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Rights: The University of Waikato
Published 3 November 2009
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In this video clip, Associate Professor Charley Fleischmann from the University of Canterbury describes how flashover and backdraught occur in fires.

Point of interest
If you were a firefighter, can you think of some safety tips you might use to keep safe from these fire behaviours?

Transcript

ASSOCIATE PROFESSOR CHARLEY FLEISCHMANN
Flashover and backdraught are phenomena that happen within a compartment or a structure.

Flashover occurs within a building or structure, and what happens is normally fire starts with a single object, and as that fire grows and develops, releases its energy, it continues to heat up the room. The hot gases collect near the ceiling, and they start to transfer their energy primarily by radiation down to the burning object. That burning object absorbs that heat, as does the other objects in the room. If you have a sofa burning, the carpet on the floor, the coffee table, the chair across the room, they start to absorb that heat and they heat up. And when they reach a point where they are as hot as nearly their ignition temperature, all of a sudden, they will burst into flames. It happens very quickly. In a matter of seconds, it goes from a single burning object to everything in the room burning.

Backdraught is a low-level explosion that can occur in building fires. It’s really quite a rare event. It’s mostly seen by firefighters because of the number of building fires they go to. The problem is, is it can have disastrous consequences for them.

What happens is you have a fire in a building where the building is closed up – there’s no doors open or large window openings or anything like that – so the fire doesn't have enough air to continue to burn. So it goes from being a well-established flaming fire into what is a smouldering fire. It doesn't have enough oxygen. It still has some energy being released, but at very low levels. And so what happens is it’s still releasing fuel but there is no oxygen to react with. Something happens, there’s a change. That change can be a window breaks, OK, and lets air in, or worse, a firefighter opens the door and comes in. Well when he comes in, he brings with him the air that the fire is looking for. That air comes in with him, he starts to move around that room, but so does the air. When that air comes in, it mixes with the fuel-rich environment caused by the fire. It finds an ignition source and you get an explosion.

Acknowledgement:
New Zealand Fire Service