Enhancing Phase Change Heat Transfer via Interfacial Engineering
Abstract
Liquid-vapor phase change processes play an essential role in maintaining the water balance in the natural water cycle and are ubiquitous in various industrial applications such as power generation and conversion, water harvesting/desalination, and electronics thermal management. The pursuit of enhancing phase change heat transfer and especially enhanced condensation has been ongoing for decades, as the improvements can have significant impacts on both the environment and the efficiency of energy systems in industries. Dropwise condensation typically facilitated with thin (<100 nm) low surface energy coatings is broadly known to enhance heat transfer due to faster shedding of the condensate. However, low surface tension fluids condense on these low surface energy coatings in the inefficient filmwise mode, and thin coatings at the thickness required to improve heat transfer typically degrade rapidly (minutes to hours) during water vapor condensation. More importantly, the degradation/failure mechanism(s) of these coatings during condensation remain unknown and/or unproven. In this dissertation, we present two pathways (i.e., surface geometry method and surface energy method) to overcoming the existing limitations of low surface energy coatings and enhancing the performance of liquid-vapor phase change heat transfer accordingly. In the surface geometry method, we introduce a scalable and robust capillary-enhanced filmwise mode where condensation occurs within a high effective thermal conductivity porous condenser and condensate removal was sustained by the capillary forces within the porous media. The semi-analytical modeling framework incorporates the non-linear pressure gradient obtained by discretization and the accurate local liquid-vapor interface (meniscus) curvature/shape, and demonstrates favorable enhancements on the heat transfer coefficients of low surface tension liquids. In the surface energy method, we present a mechanistic understanding of the condensation-mediated degradation of self-assembled monolayer coatings on silicon and copper surfaces, and we significantly extend the coating durability when condensing water vapor continuously. Elimination of water/moisture in the synthesis and proper surface terminations (i.e., cleaning, polishing, plasma modification) are experimentally validated to be essential to obtain superior coating robustness for condensation. The novel insights from this work have the potential to drastically improve transport efficiency in enhanced phase change heat transfer applications and sustainable energy/water technologies.
Citation
Wang, Ruisong (2022). Enhancing Phase Change Heat Transfer via Interfacial Engineering. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /197121.