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Unit 1: The Changing Fire Environment

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Environment   ::  Heat Transfer  ::   Behavior   ::   Predictions   ::   Summary   ::   Exercises

Heat Transfer

To better understand when and how ignition and combustion occur in a wildfire, we need to discuss the physical processes involved. Note that heat transfer refers to the physical processes by which heat energy moves to and through unburned fuel.

Heat transfer refers to the physical process by which heat energy moves from one area to another.

Heat transfer

The Heat Transfer Methods figure above illustrates various heat transfer methods. Branches above the fire are receiving heat by convection and radiation. Tree trunks and shrubs are receiving heat by radiation from the fire. Fuels on the ground are being preheated by conduction and radiation. Preheating of fuels may be occurring by all of these methods at the same time, depending on the arrangement or loading of the fuels.
We've stressed the importance of radiant heat transfer in the preheating of fuels and spread of the fire. How much heat will be received by fuels ahead of the fire? Well, this depends on the fire intensity and the distance, but how much?

Common Methods of Heat Transfer

Conduction

The first common method of heat transfer is conduction. Conduction is the transfer of heat from one molecule of matter to another. An example of this is fire smoldering through a solid piece of fuel. Since wood is generally a poor conductor of heat, conduction is the least important method of the three.

Convection

This is the transfer of heat resulting from the motion of air (or fluid). It is the natural buoyant rise of warm air over a heat source that induces an automatic circulation within an airmass. Examples of forced convection are fire spreading from surface fuels to aerial fuels, and columns of smoke rising high into the atmosphere. Convection also includes direct flame contact, a powerful heat transfer process, especially in a head fire.

Radiation

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Radiation is the transmission of heat energy by rays passing from a heat source to an absorbing material. Examples are the heat received from the sun, and the preheating of fuels ahead of a flaming front. Radiation from glowing char or flames is very strong. This is why firefighters often must shield exposed skin. Radiation is the chief source of heat transfer in a backing fire.
Do the examples given for the three heat transfer methods suggest a relationship among ignition, fire intensity, and rate of spread? Well, they should, because fire behavior is the result of, and is affected by, the method and the amount of heat energy transfer within the fire environment.

Mass Transport of Firebrands

There is a fourth method by which fire spreads that is of great concern to firefighters. This is the mass transport of firebrands which can occur as a result of convection, wind, or gravity. We call this spotting. Small embers of burning material can be lifted in a convection column and be carried some distance ahead of a fire. Wind, in addition to strong convective currents, can carry embers or firebrands considerable distances downwind from the fire. Wind without convective lifting will result in shorter range spotting of firebrands.
Gravity also is responsible for spotting of firebrands, but always down slope. Usually, the steeper the slope, the greater the spotting problems from burning materials of various sizes rolling down slope. In each of these cases, we are dealing with new ignitions outside the fire perimeter, and not the normal growth of the fire.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 20). Unit 1: The Changing Fire Environment. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_1__The_Changing_Fire_Environment_8.html. This work is licensed under a Creative Commons License Creative Commons License