This thesis discusses two topics. The first is the effect of the viscosity of a base fluid on the thermal conductivity of nanofluids. The other one is the application of Fe3O4
nanofluids on transformers.
The thermal conductivities of nanofluids with various viscous base fluids were measured. The viscosity of the base fluids, the parameter of interest, was varied by changing the mixing ratio of two base fluids. The experimental results support the following conclusions.
1. The measured thermal conductivity of nanofluids with a low viscous base fluid exceeds that predicted by Maxwell prediction model.
2. As the viscosity of the base fluids increases, the measured thermal conductivity of the nanofluids gradually approaches the value predicted by the Maxwell prediction model. This result indicates that the high viscosity of base fluid constrains the Brownian motion of suspended nanoparticles and reduces their enhancement of thermal conductivity.
3. However, in a highly viscous fluid, the Maxwell prediction model accurately predicts the thermal conductivity of nanofluids, as the Brownian motion of suspended nanoparticles is unimportant, and kstatic=kMaxwell.
4. In summary, experimental results obtained using base fluids of various viscosities
indicate that the Brownian motion of suspended nanoparticles is important to their enhancement of the thermal conductivity of nanofluids, and that a model to predict the thermal conductivity of nanofluids should be based on the Maxwell prediction model.
Although numerous predictive models have been proposed, none offers accurate predictions on thermal conductivity for all nanofluids. Apart from the size and geometry of nanoparticles, the thermal properties of materials, the temperature of the nanofluids and other factors, many unknown factors, such as surface thermal resistance between nanoparticles and the base fluid affect the thermal conductivity of nanofluids. A theoretical understanding of the mechanisms is still lacking. Moreover, experimental results from different research groups regarding the production of nanofluids and the measurement of thermal conductivity do not agree closely. Advances in the heat transfer by nanofluids must address this fact. Therefore, further theoretical and experimental investigations must be performed to understand the heat transfer characteristics of a nanofluid. More measurements of the thermal conductivities of nanofluids with base fluids of various viscosities must be made to establish a predictive model that includes the effect of Brownian motion on oil-based ferrofluids. The effect of the viscosity of base fluids on the thermal conductivity of nanofluids should be confirmed with reference to various nanofluids.
Ferrofluids were used as the magnetic cores of two transformers. The performance of transformers with magnetic cores of air, bulk Fe3O4 and ferrofluids is measured. The experimental results support the following conclusions.
1. The presence of Fe3O4 improves the inductance of the coils and the coupling coefficient.
2. However, the lag between the magnetization of materials and the external magnetic field increases resistance. Moreover, as the frequency increases, the resistance increases to a great extent, and faster than inductance, yielding a low quality factor.
3. Although ferrofluid is not suitable for use as the magnetic core, it can be applied as a carrier of ferro-nanoparticles into the microchannel. Then, a solid magnetic core can be obtained by repeatedly adding ferrofluid and removing the base fluid. The experimental results of the application of bulk Fe3O4 reveal that it slightly improves the performance of a transformer at a frequency of less than 4 MHz.
4. This fabrication process of solid magnetic core has a lower thermal budget than the sputtering and electroplating processes, and it is compatible with the MEMS process.
This thesis proposed a new process for fabricating solid magnetic cores. However, the permeability of the solid magnetic core that was fabricated in this study was not as high as that of a sintered ferrite core. Therefore, further investigations of the method of
synthesis of magnetic nanoparticles, a substituent magnetic material and the fabrication of bulk Fe3O4 are needed to improve the performance of transformers and realize this new method for fabricating magnetic cores in the future.
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