Heat Release Rates under Different Fuel and Weather Conditions

The following table depicts how different fuel and weather conditions influence heat release and fireline intensity. Table: Fuel type, wind speed, and fuel moisture effects on fireline intensity compares total heat release and fireline intensity for three fuel classes under various wind speed and fuel moisture conditions. The results were generated by the Behave Plus 2.0.2 fire behavior simulator (Andrews and Bevins 2003). The fuel models for tall grass (FM 3), southern rough (FM 7), and light logging slash (FM 11) were used to represent typical fine, medium, and large fuels found in the southeast, respectively (Anderson 1982). These fuel models are summarized in (Table: Fuel Model Parameters).

For all fuels, the highest intensity fire (highest rate of heat release) occurs under conditions of high wind speed and low moisture content. Tall grass burns at a much higher intensity than the other fuels under the same weather conditions, due to the high surface area-to-volume ratio of grass, which allows it to lose moisture more rapidly than larger fuels, the higher loading of 1-hour (<.25 in. diameter) fuels, and the faster rate of spread in the grass fuels. On the other extreme, large fuels will not burn at 18% moisture content because they are unable to efficiently lose water. Total heat release per unit area of fuel does not change with wind speed but decreases with increased moisture content, because there is less available fuel per unit area as fuel moisture content increases. Another important consideration in interpreting the table is that fine fuel consumption is by flaming combustion whereas large fuels are consumed by glowing and smoldering combustion.

For more information on the effects of fuel and weather on heat release and fire intensity, detailed physical and thermochemical process equations can be found in Johnson and Miyanishi (2001) and Drysdale (1999).