Yao, Chun-Wei (2014-04). Study of the Effects of Surface Morphology and Droplet Growth Dynamics on Condensation Heat Transfer. Doctoral Dissertation.
Thesis
Condensation heat transfer has recently received a lot of renewed attention due to the development and use of surfaces with micro- and nano-scale features. Most of the new surfaces tend to promote drop-wise condensation, which leads to higher rates of heat transfer when compared with film wise condensation. In the current study, the effects of surface morphology and surface chemistry on the condensation mechanisms have been investigated using engineered surfaces. Firstly, hybrid surfaces consisting of an array of micropillars with hydrophobic and hydrophilic sites have been designed to exhibit a distinct Cassie-Baxter wetting behavior at different temperatures. Characterization experiments have revealed that hybrid surfaces depict a unique wetting behavior. Furthermore, more types of engineered surfaces were fabricated including nanoparticle-based hydrophobic surface, polytetrafluoroethylene (PTFE) surface, and self-assembled monolayers (SAMs) surface. Experiments have been conducted to determine the heat transfer performance of all engineered surfaces under a constant humidity level, surface-to-ambient temperature difference, and laminar flow conditions. Experimental results reveal that droplet sliding can have an important effect on heat transfer performance. Also, empirical heat transfer correlations have been postulated and fitted using experimental data using condensing and air temperature difference and Reynolds number as independent variables. Results indicate that the postulated correlations are in excellent agreement with experimental data. In addition, surface temperature data obtained using an advanced IR imaging system have been analyzed to determine the effects of the surface features on droplet growth dynamics. The non-invasive IR measurement technique has been helpful in understanding the droplet growth dynamics such as droplet coalescence. Results to date show that the static contact angles and sliding angles have marked effects on droplet growth and coalescence on the surfaces in the early stages of condensation. Furthermore, results also reveal that droplet sliding angles can have an important effect on droplet sliding motion and condensed droplet dynamics play an important role during the overall condensation process. In summary, the effect of surface morphology and droplet growth dynamics on heat transfer during condensation were investigated and elucidated.
