TransForum Vol. 3, No. 4

NANOFLUIDS COULD HELP OPEN DOOR TO ADVANCED TRUCK DESIGNS

Driven by the need for higher-horsepower engines and by new emissions-reduction technologies, truck manufacturers are constantly seeking ways to improve the aerodynamic designs — and thus increase the fuel economy — of their vehicles. One component of this quest is reducing the amount of energy needed to combat wind resistance. At 70 miles per hour, about 65% of the total energy output from a typical heavy-duty truck engine goes to simply overcoming aerodynamic drag. One big reason air resistance is so high is the presence of a large radiator directly in front of a truck's engine to maximize the cooling effect of onrushing air.

The large radiators are needed because of the type of fluids that circulate in cooling systems, drawing heat from the engine and transporting it to the radiator, where it can be released to the surrounding air. These fluids are chosen, in part, because they have high heat capacities, which means that they can absorb a lot of heat while their temperatures increase only minimally. The fluids, however, also absorb and release heat very slowly. It's the slow release of heat that accounts for the size and positioning of vehicle radiators. If the fluids could conduct heat more quickly, radiators could be made smaller and configured to allow for highly streamlined designs. As a bonus, you wouldn't need as much fluid to do the same job, so coolant pumps could be reduced in size. Or truck engines could be operated at hotter temperatures to provide more horsepower while still meeting stringent emission standards. High-conductivity coolants also would be ideal for advanced fuel-cell and hybrid-electric vehicles, in which they could help keep running temperatures low.

Metallic nanofluids show dramatic enhancements in thermal conductivity compared with nanofluids based on oxide particles.

Argonne researchers are developing a way of giving traditional engine coolants the high thermal conductivities they need, without adversely affecting their thermal capacities. The researchers discovered that dispersing small amounts of solid particles in the fluids boosts their thermal conductivities by unexpectedly large amounts. The trick is to use particles that are no larger than tens of nanometers in size. The result is a new class of heat-transfer fluids, called nanofluids. Because the particles are so small, nanofluids aren't plagued by settling problems, whether or not dispersants are used, so they will not cause clogging — even with microchannel heat transfer devices.

Funded by the U.S. Department of Energy, Argonne is collaborating with the Valvoline Company to develop nanofluid coolants and lubricating oils for truck engines. Already the researchers have demonstrated that the thermal conductivity of ethylene glycol increases by up to about 20% when a small volume (4%) of cupric oxide nanoparticles (with an average diameter of 35 nm) is dispersed in it. A similar gain was seen in a study involving aluminum oxide nanoparticles dispersed in water. More recently, Argonne found that nanofluids consisting of copper nanoparticles dispersed in ethylene glycol show much higher thermal conductivities than do either pure ethylene glycol or ethylene glycol containing the same volume fractions of dispersed oxide nanoparticles.

The discovery that adding nanometer-size particles to traditional heattransfer fluids dramatically increases their thermal conductivity is as yet without a theoretical explanation, so getting a better basic understanding of the phenomenon is one goal motivating the Argonne research program. Another is to develop economical ways of producing nanoparticles in production-scale batches. Argonne researchers now use a one-step procedure for producing nanofluids based on metallic particles and a two-step procedure for oxide-based nanofluids. Both techniques are relatively straightforward and hold the potential for economical batch production.

As a spin-off from the nanofluid research, Argonne has been looking at the effect of soot in engine oil. Soot accumulates in engine oil over time in amounts that are expected to increase dramatically as engine designers resort to using exhaust gas recirculation to limit exhaust emissions. Even though soot particles are not as small as those in nanofluids, the researchers found that their accumulation in engine oil leads to a 15% increase in thermal conductivity and an increase in the oil's lubricating properties, as well. The former finding opens the door to development of a sensor that monitors engine performance by measuring thermal conductivity increases as soot builds up inside an engine.