Boiling crisis revealed

A new study carried out at a chilly 33 K explains why certain industrial heat exchangers, including those used at power plants, melt catastrophically during a so-called boiling crisis. The crisis occurs at a critical heat flux, when the bubbles that nucleate in the liquid on a heater’s surface merge into a vapor film that inhibits further heat transfer to the liquid. The same thing happens when a water droplet hits a hot frying pan: A vapor layer insulates the drop so that it evaporates slowly. In a heat exchanger, though, the heater can rapidly overheat and even melt. What Vadim Nikolayev (Atomic Energy Commission in Grenoble, and ESPCI in Paris, France) and his colleagues have done is provide both a model for and a detailed look at the boiling crisis. The model invokes vapor recoil, whereby a molecule that escapes a liquid surface pushes against that surface, analogous to the gas thrust of a rocket engine. At high enough heat flux, a growing bubble can forcefully push the liquid entirely away from the heating element. That model 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 can occur very slowly. Nikolayev says that better understanding of the boiling crisis will facilitate certain countermeasures at industrial sites. (V. S. Nikolayev et al., Phys. Rev. Lett. 97 , 184503, 2006 .)