Phase Change Heat Transfer Enhancement Using Engineered Surfaces

Official URL: https://risc01.sabanciuniv.edu/record=b2583862_(Table of contents)

Condensation is a phase change naturally occurring phenomenon and widely encountered in nature. Condensation plays a vital role in various applications, such as power generation, water collection, desalination, electronics cooling. During the last decade, the studies on condensation have been mostly limited to the working conditions, which are ideal for Electron Microscopy techniques. Therefore, the behavior of condensed droplets in the presence of vapor flow with different vapor qualities and their implementation to flow condensation heat transfer enhancement have received rather little attention. In this thesis, two different types of surfaces, namely superhydrophobic and biphilic surfaces, have been investigated. First, on superhydrophobic surfaces, besides heat transfer analysis, we performed a visualization study during flow condensation in a minichannel and investigated droplet dynamics including a histogram of droplet diameter distribution at different time intervals and stages of a condensation cycle consisting of nucleation growth and departure, droplet departure diameters, cycle time, and droplet number density. The droplet departure diameter decreases with steam mass flux, leading to a shift to smaller radii in droplet size distribution, which enhances condensation heat transfer. Enhancements up to 33% in heat transfer coefficient were obtained at lower steam qualities for the tested superhydrophobic surface compared to the reference plain hydrophobic surface. Secondly, to take advantage of the mixed wettability, we fabricated biphilic surfaces to assess their effect on heat transfer performance during flow condensation. Electron beam physical vapor deposition (PVD) technique was utilized to form hydrophobic patterns on the superhydrophobic substrate. Here, we report an optimum island diameter D of the hydrophobic spots on a superhydrophobic substrate, where heat transfer performance becomes maximum. We show that considering the optimum islands diameter, compared to the plain hydrophobic surface, condensation heat transfer coefficient is enhanced by 51, 48, 42, 40, and 36% for the steam mass flux of 10, 20, 30, 40, and 50 kg/m2s, respectively. Through visualization experiments, we demonstrate that the observed optimum points correspond to enhanced droplet nucleation and rapid sweeping region, where droplet pinning and bridging do not occur. By fitting the experimental data, a correlation for the prediction of the optimum island diameter of biphilic surfaces is provided as a function of steam mass flux for flow condensation heat transfer.