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. |