A porous-medium model for vegetation in bushfires

The vegetation that burns in bushfires consists of water, pyrolys-able material that can ultimately burn as a gas, solid combustible material (char) that is capable only of surface burning and unburnable material (ash). The vegetation is also a porous medium in which air, water vapour, pyrolysis products and combustion products flow past solid material. This flow is strongly influenced by and interacts with the flow of air above the layer of vegetation. As unburnt vegetation, the material is all contained in objects of different size and shape, such as leaves, stalks, branches and trunks, all exhibiting differing rates at which water and pyrolysis products can be generated through external heating, making some components much more susceptible to burning in a brief period while others would require longer time-scales; such material can often survive a fire. This article presents a sectional model for the vegetation in which different sizes of material are represented separately.