DETONATIONS may rip through flammable gases even faster than previously thought. They were considered to be limited to a single shock wave travelling at several times the speed of sound. But a new model shows that a few molecules could leap ahead of the main shock wave, triggering further reactions so the explosion tears outwards even faster.
Some chemical reactions – for example when flammable gases burn – happen so fast they generate a shock wave that shoots forward, triggering further reactions as it spreads. These reactions in turn fuel the shock wave, resulting in the kind of explosion known as a detonation.
Equations that simulate detonations in gases assume the gas is continuous. In these models, all parts of the reaction front travel outwards at the same speed, just behind the shock wave. But this does not necessarily give the whole picture, says physical chemist James Anderson of Pennsylvania State University. “The equations don’t allow for the existence of particles.”
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Now Anderson and his colleague, aerospace engineer Lyle Long, have developed a more sophisticated model that treats each gas molecule individually. When they ran the model, they got a surprise: it predicted something that the old equations simply did not allow. Some of the gas molecules reacted so fast they overtook the shock wave. Riding the wave like miniature surfers, these speedy reactions sparked fresh shock waves ahead of the main front, increasing the overall speed of the detonation.
But some detonation experts are sceptical of whether such “ultrafast” explosions are really possible. “People would have seen it if it could happen,” argues Craig Tarver from Los Alamos National Laboratory in New Mexico.
Betsy Rice of the Army Research Laboratory in Maryland points out that Anderson and Long’s model treats the gas particles as simple spheres. But in real molecules the motions in the chemical bonds could play a part. And the researchers’ hypothetical gases react faster than almost anything known, she says.
Anderson and Long maintain that under the right conditions, a reaction could overtake its own shock wave, and hope that chemists will now try to observe the effect in the lab. The reaction between hydrogen and fluorine would be a good bet, they say.
Whether this works or not, the new model should help scientists solve various problems such as spotting the rocket plumes of missiles, and understanding shuttle re-entry.
