Date of Award

2017

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Chen Li

Second Advisor

Jamil Khan

Abstract

The heat transfer during condensation on a surface depends on the pattern design of the surface, which can highly influence hydrophobic/hydrophilic wettability. In this study hybrid pattern designs were studied. The relationship between the droplet dynamic and the hybrid pattern design can alter the drainage rates, droplet departure frequencies, and the condensation heat transfer rates. Therefore, two series of hybrid patterned surfaces have been designed, developed, and tested during condensation of water vapor on horizontal copper tubes, and compared to complete dropwise and complete filmwise condensation samples. This is to investigate the design that provides the maximum improvement in the droplet mobility and consequently the condensation heat transfer performance. In the first series, hydrophobic circular patterns on hydrophilic background were studied, the optimum pattern sizes/ratios were found for different subcooling temperatures. However, the corresponding maximum heat transfer rates were lower than a surface with a complete dropwise condensation. In the second series, hydrophilic circular patterns on hydrophobic background were employed and strategically examined as a function of the patterns diameter and gap. The corresponding optimum diameter that provides the peak heat transfer coefficient for this series which is 12% higher than that of the complete dropwise surface was found to be 1.5 mm when the gap is 0.5 mm. In addition, findings indicate that increasing the gaps between adjacent patterns reduces the number of bridging droplets, thereby increasing the condensation rate. The optimized dimensions of 1.5 mm were found for both pattern, and gap size, which enhanced the heat transfer rate compared with the corresponding complete dropwise surface. Ultimately, changing the gap plays a more important role than changing the size of the pattern in governing the droplets departure frequency and thus the condensation heat transfer performance.

Moreover, droplet dynamics and departure characteristics during condensation on horizontal copper tubes with circular patterns have been investigated based on different patterns’ sizes and the gaps between them. Initially, series hydrophobic circular patterns on hydrophilic copper tubes are tested at various subcooling temperatures and departure frequency optimum pattern sizes are found. However, it is determined that the corresponding departure frequencies are lower than complete dropwise surface. Second, series of hydrophilic circular patterns on hydrophobic copper tubes have been systematically studied based on the patterns’ size and the gaps between them and corresponding optimum designs have been found. Results indicate that the influence of the gap between the patterns on the droplet dynamic and departure frequency is significant. The results show that when the gaps between the patterns decrease, droplets from neighboring patterns are more likely to merge, resulting in lower droplet departure frequencies, velocities, and mobility. On the other hand, increasing the gaps between the patterns promotes renewal of droplets on the condensing surfaces. The droplet departure frequency on the hybrid surface with a gap of 1.5 mm is 1.37 times higher than that of 0.5 mm gap. Moreover, the renewal droplet frequencies from the patterns are strongly affected by the gap sizes. The optimum design of the hydrophobic/hydrophilic patterns to enhance droplet dynamics is studied.

In addition, with regard of condensation on hybrid surfaces, the geometry of the patterns has a significant influence on droplets departure frequency and heat transfer performance. Therefore, different patterns geometries (circle, ellipse, and diamond) have been developed on horizontal copper tubes at atmospheric pressure. All the patterns have the same size, and the same identical gap between the adjacent patterns. Results show that the diamond hybrid surface has the best performance compared with elliptic, circular hybrid surfaces at the same pattern area with same neighbor gap distance between two patterns and complete dropwise condensation. However, the circle and ellipse hybrid surfaces outperform lower performance compared to complete dropwise surface. The gap between the patterns has a significant influence on droplets dynamic and heat transfer performance for all hybrid surfaces. The heat transfer rate increases with increasing the gap between the patterns on all hybrid surfaces. The heat transfer rate for the diamond hybrid surface is 40% higher than complete dropwise condensation surface when the gap is 1mm. However, the heat transfer rate for circle and ellipse hybrid surface increases with increasing the gap, but it does not advance the complete dropwise performance. This study clearly demonstrated that an optimal geometry and gap scale patterned surfaces exist regarding maximum condensation heat transfer rate and droplet departure frequency.

Rights

© 2017, Karim Khazal Egab

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