PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 799 November 1, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu

SLOW-MOTION BOILING. A new study, carried out at a chilly
temperature of 33 K, explains why certain industrial heat exchangers
(including those used at power plants) melt catastrophically when
steam formation undergoes a process referred to as a “boiling
crisis.” Boiling, a sort of accelerated evaporation, is usually a
very efficient form of energy transfer because of the transport of
latent heat (the heat required for a substance to change its phase);
energy moving from a heater to a liquid by the formation of vapor
bubbles. There can be an important hitch in this process, however,
and that is the poorly understood boiling crisis.

This potentially
dangerous situation comes about as follows: at high enough
temperatures the formation of bubbles becomes so great that the
entire surface of the heating element (the part of the heater in
contact with the liquid) can be covered with a vapor film, which
insulates the liquid above from absorbing heat. (Just as a water
droplet, hitting a frying pan, evaporates only very slowly.) The
result is a buildup of heat in the heater and possible meltdown.
(For a film of this process see
http://www.pmmh.espci.fr/~vnikol/boiling_crisis.html )
What Vadim Nikolayev (vadim.nikolayev@espci.fr, 33-140-79-58-26) and
his colleagues at the Ecole Supérieure de Physique et de Chimie
Industrielles in Paris, Commission of Atomic Energy in Grenoble, and
the University of Bordeaux have done is to provide the first
detailed look at the boiling crisis by performing simulations and
laboratory tests of a theory which suggests that the overheating
comes about because of vapor recoil. That is, at high enough heat
flux, the growing bubble will forcefully push aside liquid near the
heating element (much as rocket blasts provide thrust), expanding
the potentially dangerous insulating vapor layer. This theory was
upheld by experimental work performed not at the blazing temperature
of
high-pressure steam but near the chilly critical temperature of
liquid hydrogen, where boiling would occur very slowly, in a way
that could be glimpsed more completely. Thanks to the universality
of fluid dynamics, however, lessons learned at 33 K should be
applicable to fluids at 100*C.

Nikolayev believes that better understanding of the boiling crisis
will facilitate certain counter-measures. This is important since
possible boiling problems occur not just at major industrial sites
but also for such consumer electronic products as laptop computers,
where soon the rate of heat dissipation will be much higher than for
today’s models owing to further miniaturization. (Nikolayev et al.,
Physical Review Letters, upcoming article; further background at
http://www.pmmh.espci.fr/~vnikol/vnikol_pdf/Bull_DRFMC04.pdf )