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Fire Behavior Physical Principles
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What are the physics of fire spread?
A spreading fire can be examined at many scales. For example, an incident commander
working to contain a large fire observes the behavior of the fire at a level of spatial resolution that is
no more detailed than a single watershed. A firefighter working on the same fire has less of an understanding
of how the fire is moving through the various watersheds in the area and instead focuses only on the behavior
of that portion of the fire front that is most nearby and (potentially) threatening. It is impractical to
discuss the physics of fire spread from either of these scales. Instead, an idealized description of the fire
and fuel bed is helpful. Three assumptions, listed below, facilitate the physical description. A good, but
somewhat dated source, of information on the physics of fire spread is available in the book written by
Drysdale (1985).
Assumptions
a1 Assume that the material the fire is spreading through, termed the fuel bed, is completely
homogeneous. The fuel bed can then be described as a collection of completely homogeneous fuel particles.
Each atomic fuel particle has three measurable properties:
- mass of liquid water present
- oven-dried mass
- temperature-dependent specific heat
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a2 Assume that the fire is neither accelerating nor decelerating, and has thus reached an equilibrium
rate of spread, termed steady-state. Fons (1946) was the first to observe
that if four factors are held constant, the change in the rate of fire spread will be zero. Therefore, four
factors must be held constant during steady-state conditions:
- meteorology
- fuel bed moisture content
- fuel bed composition, quantity, and arrangement
- topography
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a3 Assume the existence of a fictitous unique temperature for every fuel particle in the fuel bed,
termed the ignition temperature. The ignition temperature marks the exact onset of flaming combustion for the fuel particle.
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Physical Description of Fire Spread
i Energy from combusting fuel particles at the fire front is transferred to an unignited fuel volume ahead of
the fire front via three heat transfer mechanisms:
- radiation
- convection
- conduction
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ii The energy transferred to the unignited fuel volume raises it's temperature from ambient to ignition and
in the process volatilizes all the water present in the fuel.
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iii Between the time of the onset of ignition and the reduction of the fuel particles in the fuel volume to
char or tar, radiant energy is being emitted from two sources:
- the surface oxidation of the glowing fuel particles within the fuel bed
- a flame attached to the upper surface of the fuel volume
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iv If wind is present, then contemporaneous to the conditions described in iii, there will be heated gas
and soot particulates from the oxidizing fuel particles and flame which displace the air around the unignited fuel bed
creating strong temperature gradients between the fuel particles and the surrounding air.
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v The energy released while the flame is present, the fuel particles are combusting, and heated gases are emanating
from the fuel is transferred to adjacent unignited fuel to repeat the process begining at i.
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Last Updated: Marco Morais
