Date of Award

2017

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Chen Li

Abstract

Condensation heat transfer performance can be improved by many methods including the most common used method, which is by increasing the droplet removal rate of the condensing surface. Commonly, this approached can be achieved by promoting a dropwise condensation mode in which super/hydrophobic coatings can be applied on the entire condenser surface to reduce the surface wettability degree. In this dissertation, two main approaches were adapted to enhance the condensation heat transfer performance of the condensing surface via wettability contrast mechanism. Three approaches of investigation were performed to better understand such dropwise condensation promoter methods.

In the first part, alternative mini-scale straight patterns consisted of hydrophobic (β) and less-hydrophobic (α) regions were formed on surfaces of condenser copper tubes. The existence of the two adjacent regions carrying different surface energy generates wettability gradient which can mitigate condensate and increase its removal rates. A parametric study was conducted to experimentally determine the influence of (β/α) ratio on the condensation heat transfer performance and the droplet dynamic under saturation condition near the atmosphere pressure with the presence of non-condensable gases (air). The results reveal that all patterned surfaces exhibited a drastic enhancement in terms of condensation heat transfer coefficient and heat flux compared to those of filmwise condensation. More interestingly, some (β/α) ratios significantly outperformed a surface with a complete dropwise condensation. In addition, an optimum (β/α) ratio of (2/1) exists with β and α-regions widths of 0.6 mm and 0.3 mm, respectively. The heat transfer coefficient of the sample with the optimum ratio peaked at a value of 85 kW/m2 K at subcooling temperature of 9°C, which was about 4.8 and 1.8 times that of a complete filmwise and dropwise condensation, respectively. This term of investigation also demonstrated that the β-regions served mainly as droplet nucleation sites with rapid droplets mobility; whereas the α-regions promoted droplet removal from the neighboring β-regions, and served as drainage paths, where condensate can be drained quickly under gravitational force. Furthermore, the existence of both α and β-regions on the condensing surface controls the droplets maximum diameters of the growing droplets on the β- regions. The maximum diameter is approximately 0.56 ± 3 % mm, which is 26 % the size of the droplets maximum diameter on a full β-region surface.

In the second part of the study, the main objective was to analyze the droplet dynamics during condensation on hybrid-wettability patterned surfaces of horizontal oriented tubes, and to investigate why some patterned surfaces with alternative parallel straight stripes consist of hydrophobic (β) and less-hydrophobic (α) regions at different ratios exhibited higher heat transfer rate than others. Three major outlines were found in this course of the droplets dynamic investigation. First, the existence of an optimum (β/α) ratio that maximized the condensation heat transfer rate was justified due to exhibiting the maximum droplet departure frequency and the minimum droplet area coverage rate relative to other tested samples. Second, the reduction in the heat transfer rate resulting from any deviation from the optimum ratio was also identified. We observed that by increasing the α-regions width, the condensation was dominated by a filmwise condensation mode, thus reducing the condensation rate. In contrast, decreasing the width of α-regions less than the optimum ratio was found to be unfavorable due to the increase in the bridging droplets observed and discussed herein. Lastly, the undesirable observed bridging phenomenon found to occur on all tested hybrid patterned surfaces, can significantly influence the condensation heat transfer performance. A bridging droplet can be referred to a droplet that joined or bridged by two, three, or four neighboring α- stripes. Increasing these unwanted droplets formation frequency can induce additional thermal resistance which can reduce the condensation rate. The most dominant and frequent bridging droplet type observed herein was found to be for droplets that were bridged by two α-regions, followed by those between three and four α-regions. A quantitative method (i.e. Bridging coverage area rate) was adapted herein to quantify the influence of the velocity, frequency, and size of the three types of bridging droplets on the condensation rate of the hybrid patterned surfaces.

In the third part of the investigation, the same hybrid wettability concept was applied however at a nanoscale instead. A bi-philic surface consist of nanoscale hybrid wettability regions was developed by depositing different numbers of hydrophilic nickel oxide layers on smooth nickel tubes surfaces via atomic layer deposition method (ALD). The deposition nature of the ALD method allows for a certain amount of carbon, which is hydrophobic in nature, in combination with the NiO to be deposited on the surface. The existence of the contrast in wettability degree of the condensing surface helped in droplet mitigation and improved the droplet removal rate. Moreover, the choice of nickel as a material for such investigation can be justified by its relative stability among other common condenser metals and their oxide. Most of the metal surfaces will be oxidized when exposed to the ambient containing water vapor, such as the extreme situation of saturation conditions of water vapor during condensation process. The deposition of NiO layers on the Ni surface is basically mimicking the oxide layer that would be formed on nickel surfaces during real applications. The condensation heat transfer performances for all samples with different NiO layers, i.e. 50, 100, 200, and 400 cycles of ALD. A significant enhancement was achieved especially for the sample with least deposition number of NiO ALD cycles. The maximum condensation heat transfer coefficient achieved was at subcooling temperature of about 3.5°C with a value of 100 kW/m^2 K, which is 4.2 times the FWC. While, the heat flux max out at subcooling temperature of about 11.0°C with value of about 700 kW/m^2 which is 3.9 times the FWC. The coexistence of the hydrophobic carbon and hydrophilic NiO at atomic concentration ratio of about 3 to 1 (i.e. 74.3 % to 25.7 % for carbon to NiO, respectively) allows for a proper droplets mitigation due to the existence of bi-philic condensation mode which was driven by the capillary force.

In summary, this capillary-driven mechanism allows droplets to be expediently removed from the condensing surface at higher rates, allowing more surface area to be exposed to the surrounding vapor and leading to a substantial enhancement in the condensation heat transfer coefficient. Such mechanism can be achieved by introducing two or more regions with different wettability degrees on the condensing surface following a pattern. The patterns can be studied and designed in such away it can deliver suitable scale and ratio that match wettability contrast degree of these regions.

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