A fundamental study on heat transfer characteristics of magnetite nanofluids

Abstract

Research Doctorate - Doctor of Philosophy (PhD)This study investigated the heat transfer behaviour of nanofluids with focus on a magnetite nanofluid as an alternative for the next generation of cooling applications. The very first aim of this study was to shed light on conflicting data reported in the literature about the thermal conductivity of conventional nanofluids (non-magnetic). For this purpose, the thermal conductivity of different nanofluid samples were measured under different conditions (namely particle volume fractions, temperature and pH) and results were compared with theoretical prediction of mixture theory. Shortcomings of different hypotheses were examined, analysed and explained. All of the experimental data were within the ±10% boundary of theoretical mixture theory (i.e. Maxwell model). It was shown that higher thermal conductivity in the conventional nanofluids can be achieved by controlling the aggregation size (i.e. pH value). To test this hypothesis, artificial aggregates were generated by applying an external magnetic field to the magnetite nanofluid. Thermal conductivity enhancement by up to 160% was observed for 0.86 vol% magnetite nanofluids (Fe3O4-water), subjected to the magnetic field intensity of 32 mT parallel to the temperature gradient. On the other hand, the thermal conductivity of magnetite nanofluid is almost constant under the applied magnetic field perpendicular to the temperature gradient. Convective heat transfer coefficient of magnetite nanofluid under laminar flow condition was also considered in the absence and presence of an external magnetic field. Heat transfer in magnetite nanofluids with and without magnetic field has been performed previously. However, the extent of the experiments and analysis is the main goal which has been achieved through this study. It has been shown that the experimental data in the absence of an external magnetic field follows the prediction of standard correlation for laminar flow heat transfer (Shah equation) using the thermophysical properties of nanofluid. Up to 300% enhancement in convective heat transfer enhancement has been observed at Re=745 and the magnetic field gradient of 32.5 mT/mm. Moreover, the effect of magnetic field on pressure drop was insignificant. It is concluded here that aggregation is the main mechanism of heat transfer in nanofluids. The thermal conductivity and convective heat transfer coefficient of magnetite nanofluids can be altered far beyond the prediction of traditional theories by applying an external magnetic field