Experimental investigation of dropwise condensation of water vapor under atmospheric and sub-atmospheric pressure

Steam condensation represents a key process in numerous industrial applications. Lately, the researchers’ attention is focusing on the dropwise condensation (DWC) phenomenon, which involves the formation, growth and rapid removal of small droplets over the heat transfer surface. DWC can achieve values of heat transfer coefficients 5-9 times higher compared to the most exploited filmwise condensation (FWC) that is characterized by a continuous liquid film on the surface that hinders the heat transfer. This results in an enhancement of the overall heat transfer performance, leading to lower energy consumption and greenhouse gases emission. Recently, many studies are researching new methods to further increase the heat transfer coefficient (HTC) of dropwise condensation. In this paper we will analyze DWC over two sol-gel coated hydrophilic surfaces (MTO_300 and MTO_450), making a comparison with the lower wettability surface P7M3 (studied by Parin et al. in the same experimental apparatus and in similar operating conditions). It is found an overall increase of the HTC values that can be explained considering the higher nucleation site density and the flatter droplets formed over the surface (thus lower thermal resistance). In particular, higher HTC are found for the more hydrophilic surface (MTO_450). Additionally, the lifetime of the hydrophilic coatings is tested to verify their durability, and the transition from DWC to FWC, caused by a decrease of the inlet temperature of the coolant water (thus increase of the heat flux), will be discussed. It’s discovered that this transition occurs at higher temperatures for the more hydrophilic coatings, thus dropwise condensation can be withstood only for sufficiently low heat fluxes. Lastly, starting from the literature review focused on the sub-atmospheric DWC, a new test section design for low vacuum operating conditions will be presented.