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Fabrication and characterization of the nanofluids by the electrical explosion of the wire in liquid method

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Title
Fabrication and characterization of the nanofluids by the electrical explosion of the wire in liquid method
Author
Park, Eunju
Advisor
Park, Hyung Wook
Issue Date
2015-02
Publisher
Graduate school of UNIST
Abstract
Although nanofluids have superior and unique properties, the problem of dispersion stability and the mass production of nanofluids are still limited in the industrial applications. For this study, nanofluids were prepared using the electrical explosion of wire in liquids (EEWL) method. EEWL enables the manufacture of high-purity nanoparticles without the chemical additives. Other advantages of this method are its simple evaporation and condensation processes, short production times, and the feasibility for mass production. The EEWL method has been employed in the production of pure metallic nanofluids as well as their oxides. In this study, silver, copper oxide and aluminum oxide nanofluids were synthesized under various experimental parameters, such as voltage, capacitance, length of the wire and basefluid. The size of nanoparticles depends on the deposited energy value in the wire. The phase and morphology of nanoparticles are influenced by the basefluid and the additives. The nanoparticles synthesized by the EEWL process are spherical in shape. Copper oxide and aluminum oxide nanoparticles are synthesized in the water due to a chemical reaction with oxygen and hydroxyl. In the case of copper, the copper oxide nanoparticles were synthesized to be spherical in shape in the water as basefluid, whereas the 3D copper oxide structure particles were synthesized by adding sodium hydroxide (NaOH) or ammonia (NH3∙H2O) in the water. In this study, the metallic and oxide nanofluids were prepared in the various base fluids. The structure and morphology were investigated using various analysis tools, such as X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. The optical band gap was determined using ultraviolet/visible spectroscopy. The impact of deposited energy in the wire on the size and shape of the nanoparticles were analyzed by using TEM analysis. In order to determine the optimized experimental conditions of nanofluids production, the investigation of the effects of experimental parameters are very important. As a method for the optimization of the process conditions, we used design of experiments (DOE) and analysis of calculated deposited energy values in the wire by measuring the voltage and current oscillograms. Through production of the silver nanofluids, optimized conditions can be determined by the MINITAB tool. The energy used in the explosion has to be experimentally quantified using the voltage and current oscillograms because it is difficult to directly measure the energy and to theoretically calculate it. Furthermore, the explosion phenomena can be explained by the analysis of the voltage and current oscillograms. The thermal conductivity and the viscosity of aluminum oxide nanofluids were measured and compared with the prediction models. The experimental results are similar to the prediction model, the increasing of the thermal conductivity and the viscosity could not be confirmed because of the very small concentration of nanoparticles. A pool boiling experiment was performed to investigate the effect of nanofluids on critical heat flux (CHF) enhancement. The nanofluids for experiments were prepared into two groups; 1) the water-based silver, copper oxide and aluminum oxide nanofluids synthesized by electrical explosion of the wire in liquid method and 2) xGnPs and xGnPs oxide nanofluids prepared by sonication. The silver, copper oxide and aluminum oxide nanofluids prepared by the electrical explosion of the wire in liquid process caused significant CHF enhancement during pool boiling experiments: 187% at 0.001 vol% for aluminum oxide, 58% at 0.001 vol% for silver, and 99% at 0.001 vol% for copper oxide nanofluids. The xGnPs oxide nanofluids dispersed with 0.005 vol% particles showed the largest CHF enhancement (189%). Deposition of nanoparticles on the wire surface occurs during nucleate boiling, and it can change the surface properties. In this study, the CHF phenomenon was also predicted using Kandlikar’s CHF model. The CHF results of silver, copper oxide and aluminum oxide nanofluids are in accord with the trend of the theoretical results. However, in case of the xGnPs and xGnPs oxide nanofluids, the CHF is enhanced with increasing of the contact angle value while CHF by Kandlikar’s model is enhanced with decrease the contact angle.
Description
Department of Materials Science Engineering
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MSE_Thesis_Ph.D.
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