5.3: Variables That Influence Combustion
- Page ID
- 66706
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Flame Spread
Flame spread refers to the movement of the oxidation reaction across the fuel. The speed at which flame spread occurs is determined by several factors, including orientation and fuel characteristics. Flame spread can occur rather slowly, such as across a log in a campfire, or with explosive speed such as when a flame propagates through a cloud of flammable gas.
Surface-to-Mass Ratio
Surface-to-mass ratio impacts combustion in two ways.
First, it increases the amount of surface that can be heated, and therefore can contribute fuel to the combustion reaction.
In the example below, the 1” x 2” green block has 10 square inches of surface area. The blue blocks together have the same mass as the green block; however, the blue blocks have 12 square inches of surface area.
The second way that the surface-to-mass ratio impacts combustion is by overall mass.
Remember, heat moves from hot to cold, so as the surface of the fuel is heated, the heat moves through the fuel, essentially cooling the surface. As the surface area increases relative to mass, the size of the fuel being heated decreases, reducing the ability to absorb heat from the surface and therefore allowing the fuel to ignite sooner.
Orientation and Geometry
The fuel's orientation and geometry influence the combustion process. For example, a match being held with the flame up will consume the fuel slower, because the heat is primarily moving in the opposite direction of the fuel. However, if you hold the match upside down, the fire plume is able to heat much more fuel as it moves upward, therefore, growing in size and consuming the fuel at a faster rate.
A Christmas tree, due to its many small branches and needles all evenly spaced, has a geometric arrangement that favors fire growth.
Thermal Inertia
Thermal inertia refers to the speed at which heat travels through a material. This speed is described as either thermally thick or thermally thin.
Materials that are considered thermally thick do not transfer heat readily through their mass. Thermally thick materials when heated on one side, will be slow to have heat reach the other side. Wooden blocks or boards would be a good example of something that is thermally thick.
Materials that are considered thermally thin transfer heat quickly through their mass. A metal pan is a good example of a thermally thin material. Heat is transferred from one side to the other very quickly. Thermally thin fuels will ignite faster than thermally thick fuels.
Area
Much like the surface-to-mass ratio, the area that is being heated impacts the amount of fuel that can contribute to the fire. A fire can double in size every 30 seconds, so the amount of area of fuel involved is a key aspect of fire growth.
Density
The density of fuel impacts how easily a fuel can be pyrolyzed. Denser fuels require more heat for longer periods of time to turn the solids into combustible gases.
Other Factors
In addition to the above, factors such as moisture content in the fuel and the amount of heat being transferred to the fuel can influence the rate of combustion and fire growth.