3 - Forest fire behavior

Summary

A spreading forest fire is a complex combustion process in which the flaming front is heating and then igniting unburned woody and herbaceous fuels. In this heating process (Shafizadeh 1968), the moisture in the fuel is first evaporated (fuel temperatures > 100 °C), then the cellulose is thermally broken down and its breakdown products volatilized (> 200 °C) and finally the volatiles are ignited to form a visible flame (300–400 °C). The modes of heat transfer responsible for fire spread from the flaming front (Figure 3.1) are convection and radiation. Conduction does not contribute significantly to fire spread because wood and soil are such poor heat conductors. For conduction to be effective, the flame must be maintained in one place for a long time. However, to maintain a solid flame, the fire front must be constantly moving to recruit unburned fuels.

After the flaming combustion has ignited and burned most of the volatiles, the remaining carbon may burn as a solid by surface oxidation called glowing combustion. Flaming and glowing combustion are not discrete events in forest fires because of the complex mixture of fuel sizes, moistures and arrangements. However, the flaming front is dominated by combustion of gases and glowing combustion occurs primarily after it passes.

The differences between flaming and glowing combustion are of interest to ecologists because they can have different ecological effects. Flaming combustion is primarily responsible for plant death and glowing combustion for duff consumption and seedbed preparation. These effects will be discussed in detail in Chapters 4 and 5.