A Comprehensive Evaluation of Hybrid Wetting Configurations on Dropwise Condensation

摘要

The heat transfer during condensation on surface includes hydrophobic/hydrophilic wettability can be highly influenced by the pattern design. The relationship between the droplet dynamic and the hybrid pattern design can alter the drainage rates, 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 DWC and complete FWC condensation samples. This is to investigate the design that provide 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 sub-cooling temperatures. However, the corresponding maximum heat transfer rates were lower than a surface with a complete DWC 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 provide the peak heat transfer coefficient for this series which is 12% higher than that of the complete DWC 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, with which the heat transfer rate was enhanced by 79% compared with the corresponding complete DWC surface. Ultimately, changing the gap plays a more important role than changing the patterns size 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 with different sub-cooling temperatures and departure frequency optimum pattern sizes are found. However, it is determined that the corresponding departure frequencies are lower than complete DWC surface. Second, series 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 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 developeded on horizontal copper tubes at atmospheric pressure. All the patterns have the same size, and the same identical gap as well between the adjacent patterns. Results show that the diamond hybrid surface has the best performance compared with ellipse, circles hybrid surfaces at the same pattern area with same neighbor gap between two patterns and complete dropwise (DWC). However, the circle and ellipse hybrid surfaces outperform lower performance compared to complete dropwise surface (DWC). 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 increase the gap between the patterns on all hybrid surfaces. The heat transfer rate for the diamond hybrid surface is 40% higher than complete dropwise (DWC) 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 (DWC) 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.