Breakthrough in plant efficiency

In its research, the MIT team looked at the causes of extreme heating on a surface when water boils - a phenomenon that can severely damage industrial boilers and disable entire facilities.

Cases of such extreme heating occur when critical heat flux’s thermal limit is exceeded, the researchers said.

According to the MIT team, understanding critical heat flux, and developing ways to prevent it, could be the catalyst for operating chemical facilities and power stations at higher temperatures while simultaneously making them more energy efficient.

“Roughly 85% of the worldwide installed base of electricity relies on steam power generators, and in the US it’s 90%,” said Kripa Varanasi, an associate professor mechanical engineering.

“If you’re able to improve the boiling process that produces this steam, you can improve the overall power plant efficiency,” Varansi added.

When boiling water, bubbles of vapour that are produced limit energy efficiency, the researchers found. On a hot, boiling surface, therefore, the larger the area covered in bubbles, the less transfer of heat energy becomes.

If such vapour persists in one area for too long, metal components can drastically increase in temperature, as heat is not transferred away quickly enough, which, Varansi claims, could potentially melt part of the metal.

“This will most certainly damage an industrial boiler, a potentially catastrophic scenario for a nuclear power plant or a chemical processing unit,” said Navdeep Singh Dhillon, a mechanical engineering postdoc at MIT.

“When a layer of bubbles limits heat transfer, locally, the temperature can increase by several thousand degrees – a phenomenon known as a ’boiling crisis’,” Dhillon added.

To increase efficiency, while limiting the risk of catastrophic plant failure, the MIT team claims exceeding critical heat flux can be avoided if surfaces are not overly textured - something the researchers say is contrary to prevailing views.

The research, which used high-speed optical and infrared imaging of the boiling process, showed a maximum benefit at a certain level of surface texturing; understanding exactly where this maximum value lies and the physics behind it is vital to improving boiler systems, the researchers said.

“As the bubble begins to depart the surface, the surrounding liquid needs to rewet the surface before the temperature of the hot dry spot underneath the bubble exceeds a critical value,” Varanasi said.

According to the researchers, this requires an understanding of the coupling between liquid flow in the surface textures and its thermal interaction with the underlying surface.

“If anything can enhance the heat transfer, that could improve the operating margin of a power plant,” Varanasi says, which would effectively allow a plant to operate safely at higher temperatures.

Similar research conducted by MIT professor Evelyn Wang in 2012 found that the heat transfer process in items such as electronic devices could be enhanced by increasing the roughness of a surface.

In that research, Wang’s team also suggested the process of increasing surface roughness to enhance heat transfer could be applied to industrial boilers.

The research conducted by Varanasi’s team, however, shows that more surface texturing is not always better for heat transfer in certain applications such as those found within industrial-scale power plants or nuclear reactors.

A full account of the research has been published in the journal Nature Communications.