Fire Behaviour
PD7974-1 (2003) provides the basics of initiation
and development of compartment fires. Basically, an enclosure fire
may include some or all of the following phases of development,
which are also illustrated in Figure 1.
Incipient phase
|
Heating of potential fuel is taking place through a variety
of combustion processes such as smouldering, flaming or radiant.
|
Growth phase
(pre-flashover) |
Ignition is the beginning of fire development. At the initial
growth phase, the fire will be normally small and localized
in a compartment.
An accumulation of smoke and combustion products (pyrolysis) in a layer beneath the ceiling will gradually form a hotter upper layer in the compartment, with a relatively cooler and cleaner layer at the bottom. With sufficient supplies of fuel and oxygen and without the interruption of fire fighting, the fire will grow larger and release more hot gases and pyrolysis to the smoke layer. The smoke layer will descend as it becomes thicker. |
Flashover
|
In case of fire developing into flashover, the radiation
from the burning flame and the hot smoke layer may lead to
an instant ignition of unburned combustible materials in the
compartment. The whole compartment will be engulfed in fire
and smoke.
|
Fully developed phase
(post-flashover) |
After the flashover, the fire enters a fully developed stage
with the rate of heat release reaching the maximum and the
burning rate remaining substantially steady.
The fire may be ventilation or fuel controlled. Normally, this is the most critical stage that structural damage and fire spread may occur. |
Decay phase |
After a period of sustained burning, the rate of burning
decreases as the combustible materials is consumed and the
fire now enters the decay phase.
|
Extinction |
The fire will eventually cease when all combustible materials
have been consumed and there is no more energy being released.
|
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