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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 heat transfer
coefficient 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 cooling systems 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 magnetic 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.
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 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, 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.”
The team also included Thomas McKrell, a research scientist in MIT’s
Department of Nuclear Science and Engineering, and Elham Doroodchi,
Behdad Moghtaderi, and Reza Azizian of the University of Newcastle in
Australia. The work was supported by the University of Newcastle,
Granite Power Ltd., the Australian Research Council, and King Saud
University in Saudi Arabia.
MIT; November 19, 2013
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