Why do some fires go supersonic?

Although plenty of people have looked into the physics of explosions, there have been some basic questions left unanswered. One of them is why some fires suddenly go supersonic. Now there's a new theory about DDT explosions.

DDT is the abbreviation for 'deflagration to detonation transition.' Deflagration is what happens when a material catches fire and the flames spread quickly. Already it's not an ideal situation, but things go very bad when the whole fire detonates. Suddenly the flames get a kick of speed that boosts them up to above the speed of sound. Sound travels at a speed that does not allow the pressure waves of air to pile-up. When flames go supersonic, they push a massive wall of air, a pressure wave, in front of them. This pressure wave can kill people and damage property that the flames never touch.

Plenty of people have seen this in movies or TV shows. The protagonists run clear of a burning house, and they turn to look back as part of the building suddenly explodes. The audience assumes that the flames hit a gas line or some chemicals or a kid's chemistry set. That's not necessarily true. The fire may not have hit anything at all, just been part of a DDT explosion. The fire just reached conditions under which it could spread faster than the speed of sound. This, worryingly, happens often in coal mines and nuclear power plants.

Scientists have been baffled as to why it would reach these conditions, and explosions are a tough subject to research, but recently teams in Russia and Sweden have a theory as to what causes these explosions. One major factor is space. DDT explosions rarely happen in spaces where the flames are confined, even though the small space seems like it would build up more pressure faster. For pressure to happen, there need to be long stretches of space for the fire to build up heat. The heat causes waves of pressure, which expand the fire, which creates more heat and therefore more pressure. Hallways are good for these types of explosions, with plenty of surface area and room to expand the flames. When enough heat and pressure build up, the whole thing detonates, pushing flames out faster than sound and creating the supersonic shock wave that does so much damage.

Image: Jon Sullivan

Via Physics Central.