Investigation of dropwise condensation on engineered surfaces

The scientific objective of the present thesis is to investigate dropwise condensation (DWC) over water repellant surfaces. Creation of surfaces which can promote dropwise condensation is one of the main issues. This type of surfaces, in presence of phase change, vapour-liquid or liquid-solid, can bring significant benefits to various applications, for example in thermoelectric power plant condensers, in the production of drinking water, or in applications that require defrosting for their correct operation (e.g. air source heat pumps). The research activity is focused on metallic surfaces such as aluminum and copper, as they are widely used in industry. The advantage of dropwise condensation of steam is related to the promotion of liquid droplets, which leads to higher heat transfer coefficients by one order of magnitude compared to film condensation. The development and characterization (pre and post experimental condensation tests) of several surfaces with different wettability, from hydrophilic to superhydrophobic, is presented. In particular, hydrophobic surfaces obtained via sol-gel method using hybrid organic-inorganic sol-gel silica coatings functionalized with methyl and phenyl groups are analyzed. This type of surfaces paves the way to a cheap and green route to promote stable DWC on metal substrates without using fluorocarbons or controlled roughness patterns. Indeed, the hydrophobic behavior due to methyl/phenyl groups allows to promote DWC and at the same time SiO2 represents a good barrier to chemical agents and provides resistance to mechanical stress. Condensation tests proved that, while these coatings are barely hydrophobic, exhibiting contact angles similar to untreated aluminum, condensation of steam occurs in dropwise mode reaching values of heat transfer coefficient (HTC) up to 250 kW m-2 K-1, among the highest obtained during DWC on metallic substrates, and duration of more than 100 hours with heat flux of 400 kW m-2. Furthermore, the robustness and performances of such coating at different vapor velocities, from 2.7 m s-1 to 11 m s-1, is assessed. The experimental data are compared against DWC models proposed in the past 50 years by different research groups. Such models describe the phenomena that take place during dropwise condensation: the nucleation of a droplet until its departure, the heat exchanged by the drop during its lifetime and the droplets population on the surface. The conduction through thin film (≈ 200 nm - 300 nm) is also discussed for its importance to the process. The experimental campaign is conducted at the Two-phase Heat Transfer Laboratory of the University of Padova with the collaboration of the Material Science Engineering laboratory for the surfaces development.