Towards dropwise condensation of low surface tension fluids: evaluation of the contact angles and design of a new test section for heat transfer measurements

Climate change and sustainability are becoming increasingly important and widely recognized, leading to the implementation of measures across almost every field. In sectors such as energy production, heating, refrigeration and the process industry, condensation plays an important role and often involves refrigerants that can be flammable, toxic or have a high environmental impact. Dropwise condensation (DWC) is a heterogeneous phase-change process where vapor changes to liquid in the form of discrete drops on a surface. Compared to filmwise condensation (FWC), DWC can achieve up to ten times higher heat transfer coefficients, allowing to reduce the size of heat exchangers and improve their effectiveness, reducing the refrigerant charge in refrigeration systems and thus addressing all the aforementioned environmental issues. While various studies in literature focus on DWC of water, only a few investigate commonly exploited refrigerants. The lack of study on this topic is related to the fact that refrigerants of practical interest have very low surface tension, which is an important parameter affecting the achievement of DWC. Dropwise condensation occurs when a vapor condenses on a surface forming droplets that increase their size through coalescence and condensation of vapor and then are removed by gravity when the departure radius is reached. Low surface wettability, that is quantified by the contact angle, is required to achieve the formation of droplets but becomes more challenging as the fluid surface tension decreases. Moreover, these fluids often have a normal boiling point below ambient temperature, so common techniques to measure contact angles cannot be employed in the atmosphere. Since wettability characterization is the first step towards the promotion and industrial implementation of DWC, the thesis focuses on the development of an experimental setup based on the Wilhelmy plate method to measure contact angles inside a pressurized chamber by means of a force sensor. The work includes validation in atmospheric conditions using water, ethanol and ethylene glycol (with varying surface tension), on various samples made of aluminum and steel functionalized with sol-gel coatings. The last part of the thesis is dedicated to the design of a new test section to study condensation of refrigerant fluids. The test section will house metallic samples treated with different coatings. The study covers a preliminary model developed in Excel and Matlab to evaluate some initial design parameters. Based on the preliminary model, CFD simulations are carried out to optimize the geometries of water and refrigerant channels. Next steps of the design are mechanical verifications to assess the feasibility and the integrity of the test section. Eventually the three-dimensional model is carried out. The design phase also includes mechanical analyses to assess the feasibility and structural integrity of the test section. Finally, a three-dimensional model is developed.