Condensation on hybrid-patterned copper tubes (II): Visualization study of droplet dynamics

Condensation heat transfer performance can be significantly enhanced by patterning the condenser surface with different wettability regions as shown by numerous studies, including part I of this study. In part I of this study, 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. In this Part II of the study, our objective is to analyse the droplet dynamics during water vapor condensation on hybrid-wettability patterned horizontal tube surfaces under saturation conditions near the atmospheric pressure. Three major outlines were found in the course of the droplets dynamic investigation. First, the existence of an optimum (β/α) ratio that maximized the condensation heat transfer rate, as demonstrated in part I of a sample carrying β and α-regions widths of 0.6 mm and 0.3 mm, respectively is justified. This is because the optimum ratio exhibits the maximum droplet departure frequency and the minimum droplet area coverage rate among other samples. Second, the reduction in the heat transfer rate resulting from any deviation from the optimum ratio is also identified. We observed that by increasing the α-regions width on the hybrid patterned surface, the condensation was dominated by the filmwise mode, thus reducing the condensation rate. In contrast, decreasing the width of α-regions less than the optimum ratio was found to be unfavourable 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 joined (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.