Nanofluids with Enhanced Thermal Transport Properties
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Heat transfer fluids have inherently low thermal conductivity that greatly limits the heat exchange efficiency. While the effectiveness of extending surfaces and redesigning heat exchange equipments to increase the heat transfer rate has reached a limit, many research activities have been carried out attempting to improve the thermal transport properties of the fluids by adding more thermally conductive solids into liquids. Liquid dispersions of nanoparticles, which have been termed "nanofluids", exhibit substantially higher thermal conductivities than those of the corresponding base fluids. In this study, new nanofluid systems have been developed by utilizing semiconductor nanorods, hybrid nanoparticles, phase-change liquid nanodroplets and phase-change metallic nanoparticles as the dispersed phases. A nanoemulsification technique has been developed and used to synthesize nanofluids. The thermal transport properties of nanofluids, including thermal conductivity, viscosity, heat capacity and heat transfer coefficient in convective environment were characterized and modeled. Obvious thermal conductivity increases have been observed in these nanofluid systems, e.g., 52% enhancement in thermal conductivity was found in water-in-FC72 nanofluids. This anomalous enhancement can not be well explained by the Effective Medium Theory. Theoretical Models based on Ordered Liquid Layering, Brownian motion and Nanoparticle Aggregates theories are used to describe the thermal conductivity enhancement in nanofluids. Since the heat capacity of heat transfer fluids is another important thermal transport property, phase-change nanodroplets and nanoparticles are thus used to synthesize phase-change nanofluids. Up to 126% and 20% increases in the effective heat capacity were experimentally found in water-in-FC72 nanoemulsions and indium-in-PAO nanofluids，respectively, due to the large amount of latent heat absorbed in phase transition from nanoparticles to nanodroplets and released in reverse transition. The viscosity of nanofluids is increased as a result of the addition of nanoparticles, which can be described by the Einstein-Batchelor model. But due to the enhanced thermal conductivity of nanofluids, 15% increase in heat transfer coefficient of natural convection has been observed in water-in-FC72 nanofluids. The results show that nanofluids possess improved thermal transport properties and it has been experimentally proved that nanofluids have potential as next-generation advanced heat transfer fluids.