Slow-Motion Boiling

A new study, carried out at a chilly temperature of 33 degrees Kelvin, 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 École 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 degrees Kelvin should be applicable to fluids at 100 degrees Celsius.

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
Contact Vadim Nikolayev
École Supérieure de Physique et de Chimie Industrielles
Tel: +33-140-79-58-26
vadim.nikolayev@espci.fr
For further background, see Boiling Crisis: Theory, Simulation and Experiments (PDF), by V. Nikolayev and D. Beysens