Condensation on hybrid-patterned copper tubes (I): Characterization of condensation heat transfer

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Condensation heat transfer performance can be improved by increasing the condensate removal rate. Commonly, this can be achieved by promoting dropwise condensation mode in which super/hydrophobic coatings applied on the entire condenser surface. Herein, alternative mini-scale straight patterns consisted of hydrophobic (β) and less-hydrophobic (α) regions were formed on the condenser tubes. The existence of the two adjacent regions 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) ratios on the heat transfer performance and 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 0.6mm and 0.3mm, respectively. The heat transfer coefficient of the optimum ratio is peaked at a value of 85kW/m2K at a subcooling of 9°C, which is 4.8 and 1.8 times that of a complete filmwise and dropwise condensation, respectively. Our study also reveals 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 summary, this wettability-driven mechanism allows droplets to be removed from the condensing surface at higher rates, leading to a substantial enhancement in the condensation heat transfer coefficient.