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.
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:
In addition, some researchers have found that the addition of the nano-particles can improve lubricating performance of both coolant and oils.
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...