Dropwise condensation from flowing humid air: an experimental study on the relation between heat transfer and droplet population

Condensation is a phase change process present in many engineering applications, including power generation, refrigeration and air-conditioning units, humidity control, thermal management equipment and water recovery technologies. It has been shown that promoting dropwise condensation (DWC) instead of the traditional filmwise condensation (FWC) can enhance the heat transfer performance enabling more compact components, lower energy consumption and higher system efficiency. Although DWC was discovered in the 1930s and it has been the focus of extensive research, experimental datasets under flowing humid air conditions remain limited and the quantitative link between droplet population (nucleation site density, droplet growth rate and droplet distribution) and heat transfer coefficient is still not fully established. Moreover, while several predictive models for DWC have been proposed, their validation under flowing humid air conditions is often hindered by the lack of synchronized measurements of thermal data and droplet population dynamics. This thesis addresses this gap by coupling simultaneous high-speed imaging and thermal measurements to relate droplet dynamics to the overall heat transfer across a wide range of operating conditions. The resulting dataset supports model validation and clarifies how changes in droplet population translate into changes in heat transfer performance. The present work experimentally investigates DWC from flowing humid air over a wettability-controlled surface. The data were acquired using the experimental setup at the Two-Phase Heat Transfer Laboratory of the Department of Industrial Engineering of the University of Padova, where simultaneous measurements of heat flux and heat transfer coefficient are combined with high-speed optical imaging, allowing to assess the effect of relative humidity, dew-to-wall temperature difference and the air and air velocity. Data are analyzed via custom programs to characterize key microscopic features of the process, including droplet nucleation site density, droplet growth rate and droplet size distribution as a function of droplet radius. This thesis is organized into six chapters. Chapter 1 introduces FWC and DWC, describes the physical mechanisms underlying DWC and discusses its potential applications. It also reviews surface wettability concepts and summarizes the predictive models for dropwise condensation available in the literature. Chapter 2 is divided into two parts: the first describes the experimental apparatus and instrumentation, while the second details the methodologies adopted for the analysis of the acquired data, including nucleation-site identification, droplet growth-rate measurements, thermal data processing and droplet size distribution analysis. Chapter 3 presents the experimental results, which are then discussed in detail in Chapter 4. Chapter 5 summarizes the main conclusions of the work.