Textured Surface Analysis May Yield Safer Power Plants
Engineering360 News Desk | September 10, 2015What happens at the surface of boiling water, used in a wide range of industrial processes, is far from understood. However, what is known is that hot spots have the potential to melt equipment and disable plants.
Research at the Massachusetts Institute of Technology (MIT) is focused on understanding what causes the extreme heating when critical heat flux (CHF) is exceeded and how to prevent it. If the research is successful, it could potentially make it possible to operate power plants at higher temperatures.
Navdeep Singh Dhillon studies physical mechanisms that dictate the boiling crisis phenomenon. The rig measures the critical heat flux of thermally saturated water boiling on thin textured substrates. The journal Nature Communications has recently published a paper on the research co-authored by mechanical engineering postdoc Navdeep Singh Dhillon, professor of nuclear science and engineering Jacopo Buongiorno and associate professor of mechanical engineering Kripa Varanasi.
The researchers say that bubbles of vapor that characterize boiling actually limit energy efficiency. Gas, whether it is air or water vapor, is highly insulating, whereas water is a good absorber of heat. On a hot surface, the more area that is covered with bubbles, the less efficient the transfer of heat energy becomes.
If the bubbles persist too long at a given spot, they can increase the temperature of the metal underneath. That is because heat is not transferred away fast enough, meaning it can potentially melt part of the metal.
The increase in metal temperature will damage an industrial boiler, a potentially catastrophic scenario for a power plant or a chemical processing unit. When a layer of bubbles limits heat transfer, the temperature can increase by several thousand degrees, known as a "boiling crisis.”
More texturing is not always better. MIT experiments using simultaneous high-speed optical and infrared imaging of the boiling process show a maximum benefit at a certain level of surface texturing. The researchers say that understanding exactly where this maximum value lies and the physics behind it is the key to improving boiler systems.
The research used a combination of precision micro- and nano-fabrication, probative experimental techniques and innovative analysis to investigate the existence of an optimal surface geometry.
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