INVESTIGATION OF PHASE CHANGE HEAT TRANSFER ON NANOSTRUCTURED SURFACES

Phase change heat transfer, such as evaporation, boiling, and condensation, is critical to various applications in thermal management, power generation, and water harvesting. Enhancing phase change heat transfer through micro/nanoengineered surfaces has been effective but suffers from manufacturing complexity and long-term performance degradation. This study focuses on utilizing a cost-effective and mature nano-tubular structure, titanium dioxide (TiO2) nanotube, to enhance two major modes of phase change heat transfer - evaporation and condensation. The unique hollow morphology of TiO2 nanotubes results in significant changes in surface roughness, solid fraction, and porosity, which influence the capillary pressure and permeability and accelerates liquid spreading over the surface. A comprehensive theoretical model is developed and validated by experiments, which will serve as a predictive tool for the design of future nanotube-based wetting surfaces. Subsequently, this work proves that TiO2 nanotube-enabled fast liquid spreading raises the Leidenfrost point, delaying the formation of continuous vapor film and, therefore, increasing heat transfer efficiency. Finally, a condensation apparatus is constructed to explore if and how TiO2 nanotube surface enhances condensation heat transfer. The data suggest that filmwise condensation persists on the TiO2 nanotube surface, but the ultra-thin thickness of the liquid film guarantees heat transfer performance comparable to those achieved in jumping droplet condensation on superhydrophobic micro/nanostructured surfaces. The interfacial slip model is hypothesized to explain the observed condensation enhancement on TiO2 nanotube surface. However, more research is necessary to fully elucidate the underlying mechanisms. In summary, this study reveals the unique liquid wetting and spreading properties enabled by TiO2 nanotube structure and their potentials in enhancing phase change heat transfer. The findings provide a promising direction for creating efficient, robust, and low-cost functional surfaces to improve the performance and efficiency of various energy systems that involve phase change heat transfer.