Understanding the Role of Wettability-controlled Surfaces on Multiphase Heat Transfer Phenomena: From Dropwise Condensation to Frosting

Condensation and frosting are multiphase heat-transfer processes that are central to many thermal-engineering applications. Their characteristics and performance are strongly modulated by surface wettability. In condensation of pure steam, promoting dropwise condensation (DWC) on low-wettability surfaces can significantly outperform filmwise condensation (FWC). Regarding condensation from humid air, the use of wettability-controlled surfaces can, under specific operating conditions, reduce the mass-transfer resistance associated with non-condensable gases and thereby enhance heat transfer. In frosting, wettability-controlled surfaces can delay heterogeneous ice nucleation and reduce ice adhesion, thereby mitigating performance degradation. Understanding the role of wettability in these processes is therefore paramount. Accordingly, this doctoral dissertation examines how surface wettability and operating conditions influences: (i) DWC of steam, (ii) DWC from humid air, and (iii) frost formation, integrating experimental and numerical approaches to address specific gaps in the literature. The thesis is organized in five chapters. Chapter 1 introduces the concept of surface wettability and the main functionalization techniques, reviews DWC from governing mechanisms to modeling frameworks, and outlines key concepts in frosting. Chapter 2 presents two experimental campaigns of condensation of steam. The first was conceived to investigate the coupled effects of vapor velocity and surface inclination for DWC of steam over hydrophobic coatings. The resulting data were used to validate a new model for droplet’s departing radius predictions. The second campaign addresses the possibility to promote DWC on hydrophilic substrates engineered for low contact-angle hysteresis and clarifies the mechanisms underlying the transition from DWC to filmwise condensation. Furthermore, we propose a simplified model to predict the condensation mode and the heat flux during DWC and validate it against the collected measurements. Chapter 3 describes the investigation of sub-atmospheric dropwise condensation of steam through an efficient individual-based model (IBM). The role of saturation pressure on overall heat transfer, drop-size density distribution, and single-drop transport is quantified. This chapter also presents the design of a new experimental apparatus, developed in collaboration with Cornell University. The facility will allow the experimental investigation of sub-atmospheric DWC of steam, simultaneously enabling resolution of droplets on the order of one micrometer (and potentially smaller), thereby overcoming the technological limitations of conventional setups. Chapter 4 outlines a study on DWC from humid air. The experimental campaign was conceived to establish whether, and under which conditions, superhydrophobic surfaces can deliver measurable heat-transfer enhancement during condensation from humid air when compared to hydrophilic samples. Several tests were done on both surfaces varying the operating conditions: relative humidity, air temperature, surface subcooling and air velocity. Chapter 5 presents a preliminary investigation of frost formation. Experiments were conducted on a hydrophilic surface while systematically varying the operating conditions to assess their effect on frost thickness, mass, and density. The resulting data are compared with those obtained on a hydrophobic surface to determine whether changes in surface wettability, under the tested operating conditions, lead to measurable differences in the aforementioned frost metrics.