The aim of this study is to understand the temperature and viscosity effects on the thermal conductivity of ferro-nanofluid. The base-fluid of ferro-nanofluid is made of polydimethyl-siloxane (PDMS) and diesel fuel which have similar thermal property but different viscosity. The viscosity of base-fluid is controlled by changing the volume ratio of both fluids. The measured results show that the thermal conductivity is smaller when the base fluid is highly viscous, and the thermal conductivity approaches to the value predicted by the Maxwell equation. It should be that the Brownian motion effect on highly viscous fluid is not as important as on lower viscosity fluid. Usually rising temperature will decrease viscosity of base fluid. In our study, rising temperature to 58°C can reduce the viscosity of 90%diesel fuel+10%PDMS to pure diesel fuel base fluid at 23 °C. At the same viscosity condition, rising temperature will decrease the thermal conductivity of ferro-nanofluid. The reason is that the thermal conductivity of base-fluid decrease dramatically. Comparing thermal conductivity of 2% volume fraction ferro-nanofluid to base-fluid at 23°C and 58°C respectively, the value increases 8.3% at 23 °C but increases 18.8% when the temperature is 58°C. This means the Brownian motion is more active at higher temperature than lower temperature.

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