| Nanofluids, homogeneous and stable suspensions which contain nanoparticles dispersed in base fluids like deionized water (DW), ethylene glycol EG, and oil have been proposed as a new class of heat transfer media. Due to the enhancement in heat transport and energy efficiency, nanofluids have potential application prospects in the field of heat transfer systems, such as solar heat exchangers, coolers of engines and chips, and so on. Magnetic nanofluids possess not only the magnetic properties of common magnetic fluids but also the heat transfer enhancement performance of nanofluids when magnetic nanoparticles are dispersed into the base fluids.It is expected that nanofluids be more stable than those suspensions containing micron-sized particles. However, it is still not fit for settling for a long time due to the electric charges and chemical groups on the surfaces of nanoparticles, agglomerations etc. Preparation of homogeneous and stable nanofluids is the precondition of practical applications of nanofluids. Thermal conductivity and heat transfer coefficient would be improved obviously and the rheological behavior would change due to the dispersion of the nanoparticles. In this study, the magnetic nanofluids have been prepared. The thermal conductivity, heat transfer coefficient, and rheological behavior were further investigated. The results are summarized as follows:Firstly, when the volume fraction of nanofluids was 2.0 vol.%, the thermal conductivities enhancements were obtained to be 3.1%,8%,and 3.7% for EG/DW mixture, EG and DW as base fluids, respectively. For EG based nanofluid at a volume fraction of 5.0 vol.%, the thermal conductivity enhancement amounted up to 12%. Furthermore, it is demonstrated that the temperature has no obvious effect on the thermal conductivity enhancement of nanofluids in our experiments. Magnetic field was shown to have a critical effect on the thermal conductivity enhancement of the magnetic nanofluids. For instance, the thermal conductivity enhancement of y-Fe2O3/ DW magnetic nanofluid was 3.7% without magnetic field and 23.3% under magnetic field (550 Gauss), with 2.0% nanocomposite loadings.Secondly, in order to improve the dispersity of y-Fe2O3 nanopartciles into oil, core-shell structure nanocomposite (y-Fe2O3/PMMA) was fabricated which dispersed well in oil. The thickness of PMMA layer was observed to be about 5nm. Magnetic nanofluids which consisting of core-shell strctured nanocomposites as additives and engine oil as base fluid were prepared and their thermal transport properties were investigated. It is showed that the thermal conductivity of the nanofluids increase with the particle loading. Furthermore, the magnetic field has a critical effect on the thermal conductivity of nanofluids. For instance, the thermal conductivity enhancement of (y-Fe2O3/PMMA)/3GS oil magnetic nanofluid was 7% without magnetic field and 21.8% under magnetic field (770 Gauss), with 2.0% nanocompositc loadings.Thirdly, rheological behaviors of magnetic nanofluids were demonstrated Newtonian behavior and the viscosity of magnetic nanofluids depended strongly on the tested temperatures and the nanoparticle loadings. It is shown that the viscosity of the magnetic nanofluids increases with the augment of the volume fraction, but decreases rapidly with an increase in the temperature. Furthermore, convective heat transfer coefficients measured in a laminar flow showed that the coefficients increased with the augment of Reynolds number and the volume fraction. For the nanofluid with a volume fraction of 0.02 at Reynolds number of 1000, the convective heat transfer coefficient and Nusselt number can be enhanced by more than 60% and 55%, respectively. For the nanofluid with the flow rate of 100mL/min, the convective heat transfer coefficient can be enhanced by more than 50% when the volume fraction is 0.02.
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