Taking the Heat Off - Transfering of Heat in Wildland Fires

Hello Everyone!

It has been said before but I will restate the obvious, “play with fire…and you will get burned”.  My experience with wild land fire is so limited that there isn’t even a spark that could kindle a fire. Most of my fire knowledge is based off of second hand knowledge that has been passed onto me from being around firefighters most of my EMS career. I do understand how heat is transferred and the damage it can do.

            The fire triangle describes the ingredients that are involved in making fire.

I would like to remind everyone that wildland fire is not an exact science and it involves multiple factors than can produce complex behaviors.

 

            I would like to describe how heat is transferred. This transfer of heat is the basis of how people can become burned. Understanding the mechanism of being burned helps us travel down the right path for treatment and give us a better understanding of burn pathophysiology. This method of understanding heat transfer gives us an insight of how combustion may occur in a wildland fire situation.

Three Heat Transfer Methods

 

Conduction: Transfer of heat from one molecule to another

 

EXAMPLE: Touching your hand to a hot object

Conduction is one of the only methods of transferring heat to the interior of fuels, such as wood, debris and forest litter. High density fuels (green wood) have greater conductivity which means that they need more heat to raise the surface temperature and are harder to combust. Low density fuels are considered decayed wood and forest litter, these are easier to combust.

 

Radiation: Transmission of heat by electromagnetic waves.

 

EXAMPLES: Heat from the sun, fireplace or a stove

Having direct contact between radiation source and affected area is not necessary. An example of this would be the pre-heating of fuels in front of a fire front. There is some absorption of radiation by woody fuels, this occurs within a thin layer of the surface. Most heat is transferred by conduction.

 

 

 

Convection: Transfer of heat by movement of gas or liquid

(air).

 

EXAMPLES: Heating a pot of water (steam column) or smoke from a fire.

 

Hot air moves vertically, most of the time, an exception to this could be slopes or prevailing winds may affect this heat transfer. This type of heat transfer is important when thinking about pre-heating of shrub layers and crown canopies.

There is a factor that can increase heat transfer. The proximity of the flames is brought closer to the fuel due to slope.

 

Increasing Heat Transfer

 

The wind pushes the flames over...

 

...bringing the flames closer to the fuel.

 

 

 

Information Cited

 

The San Diego Wildfires Education Project, accessed site Jan4, 2016, http://interwork.sdsu.edu/fire/resources/fire-burns.htm

 

Frankman, D., Webb, B., Butler, B.,  Jimenez, D.,Forthofer, J., SopkoB,. P.. Shannon K.,  HiersC, J., and Ottmar R., Measurements of convective and radiative heating

in wildland fires,Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602, USA.

BUS Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory,

5775 Highway 10 W, Missoula, MT 59808, USA.

CEglin Air Force Base, Jackson Guard, 107 Highway 85 N, Niceville, FL 32578, USA.

DUS Forest Service, Pacific Northwest Research Station, 400 N 34th Street, Suite 201,

Seattle, WA 98103, USA.

ECorresponding author. Email: bwbutler@fs.fed.us

http://www.firescience.gov/projects/07-2-1-20/project/07-2-1-20_Frankman_et_al_2013_field_study.pdf

 

English, J., "HEAT TRANSFER CHARACTERISTICS IN WILDLAND FUELBEDS" (2014). Theses and Dissertations--Mechanical Engineering. Paper 52. http://uknowledge.uky.edu/me_etds/52