By Josh Perry, Editor
Researchers from the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. analyzed the heat transfer mechanisms of boiling water to explain how to predict and prevent a boiling crisis, which is the point when enough bubbles form on the surface that the resulting sheet of vapor blocks further heat transfer to the surface.
Image shows the rate of heat transfer from a metal surface, with red the highest and blue the lowest. The large blue areas show the beginning of a boiling crisis. (MIT)
According to a report from MIT, this research will boost the efficiency of nuclear power plants, which currently operate at levels far below critical heat flux (CHF) where a boiling crisis could occur.
“In a nuclear plant, water is heated by the fuel rods, which heat up through nuclear reactions,” the article explained. “The spread of heat through the metal surfaces to the water is responsible for transferring energy from the fuel to the generating turbine, but it also is key to preventing the fuel from overheating and potentially leading to a meltdown. In the case of a boiling crisis, the formation of a layer of vapor separating the liquid from the metal can prevent the heat from being transferred and can lead to rapid overheating.”
Scientists documented that once the number of bubbles on the surface reaches a certain threshold there is a greater likelihood of them coalescing into an insulating layer. The article noted that it is similar to traffic patterns where the increase in vehicles makes it more likely that there will be congestion and slowdowns.
“The nanoscale texture of the surface plays an important role, the analysis shows, and that’s one of several factors that might be used to make adjustments that could raise the CHF, and thus potentially lead to more reliable heat transfer, whether for power plants, liquid cooling for advanced computer chips, or many other processes where heat transfer is a crucial factor,” the article concluded.
The research was recently published in Physical Review Letters. The abstract stated:
“We present the first experimental observations of scale-free behavior in the bubble footprint distribution during the boiling crisis of water, in pool and flow boiling conditions.
“We formulate a continuum percolation model that elucidates how the scale-free behavior emerges from the near-wall stochastic interaction of bubbles and provides a criterion to predict the boiling crisis. It also offers useful insights on how to engineer surfaces that enhance the critical heat flux limit.”