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October 20, 2005

Have you ever wondered ....

As most of my friends know, I have spent a lot of my career trying to prevent people from doing stupid things with fire, putting them out when they occur, or investigating them after someone else has dealt with it, and now teaching others to carry on the good work.

Backdraft full force.BMP
This is a backdraft as it happens. The "deflagration" is often sufficiently powerful to extinguish the original fire by expelling all the fuel vapour.

Much was made of the explosive force of the backdraft phenomenon in the film of that name and there were a number of scenes which showed particularly violent explosions. Well, backdrafts are basically a deflagrating explosion of the unburned combustion or pyrolysis products in the smoke plume. They are seldom as violently powerful as Hollywood's version, but they are incredibly dangerous and pretty deadly if you happen to be the idiot in the wrong place when it happens. Anyone stood in the path of the flame emitted is going to be seriously burned, anyone caught in the pressure wave it will generate will probably be very aware of the explosive power.

An explosion of this type is generally at it's most powerful when it is in the most efficient air/fuel range of mixture - just like the carburation principle in a petrol engine. Too rich and it loses force, to lean and it may not even ignite. The range of materials stored in our homes that can produce the vapours to do this sort of "explosion" are fairly large. Plastics, expanded foam cusions, "Melamine" finishes, gas, some household cleaners, are among the things that can all produce the fuel which makes a stoichiometric mix and - BANG!

These sorts of explosion usually produce more of a "Woof!" than a "Bang!", but they are no less dangerous for that!

Posted by The Gray Monk at October 20, 2005 12:27 PM

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