speed, terrain slope, and fuel bed arrangement.