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This article was first published in 2004.
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It's such a simple idea that it'll take just
moments to wonder why it hasn't been done before. Consider the function of
coolant in a car - it transfers the heat from the engine to the radiator, where
it's dissipated to the air. But think how much better that coolant would work if
it was full of tiny suspended particles of a conductor like copper. Suddenly
you've got not only more heat transfer ability but also better heat
conductivity.... And it needn't apply just to coolant. One of the functions of
lubricating oils is to transfer heat away from hot surfaces - and tiny suspended
particles can work to improve the performance of oil as well.
And it's not just an idea - development of this
nanofluid technology is happening right now.
Nano What?
Engineers have been working for decades to develop
more efficient heat transfer fluids for car motors and industrial equipment. But
despite the known improvements in thermal performance able to be gained by
suspending small solids in fluid, until recently engineers could only create
microparticles - large enough still to be visible to the naked eye and with a
diameter a thousand times greater than nanoparticles.
These microparticles were so large that they would
quickly settle out of the fluid and sink to the bottom of a pipe or tank. Even
if the fluid was kept circulating rapidly enough to prevent much settling, the
microparticles would damage the walls of the pipe, wearing them thin. The
abrasive particles would also quickly wear out pumps and bearings.
But the use of nano-sized particles has apparently
overcome these problems.
[A nanometer (nm) is one-billionth of a meter - about
1/50,000th the width of a human hair, much smaller than can be seen unaided by
the human eye.]
Nanofluids are made by suspending nano particles
of materials such as carbon, copper or copper oxide in liquids such as oil,
water and engine coolant (a mixture mostly of water and ethylene glycol).
At the Argonne National Laboratory in the US,
researchers found that the addition of 3 percent (by volume) of copper oxide
nanoparticles to ethylene glycol increased its heat conduction by 15 percent.
However, the heat-transfer capability of ethylene glycol grew by a stunning 40
percent when only 0.3 percent of 10 nanometer diameter spheres of pure copper
were suspended in it.
The two Argonne scientists - Steve Choi and Jeff
Eastman - who developed nanofluids, started collaborating in 1993 when Choi, a
heat-transfer specialist, heard Eastman was studying nanometer-sized crystals.
Choi had long been frustrated with the limitations of mixing traditional fluids
with traditional small particles.
The breakthrough came in two new methods of
creating the nanoparticles.
Getting the Particles into the Fluid
"We found that you could first condense gaseous
metal oxide into particles between 30 and 50 nanometers in diameter, then mix
them into a fluid afterward," Choi said. "It was a two-step process. But our
best results came when we made the nanoparticles from pure copper in a single
step."
The one-step method employs direct
evaporation-condensation that results in very small nanoparticles that disperse
well. The metal nanofluids it produces are the only ones to date that are
extremely stable as a result of particle size alone - no dispersants are needed
to achieve long-term stability. In fact, the particles are so small that in some
cases, there is little or no settling of the particles after even months. It is
this technique which produces the highly conductive metal nanofluids.
The simpler, less expensive two-step method
produces oxide and non-metallic nanofluids. This method first produces
nanoparticles and then disperses them in a base fluid. It is simple, and it is
less costly and works with more fluids than the one-step method.
(Keeping the nanoparticles suspended is a key to
success. In the case of engine coolant, the suspension of nanoparticles has
found to be improved with the use of the commercially-available additive,
Redline Water Wetter.)
Eastman and Choi hold a patent on the idea that
dispersing nanoparticles into fluids can improve heat transfer properties.
Why They Work
The improved heat transfer performance of
nanofluids is due to the fact that the nanoparticles:
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- Increase the surface area and heat capacity of the
fluid
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- Improve the thermal conductivity of the fluid
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- Cause more collisions and interactions between the
fluid, particles and flow passages
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- Cause more turbulence and mixing of the
fluid
In addition, some researchers have found that the
addition of the nano-particles can improve lubricating performance of both
coolant and oils.
Cars
Tests have shown that nanofluids have
characteristics that make them ideal for engine cooling systems. Their ability
to respond quickly to temperature changes allows for the dissipation of more
heat using less coolant and in a shorter time.
In addition, engine oils, automatic transmission
fluids, and other synthetic high-temperature heat transfer fluids all currently
possess poor heat transfer capabilities and so could benefit from the high
thermal conductivity offered by nanofluids.
Nanofluids are currently expensive, in part
because the equipment used to manufacture them is one-of-a-kind. The fluids must
be made more affordable before they will see widespread use.
But when they do, expect to see radiators
literally halve in size...
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Argonne
National Laboratory Transportation Technology R&D Centre
The
Effect of Nanoparticle Additions on the Heat Capacity of Common Coolants - SAE
paper 2002-01-3319
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