Heat Transfer

How much heat is required to ignite forest fuels? Vegetative material such as forest fuels ignites at relatively low temperatures provided the amount of moisture in the fuel is low and it is exposed to the air so that sufficient oxygen is available. Actual heat requirements for ignition of dead forest fuels varies from about 500° to 750° F.  Many common ignition sources will provide enough heat including a burning match and even a glowing cigarette when in contact with fine fuels that are dry.

IGNITION TEMPERATURE FOR DRY FOREST FUELS = 500º - 750º F.

We know of many methods where heat can be applied to forest fuels to start the combustion process; but how is the process continued?  A fire spreads by transferring heat energy in three ways:  Radiation, Convection, and Conduction.

Radiation

Radiation refers to the emission of energy in rays or waves.  Heat moves through space as energy waves.  It is the type of heat one feels when sitting in front of a fireplace or around a campfire.  It travels in straight lines at the speed of light.  This is the reason that when facing the fire, only the front is warmed.  The backside is not warmed until the person turns around.  The earth is heated by the sun through radiation.  Sunburns are a “fact of life” when people are exposed to the sun very long.  Most of the preheating of fuels ahead of a fire is by radiation of heat from the fire.  As the fire front gets closer, the amount of radiant heat received is increased.

Convection

Convection is the transfer of heat by the physical movement of hot masses of air.  As air is  heated, it expands (as do all objects).  As it expands, it becomes lighter then the surrounding air and it rises.  (This is why the air near the ceiling of a heated room is warmer than that near the floor.)  The cooler air rushes in from the sides.  It is heated in turn and it also rises.  Soon a convection column is formed above the fire which can be seen by the smoke that is carried aloft in it.  This “in-draft” of cooler air from the side helps to supply additional oxygen for the combustion process to continue. 

Conduction

Conduction is the transfer of heat within the material itself.  Most metals are good heat conductors.  Wood is a very poor conductor so it transmits heat very slowly.  This can be illustrated by the fact that a wooden handle on a hot frying pan remains cool enough to be held by the bare hands.  Conduction is not an important factor in the spread of forest fires.

Demonstration

Light the candle again that we used in the previous demonstration.  (Note that you can hold the match while the other end burns because wood is not a good conductor  of heat.) Now hold your hand out beside the candle and move it closer until the heat can be felt. The heat from the candle is reaching your hand by radiation. Move your hand in closer to the candle. What happens to the hand? It gets warmer because the radiant heat did not have to travel as far. Now hold your hand over the candle and move it as close as you can. Can you hold it as close as you can from the side? You can't because of the convection heat rising from the candle in addition to the radiant heat.

  1.  The three ways heat is transferred are

7.  The ignition temperature for dry forest fuels ranges between