Stages in Fire Behaviour III
- Published: Saturday, 27 December 2014 11:41
- Written by Lance Hartley
Reference: Adapted from CFBT–US (Ed Hartin) and Village Handbook (2005)
Flashover is the sudden transition from a growth stage to fully developed fire. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment. Conditions for flashover are defined in a variety of different ways. In general, ceiling temperature in the compartment must reach 500 ° – 600 °C.
When flashover occurs, burning gases will push out openings in the compartment (such as a door leading to another room) at a substantial velocity.
Flashover is the sudden transition to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment….Flashover may occur as the fire develops in a compartment or additional air is provided to a ventilation-controlled fire (that has insufficient fuel in the gas phase and/or temperature to backdraft).
It is important to remember that flashover does not always occur. There must be sufficient fuel and oxygen for the fire to reach flashover. If the initial object that is ignited does not contain sufficient energy (heat of combustion) and does not release it quickly enough (heat release rate), flashover will not occur (e.g., small trash can burning in the middle of a large room). Likewise, if the fire sufficiently depletes the available oxygen, heat release rate will drop and the fire in the compartment will not reach flashover (e.g., small room with sealed windows and the door closed). A fire that fails to reach a sufficient heat release rate for flashover to occur due to limited ventilation presents a significant hazard as increased ventilation may result in a ventilation induced flashover.
A backdraft can be defined as a rapid combustion propagated through a gas at explosive speed and accompanied by intense heat, following the introduction of oxygen into a compartment filled with accumulated unburned fuel.
The first step in the development of a backdraft is the formation of a fuel-rich atmosphere in an oxygen-depleted enclosure as represented by diagram 1.
The second step is the sudden introduction of air into the enclosure by opening a door or window as per diagram 2.
As the air flows into the enclosure under the hot fuel-rich gases flowing out, a mixing region develops at the boundary between the two streams as illustrated in diagram 3.
If the mixture becomes large before it encounters a sufficiently hot source to ignite it, then an explosion occurs as illustrated in diagrams 4 and 5.
The exploding flame front generated in the explosion, pushes fuel-rich gases out through the enclosure opening followed by a fireball. In the last diagram, a shock wave propagates into the air outside the enclosure away from the enclosure at a great speed.