Nucleate Pool Boiling of Surface-Modified Nano-Porous Alumina
AuthorZhang, Bong June
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In nucleate pool boiling, heat transfer augment is an important issue. Among various techniques for enhancement, porous surfaces with millimeter to micron-sized diameter have been studied in the past few decades. In this study, the nano porous surface (NPS) of alumina was created to address technical issues in submicron regime pool boiling. Especially, influence of surface modifications (structural and chemical treatments) of the NPS over heat transfer enhancement was the subject of the reported research. There are a few important aspects of the research reported in this dissertation: to develop the optimal NPS to increase heat transfer in nucleate pool boiling, to suggest the mechanistic model to explain heat transfer augment in submicron regime, and to assess the feasibility of the NPS for nucleate pool boiling enhancement. First of all, an optimal NPS with surface modifications for heat transfer in nucleate pool boiling was developed. The surface-modified NPS provides enlarged heating surface area, active nucleation site enhancement, and improvement of vapor-liquid menisci through reentrant-shaped cavities. Secondly, the mechanistic model suggested in this study explained that the combination of convection-driven and phase change-induced heat transfers and liquid thin film evaporation (LTFE) achieves heat transfer augment. Especially, the model supported the hypothesis that the NPS is appropriate to dissipate heat through the LTFE at lower heat flux regime. Lastly, this study shows that heat transfer enhancement phenomena of the NPS that was evaluated and quantified by using various physical and electrochemical methods. For instance, most contributing factor of the CHF enhancement was assumed the liquid spreading and the absorption-induced capillary pressure. They were assessed by using Apparent Water Contact Angle (AWCA) and Electrochemical Impedance Spectroscopy (EIS) measurements of which trends were well matched with CHF experimental data.