Combustion and Heat Transfer

Fire releases heat through combustion. Combustion is a physical and chemical process that unleashes the solar energy stored in chemical form in fuels as heat. Oxygen, heat, and fuel often called the fire triangle must be present in the proper combination for a fire to ignite and initiate combustion. Sufficient oxygen is found in our atmosphere, fuel accumulates in forested ecosystems so when it is dry enough all that is required is the heat or ignition source, which can be either from a natural or anthropogenic source.

A common sequence of physical processes occurs in all fuels before the energy contained in them can be released and transferred during combustion. The changes a fire goes through are traditionally organized into five phases of combustion: pre-ignition, flaming combustion, smoldering combustion, glowing combustion, and extinction. Although flames are the most recognizable and significant phase of the combustion process, these other phases have important impacts on ecosystem resources as well.

For a fire to grow and spread, heat must be transferred to surrounding fuel. Heat produced during combustion is transferred to other fuels, the soil, and the atmosphere primarily by convection, radiation, and conduction. As wildland fires spread, radiation and convection are the primary methods of heat transfer from one fuel to another, while conduction transfers heat within large pieces of fuel. Mass transport by spotting or fuels rolling downslope can also be an important means of heat transfer during intense fires. When water is present, heat transfer can also occur by vaporization. Fire intensity is an important measure of the rate of heat released by a fire.

Also see Heat Transfer.