Date of Award

Spring 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

The aim of this study is to enhance and sustain the thin liquid film evaporation (TLFE) as well as dropwise condensation (DWC) on scalable copper substrates. The current dissertation consists of two sections. The objective of the first section is to enhance the dry cooling technology which is widely used in industrial applications such as in the air side of power plants, AC unit, or electronic cooling. However, the thermal dissipation performance is limited with low heat transfer coefficients (HTCs) due to the low thermal conductivity and density of the air. Inspired by the phase change heat transfer during the perspiration of mammals, a sweating-boosted air cooling strategy was proposed to improve the thermal dissipation during the dry cooling process. In our previous research, we have demonstrated the effectiveness of a nanoscale ALD TiO2 and CuO wick structures with grooves on copper on the sweating-boosted air cooling performance. The results showed that this technique could effectively enhance the heat transfer rates much higher than the dry cooling. However, for practical applications, developing practical heating surfaces with high wettability and for the long-term use against organic contamination is a critical issue. Therefore, it is crucial to develop robust superhydrophilic surfaces with excellent mechanical and chemical stability for long-term operations.

In this study, durable superhydrophilic green patina microstructures were successfully fabricated over large surfaces on the copper substrate by a two-step chemical oxidization process. Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Nanoindentation tests were employed to analyze the morphology, the surface chemical compositions, and the mechanical properties of these green patina microstructures, respectively. The effect of the green patina thickness on the mechanical properties, liquid spreading, and the capillary rise was investigated. The results showed the high wettability of the green patina surface, and the highest capillary rise was achieved with the thinner green patina thickness. Additionally, Experiments were conducted in a wind tunnel system to evaluate the convective and evaporative heat transfer enhancement using different green patina surfaces at various air flow velocity, water dripping rate, and thermal loads. The optimized heat transfer coefficients were achieved at the saturated wetting condition, where the thin liquid film covers the whole heat transfer surface. As a summary of the results obtained by this research, the superhydrophilic green patina surfaces showed the high durability and reliability after testing each sample more than one month, which demonstrates that the green patina surface can be implemented for long-term operation with high performance in phase change heat transfer applications. In this study, we have demonstrated 614.17 % enhancement of HTC, which is about 6 times higher than that of the dry cooling process. Therefore, an enormous benefit in environment and energy efficiency of air cooling can be increased an order of magnitude due to the current research

The objective of the second section of this dissertation is to enhance dropwise condensation (DWC) on scalable copper substrates. In the past several decades, DWC on metal and metal oxides surfaces had aroused significant attention due to 10 folds higher heat transfer rate than that of filmwise condensation (FWC). Recently, numerous effects have been made to enhance DWC by fabricating superhydrophobic surfaces due to their chemical stability and high droplet mobility. However, the application of these surfaces for long-term operations has been hindered due to the highly pinned condensate droplets into the micro/nanostructures, which could turn the mobile Cassie state to Wenzel state and then shifting the condensation from DWC to unfavorable FWC.

In this study, to systematically understand hydrophobic porous coatings in promoting DWC, a series of studies have been carried out to examine effects of key parameters such as coating thickness, pore size, surface chemistry, and additional thermal resistance from coatings. The study consists of three parts. In part (I), thickness effects and role of oxygen of the superhydrophobic coatings on DWC has been investigated.

Two types of coatings, i.e., microstructured green patina (Cu4SO4(OH)6) and nanostructured copper oxide (CuO) have been employed. Experimental results clearly showed that the coating thickness of the superhydrophobic patina plays a critical role in determining the condensation heat transfer rate. When coating thickness exceeds 1.0 ��m, the condensation rate would greatly decrease. At the low and high subcooling degrees, the condensation heat transfer rate was downgraded below FWC by 12.08-29.6 % with patina coating thicknesses of about 50.0 ��m. However, the performance was enhanced by

40.1-87.1 % on the patina surface with a coating thickness less than or about 1.0 ��m. In

part (II), sub-microscale thick (300 nm-1.6 µm) cuprite (Cu2O) coatings were developed to further promote DWC owing to small thickness and nanoscale pore size. The maximum improvement in terms of heat flux on the fresh samples of the cuprite surface is 212.04% compared to FWC, but still lower than complete DWC on smooth copper surfaces. The cuprite coating with the sub microscale porous structures and nanoscale pores size can generate high capillary pressure and hence prevents the droplets from sucking into pores In addition, further oxidization of cuprite surface in the hydrogen peroxide solution could greatly improve the chemical stability of the surface. In part (III), in order to maximize DWC performance, the cuprite coatings were completely removed to eliminate the additional thermal resistance from coatings. The cuprite coatings were removed ultrasonically by acid, acetone, and ethanol, and then washed with distilled water to form a yellow rough copper surface. As a result, a novel superhydrophobic copper surface has been created with microscale porous and microgrooves structures. Compared to FWC, DWC heat transfer rate was enhanced 332.8%, on the superhydrophobic copper surface. Furthermore, the enhancements in terms of heat flux and the heat transfer coefficient (HTC) compared to the plain hydrophobic surface were about 12.65 % and 13.43 %, respectively. The maximum heat flux and HTC were about 797.05 KW/m2 and 76.72 KW/m2·K, respectively, at a subcooling degree of 10.6 ℃.

In this study, the effects of key parameters of porous coatings on DWC have been systematically and successfully characterized. This systematical study eventually leads to the development of an innovative superhydrophobic surface with microscale porous and microgrooves structures, which outperforms completely DWC on smooth hydrophobic copper surfaces. It is because of two primary reasons: first, the additional thermal resistance of coatings has been removed; the other one is that the porous structures with microgrooves can also generate high capillary pressure and prevent droplets from penetration into the cavities of the structures.

Rights

© 2019, Raikan Saad Dawas

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