Droplet size distributions in dropwise condensation heat transfer: Consideration of droplet overlapping and multiple re-nucleation

Abstract

Evolution and distribution of condensate droplets are the most important information in dropwise condensation. For surfaces with larger contact angles, droplet interactions and distributions become more complicated due to their three dimensional profiles. In this study, the effect of droplet overlapping and multiple re-nucleation on droplet size distribution and heat transfer are analyzed quantificationally by numerical simulations. Experimental droplet images obtained by environmental scanning electronic microscope are also presented to support the analyses. The simulated droplet size distributions show that droplet overlapping and multiple re-nucleation can improve the spatial distribution of small droplets for larger contact angle surfaces, and the difference in droplet size distributions decreases as droplet radius is increased. The distributions of larger droplets with radii larger than the critical coalescence size are also affected by the overlapping and multiple re-nucleation effect. The difference between droplet size distributions finally diminish for different contact angles when droplet radius is increased to dozens of microns and the effect of overlapping and multiple re-nucleation become less important in that size range. As contact angle is increasing, surface coverage decreases, while the number density of small droplets is greatly increased, and the overall effect leads to an improved heat transfer performance. The present simulations reveal the effect of droplet overlapping and multiple re-nucleation quantificationally from the perspective of three dimensional droplet profiles, which provide insight into the better understanding of droplet size distributions and extend the dropwise condensation theory.