Backdraft

Underventilated fire.

In some modern buildings its construction limits air flow. Closed doors and hermetic windows causes the situation, when during the fire growth there is not enough oxygen. Fire changes very fast from fuel controlled to ventilation controlled.

Despite this, the temperature of smoke is still high and there is a lot of unburned fuel in smoke layer. Because of high temperature, the pyrolisis process continues and more fuel is being produced.

If the ventilation doesn’t change, fire could self-extinguish.

But if fresh air will come through opened door, opened or broken window or any other opening, it can cause the phenomenon of backdraft.

Air can form a mixture with pre-heated smoke, which ignites, when will become between LEL and UEL. Huge energy concentrated in hot, flammable smoke is now released, what makes the ignition powerful and creates pressure flow in the opposite direction to the air supply.

Limited ventilation during an indoor fire can lead to the production of large amounts of unburned gases. When an opening is suddenly introduced, the inflowing air can mix with these creating a combustible mixture of gases in some part of enclosure. Any ignition sources, such as a glowing ember, can ignite this flammable mixture, resulting in an extremely rapid burning of the gases. Expansion due to heat created by the combustion will expel the burning gases out through the opening and cause a fireball outside the enclosure”.

Source: Enclosure Fire Dynamics, CRC Press – Quintiere J and Karlsson B (1999)

3.5. Phases of fire development – backdraft