Physical mechanisms for the propagation of surface fires

Abstract

The various physical processes which affect the rate-of-spread of fires on surface fuel beds are discussed. Specifically, these include conduction, radiation from the flame and burning fuel bed, and convective heating influenced by wind. A new interpretation of a laboratory experiment is given, and from this a balance between radiative heating from the flame and convective cooling of the fuel bed is established. The ratio between these two effects may be expressed as a new dimensionless number, P (defined by equation 5.12 or 5.16). If P < 1, radiation from the flame of the fire alone is not capable of causing the fire to spread, whereas if P #> 1, generally it is. Many laboratory experiments have P < 1, whereas P #> 1 for moderate to large fires in the field. The most significant factor affecting fire propagation is wind, which increases both radiative and convective heating. Much of this process is turbulent and involves large eddies in the boundary layer and fuel canopy, and their interaction with fires is currently poorly understood.