November 21, 2013
Protecting Systems From Heat May Rely On Nano-Magnetic Fields
April Flowers for redOrbit.com - Your Universe Online
Generally, cooling systems rely on water pumped through pipes to remove unwanted heat. A research team from MIT and the University of Newcastle in Australia has discovered a method for enhancing heat transfer in such systems using magnetic fields.This new method could prevent hotspots that lead to system failures.
The innovative new system, which relies on tiny particles of magnetite -- a form of iron oxide - could be applicable to cooling everything from electronic devices to advanced fusion reactors.
Lin-Wen 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 current study involved experiments where the magnetite nanofluid flowed through tubes and was manipulated by magnets placed on the outside of the tubes. The results of this study were published in the International Journal of Heat and Mass Transfer.
According to Hu, the magnets “attract the particles closer to the heated surface” of the tube. This enhances the transfer of heat from the fluid, through the walls of the tube, and into the air outside. Without the magnets in place, the fluid performs just like water, with no change in its cooling properties. With them, however, the heat transfer coefficient is higher, in the best case, about 300 percent better than with plain water.
“We were very surprised” by the magnitude of the improvement, Hu says.
Prior, conventional methods for improving the heat transfer in cooling systems employ features such as fins and grooves on the surfaces of the pipes, increasing their surface area. Methods such as these provide some improvement in the heat transfer, but not as effectively as the magnetic particles. The fabrication costs of these features are prohibitive as well.
Hu says that the reason the new system works is that the magnetic field tends to cause the particles to clump together — possibly forming a chain-like structure on the side of the tube closest to the magnet, disrupting the flow there, and increasing the local temperature gradient.
The concept has been suggested before; however it had never been proven in action. "This is the first work we know of that demonstrates this experimentally,” Hu says.
Hu notes that such a system would be impractical for application to an entire cooling system. However, it could be useful in any system where hotspots appear on the surface of cooling pipes. One method for dealing 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 Jacopo 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.”
Heat transfer 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, however, which may not be desirable in some situations.
Buongiorno says that there can be numerous applications for such a system. "You can think of other systems that require not necessarily system-wide cooling, but localized cooling.”
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.
In the future, this approach might even have applications 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."