Studio della bagnabilità di superfici per il potenziamento dello scambio termico bifase con fluidi a bassa tensione superficiale

Dropwise condensation offers a considerable enhancement in heat transfer coefficients as compared to the typical film condensation observed on metallic surfaces. Various approaches have been proposed in the existing literature to engineer metallic surfaces that facilitate dropwise condensation with high surface tension fluids like water. However, there are numerous fluids frequently employed in industrial applications, including refrigerants, that possess exceptionally low surface tension values. The use of these fluids in heat exchangers could greatly benefit from the promotion of dropwise condensation, resulting in reduced heat transfer surfaces and increased efficiency. Currently, research efforts are focused on the development of surfaces capable of promoting this type of condensation with low surface tension fluids. Experimental investigations require three essential steps: fabricating a surface with appropriate wettability characteristics, characterizing it by determining contact angles, and evaluating the achieved heat transfer performance. The objectives of this thesis are two: the design and construction of an accurate system for the measurement of surface wettability with low surface tension fluids, in particular refrigerants; the design of an experimental section for the measurement of heat transfer coefficient during condensation of saturated vapor under approximately zero velocity conditions. Limited data exists in the literature on the measurement of contact angles between refrigerants and hydrophobic surfaces. Optical methods for measuring contact angles can introduce significant errors, particularly when working with small angles. For these reasons, it has been developed a measurement system based on the Wilhelmy Plate Method, which determines contact angles through a force measurement. Accurate measurement of the force is crucial, as it can range in the order of hundreds of micronewtons. Furthermore, the instrument has been designed to measure contact angles under pressure within a sealed chamber. This instrument allows for the characterization of surface wettability in terms of advancing and receding contact angle. Understanding this information is crucial for assessing whether a specific surface coating promotes dropwise condensation. In the case of dropwise condensation, avoiding the presence of thermal resistance of the continuous liquid film enables the attainment of much higher heat transfer coefficients. Therefore, once surfaces with high contact angles and good droplet mobility have been identified (with minimal contact angle hysteresis), it becomes necessary to evaluate their heat transfer performance. For this reason, the design of a chamber for measuring heat transfer coefficients during saturated vapor condensation has been studied in the second part of this thesis. The chamber has been designed to perform measurements on a horizontal tube and a vertical surface. Additionally, it allows for the observation of dropwise condensation phenomena using a high-speed camera. This feature enables the study of droplet population (for theoretical model development and validation) as well as the durability of the surface coating, which is crucial for future industrial applications of this technology.