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

•The condensation heat transfer rate is significantly enhanced owing to the existence of gradient wettability on condensing surface.•There exists an optimum (β/α) ratio corresponding to both regions wettability degree.•Two mechanisms are responsible for the substantial condensation enhancement: first, the direct migration of condensate from β to α regions, and the second, the coalescence/bouncing actions of larger migrating droplets from β to α-regions.•The unwanted bridging phenomenon have been identified in increasing thermal resistance, yet reducing heat transfer rate. 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 t