Magnetic nanoparticles could aid heat dissipation.


Cooling systems generally rely on water pumped through pipes to remove unwanted heat. Now, researchers at MIT and in Australia have found a way of enhancing heat transfer in such systems by using magnetic fields, a method that could prevent hotspots that can lead to system failures. The system could also be applied to cooling everything from electronic devices to advanced fusion reactors, they say.

The system, which relies on a slurry of tiny particles of magnetite, a form of iron oxide, is described in the International Journal of Heat and Mass Transfer, in a paper co-authored by MIT researchers Jacopo Buongiorno and Lin-Wen Hu, and four others.

Hu, associate director of MIT’s Nuclear Reactor Laboratory, says the new results are the culmination of several years of research on nanofluids—nanoparticles dissolved in water. The new work involved experiments where the magnetite nanofluid flowed through tubes and was manipulated by magnets placed on the outside of the tubes.

The magnets, Hu says, “attract the particles closer to the heated surface” of the tube, greatly enhancing the transfer of heat from the fluid, through the walls of the tube, and into the outside air. Without the magnets in place, the fluid behaves just like water, with no change in its cooling properties. But with the magnets, the  is higher, she says—in the best case, about 300 percent better than with plain water. “We were very surprised” by the magnitude of the improvement, Hu says.

Conventional methods to increase heat transfer in  employ features such as fins and grooves on the surfaces of the pipes, increasing their surface area. That provides some improvement in heat transfer, Hu says, but not nearly as much as the particles. Also, fabrication of these features can be expensive.

The explanation for the improvement in the new system, Hu says, is that the magnetic field tends to cause the particles to clump together—possibly forming a chainlike structure on the side of the tube closest to the magnet, disrupting the flow there, and increasing the local temperature gradient.

While the idea has been suggested before, it had never been proved in action, Hu says. “This is the first work we know of that demonstrates this experimentally,” she says.

Magnetic nanoparticles could aid heat dissipation

Such a system would be impractical for application to an entire cooling system, she says, but could be useful in any system where hotspots appear on the surface of cooling pipes. One way to deal with that would be to put in a magnetic fluid, and magnets outside the pipe next to the hotspot, to enhance heat transfer at that spot.

“It’s a neat way to enhance heat transfer,” says Buongiorno, an associate professor of nuclear science and engineering at MIT. “You can imagine magnets put at strategic locations,” and if those are electromagnets that can be switched on and off, “when you want to turn the cooling up, you turn up the magnets, and get a very localized cooling there.”

While  can be enhanced in other ways, such as by simply pumping the cooling fluid through the system faster, such methods use more energy and increase the pressure drop in the system, which may not be desirable in some situations.

There could be numerous applications for such a system, Buongiorno says: “You can think of other systems that require not necessarily systemwide cooling, but localized cooling.” For example, microchips and other electronic systems may have areas that are subject to strong heating. New devices such as “lab on a chip” microsystems could also benefit from such selective cooling, he says.

Going forward, Buongiorno says, this approach might even be useful for fusion reactors, where there can be “localized hotspots where the heat flux is much higher than the average.”

But these applications remain well in the future, the researchers say. “This is a basic study at the point,” Buongiorno says. “It just shows this effect happens.”

How liquids boil without bubbling?


Explosions caused by boiling liquid could be reduced by suppressing the liquid from bubbling, according to a new University of Melbourne study.

The research, which is the first of its kind, has identified a specially engineered steel surface that allows liquids to boil without bubbling.

“This would be advantageous for use in industrial situations such as nuclear power plants, where vapour explosions are best avoided, or where gentle heating is desirable” said Professor Derek Chan, from the University’s Department of Mathematics and Statistics.

The study suggests that the new surface could also be applied to other situations that involve the transfer of heat, such as reducing fogging and preventing ice or frost formation on windows.

“Our results show the potential of using this textured surface to control heating and cooling events that affect the formation of frost on windows and ice on the control surfaces of aircrafts or even refrigeration units,” he said.

The international study was done in collaboration between the University of Melbourne and Dr Neelesh Patankar from the Northwestern University in the United States and Dr Ivan Vakarelski and his team at the King Abdullah University of Science and Technology in Saudi Arabia where the experimental studies were carried out.

The study was published in the journal Nature.

The research found that a textured, highly water-repellent steel surface controls the boiling process of a liquid and stops it from bubbling up the sides of a container and boiling over.

This is achieved by using a textured surface structure to control the stability of the vapour layer, that is, the layer of steam that forms on a surface when water is boiled.

“In most smooth surfaces, heat transfer from the surface to the liquid is prevented by the low thermal conductivity of the vapour layer,” said Professor Chan.

“This vapour layer collapses when the surface cools, which could result in an explosion.”

Professor Chan said that in textured surfaces, the vapour layer is maintained until the surface is completely cooled, preventing the liquid from bubbling and boiling over.

“The discovery shows how the texture of surfaces can combine to control the boiling of liquid in a way that was not thought to be possible”, he said.

Source: Science Alert