Nanofluid Research Can Lead to Cooler Engines

Diesel engine components are evolving rapidly in response to emission restrictions and continuing demands for more horsepower and better fuel efficiency. Cooling systems must operate at higher temperatures while transferring larger amount of heat to the surrounding environment. Radiator size must be decreased to make trucks more aerodynamic and to improve driver visibility. Realistically, it will be possible to pack more cooling power into smaller spaces only through the use of new technologies, such as nanofluids.

What are nanofluids? They are traditional heat transfer fluids in which nanometer-sized solid particles of some material have been dispersed to increase thermal conductivity and heat transfer. 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 have shown much higher thermal conductivities and heat transfer coefficients than traditional heat transfer fluids can achieve alone.

Argonne researchers are examining new ways to improve thermal conductivity in truck engines through the use of nanofluids. In previous research, they demonstrated that the thermal conductivity of ethylene glycol increased by up to about 20% when a small volume (4%) of cupric oxide nanoparticles (having an average diameter of 35 nm) was dispersed in it. A similar gain was seen in a study involving aluminum oxide nanoparticles dispersed in water.

Reseachers are focusing on nanoparticles as small as 10 nm in diameter (on average). Several processes to produce nanofluids are being explored. One technique involves changing copper vapor into nanoparticles by allowing the vapor to come into direct contact with a flowing liquid. Another technique involves introduction of solid materials which are chemically reduced into a base fluid.

Argonne staff also pursues theoretical research to inform and guide the empirical studies. One outcome of this work is a theoretical model of the dynamic behavior of nanoparticles in fluids. The modeling showed that the thermal behavior of nanofluids is governed primarily by Brownian motion, the erratic and constant movement of tiny particles suspended in a fluid or gas. The model can also be used to predict a nanofluid's thermal conductivity on the basis of concentration, operating temperature, and nanoparticle size. Furthermore, the properties of nanolayers that form on the surface of suspended nanoparticles may provide a means to further increase the thermal conductivity of nanofluids. A model based on near-field radiation for very close-spaced nanoparticles is also under investigation.

In a related research effort, erosion studies of nanofluids are being conducted. One critical question is: "Will nanofluids cause damage to radiator systems?" Argonne researchers are answering this question. They have built an apparatus that can emulate the coolant flow in a radiator and are currently testing and measuring the erosion (material loss) of typical radiator materials by various nanofluids.

Future research will focus on nanoparticle materials containing aluminum and oxide-coated metal nanoparticles. Research results will assist developers of engine cooling and other thermal management systems that might benefit from the use of nanofluids for cooling.

Facilities

• Unique nanofluid production system designed for direct evaporation of metals into low-vapor-pressure liquids
• Nanoparticle characterization tools, including transmission electron microscopes, small-angle X-ray scattering using the Advanced Photon Source, laser interferometry, viscometer
• Transient hot-wire cell designed and built for the measurement of the thermal conductivities of nanofluids
• Apparatus to measure heat transfer in both single-phase and two-phase flows
• Computer simulations of heat flow in truck radiators

June 14, 2007