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New theory explains strange events during 2003 Canberra fires: changes in pressure create shock waves, lead to explosions

Posted by gasweek on 28 September, 2007

The Canberra firestorm that ravaged Australia’s capital in January 2003 surprised everyone, reported New Scientist (4/8/2007, p38). Strange phenomena: Nobody expected the blaze to reach the city and enter the suburbs with such force, killing four people and destroying almost 500 homes. But for some scientists the biggest mystery of all was what firefighters saw. Bare earth shouldn’t be able to sustain a fire of such ferocity at ground level, let alone metres up in the air, and flames burning vegetation should be yellow or orange, not blue. As far as most scientists were concerned, what the firefighters saw was impossible. So how did the flames erupt across areas where there was no vegetation to release pyrolysis gases?

Details of theory: John Dold, a combustion researcher at the University of Manchester, UK, had a new theory to explain the strange events observed during the Canberra fire. He said gases produced by pyrolysis must have somehow escaped from the fire front and accumulated away from the flames, forming an invisible and highly flammable mixture with the air. Flames could propagate so quickly through such a mixture that the changes in pressure created shock waves, leading to an explosion. For Dold, the snapped trees, blue flames and strange shimmering fire all added up to one thing – the advancing flames must have caused just such a violent combustion.

Strong evidence: “The evidence seems quite strong that it did happen,” he said. Now he was trying to work out how. Explosive combustion of unburnt gases certainly happens in house fires, where furniture and other household items were heated in a closed space with limited oxygen. When hot pyrolysis products from furniture can’t find enough oxygen to burn in a flame, they rose and concentrated in the ceiling. If more oxygen entered the room, say from a door opening or someone smashing a window, the gases ignited, causing a potentially deadly flashover. But could this happen in open air?

New Scientist, 4/8/2007, p. 38-39

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