THE last place most people would try to start a fire is inside a tornado. But
two researchers who have pulled off a similar trick in a lab in New Zealand say
their experiment may explain enigmatic weather phenomena such as ball
lightning.
At first glance, tornadoes don鈥檛 appear fire-friendly. Even at the calm
centre of the whirlwind, there is enough of an updraft to make any flame
tenuous, and the fast winds at its edge would blow out any blaze.
Yet fireballs have been reported in some tornadoes, such as the twister that
struck Dorset in Britain in 1989. Vortices have also been associated with
floating spheres of ball lightning, which sometimes disappear with a loud
explosion, suggesting they, too, contain combustible material.
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So John Abrahamson, a chemical engineer at Canterbury University in Christ-
church, was intrigued when his former student Peter Coleman proposed trying to
create a fireball in a mini tornado. They reckoned one might form in the vortex
breakdown region, where air moves relatively slowly. 鈥淚f it was coloured, you鈥檇
see this doughnut of air,鈥� says Abrahamson.
Intriguingly, the vortex breakdown region is used in 鈥渧ortex burners鈥�, in
which a flame burns in a closed, horizontal cylinder. A horizontal vortex mixes
and contains hot gases so that the fuel burns efficiently. But it was unclear if
the combustion would be stable in a free-standing, vertical vortex.
To find out, Abrahamson and Coleman built a circular chamber about a metre
wide. Slats at the base allowed air to enter at various angles and an extraction
fan pulled air upwards from above. This created a vortex 10 centimetres wide.
Liquefied petroleum gas was introduced into the breakdown region through a pipe
and was ignited with a spark plug.
The vortex produced a stable fireball if the air entered at an angle of 66
degrees. Whether the fuel pipe was above, below or to the side of the vortex
breakdown region, the fuel was drawn into the doughnut of air and burnt as a
sphere.
Abrahamson concludes that if a natural vortex swept up fuel from the ground,
and if something like a lightning strike or power line ignited it, this could
form a stable fireball. His experiment will be described next month at a meeting
of the American Geophysical Union in Boston.
鈥淭here are many theories about ball lightning, but not many of them can be
studied so thoroughly in a lab as this one,鈥� notes Stanley Singer, president of
the International Committee on Ball Lightning in Pasadena, California. However,
he adds that some reports suggest ball lightning can pass through solid
objects鈥攕omething that is hard to reconcile with a combustion theory.
Abrahamson points out that many of those reports have been called into
question. Even if his experiment fails to explain meteorological mysteries, he
believes it may find uses in industry. 鈥淚 don鈥檛 think anyone has ever created a
vortex to control an open flame before. It could be useful.鈥� The experiment
could even explain some UFOs, adds Coleman. 鈥淪ome pictures of supposed UFOs I鈥檝e
seen look like classic fireballs.鈥�
