In the early morning hours of December 11, 2005, Europe experienced its biggest peacetime explosion ever. The vapor from almost 200 tons of petrol at a fuel storage depot in Buncefield, UK had ignited, obliterating the facility in a colossal combustion reaction that defied reason.
That's not an exaggeration, by the way — the explosion was actually inexplicably huge. The detonation pancaked many of the surrounding metal structures, damaged buildings as far as 8 kilometers from the blast site, and was audible in parts of France and Belgium. All told, the damages totalled £1.5billion (there were, remarkably, no deaths), but by all calculations, the vapor cloud should not have been capable of such devastation. For years, no one has been able to explain how the Buncefield blast happened. Until now.
Over on New Scientist, Will Grey tells the story of the investigation team that today, after seven years, thinks it knows the answer to the myterious Buncefield blast. That answer, according to the investigators, is trees — and they say the explosive video featured below proves it.
The reasoning goes like this: in an unconfined cloud of fuel vapor, your typical combustion reaction will propagate at around 10 meters per second. That's over 22 mph. For those of you playing out "what-if" scenarios in your heads, the fastest human footspeed on record comes in at just under 28mph. And that was during an olympic sprint; in a real-world scenario, Nic Cage and his mullet probably wouldn't stand a chance, and neither would you.
But forget about trying to outrun a typical combustion reaction for a second, because to wreak the kind of devastation witnessed at the Buncefield blast, you'd need a very atypical reaction — the kind of reaction that can travel through a vapor cloud at speeds approaching 1800 meters per second. That's where the trees come in.
Trees surrounding the storage depot, reason investigators, could multiply the reaction rate in two ways:
1) When the flame of a combustion reaction wraps around trunks, branches, and twigs, its surface area gets significantly larger, helping it burn faster.
2) At the same time, the branches create turbulence, causing flame and vapor to mix more readily.
Combine these two effects and the combustion rate skyrockets, the flame front accelerates, and you get a tremendous pressure wave so powerful, it actually enables the petrol vapor to ignite in the absence of any flame whatsoever.
Computer simulated models jelled with the investigators' hypothesis; but you're not here to read about computer simulations. Featured below are two field tests, recently conducted to test the effect that a 100-meter-long hedgerow of trees can have on flame acceleration. The trees were covered in plastic sheeting, and the resulting tunnel was slowly filled with propane. Ignition commenced when the gas reached a specific concentration (the beginning of the video shows footage from the morning of the actual blast, field tests begin around fifteen seconds in):
Holy crap, right? The video reveals three important things. One: the type of tree you use matters. The first test was conducted using flexible pine trees. Under these conditions, the flame propagates quickly throughout the gas, but there's no detonation. But the trees involved in the Buncefield blast were dense, deciduous trees. In the second test, investigators made this swap, leading to conclusion number two: hedgerows of dense, deciduous trees can, in fact, propagate a flame front and accelerate combustion, resulting in an enormous detonation.
Revelation number three, of course, is that I will never, ever plant a hedgerow of deciduous trees.
Read more about this fascinating investigation (which is still ongoing — another hedgerow test is schedule for 2012) over on New Scientist.
Top photo via Getty