Heat transfer and droplet population during dropwise condensation on durable coatings

Long-term sustainability of dropwise condensation (DWC) on treated surfaces is a key point for the exploitation of this heat transfer mechanism in industrial applications. A viable solution to achieve DWC, consisting of hybrid organic-inorganic sol-gel silica coatings containing hydrophobic moieties (methyl or phenyl group) is here presented. Different sol-gel coatings for DWC promotion were tested during condensation of steam in saturated conditions exhibiting heat transfer coefficient (HTC) around 100 kW m−2 K−1 in the heat flux range 100–500 kW m−2. Endurance tests have been performed at 400 kW m−2; an optimized sol-gel coating deposited on an aluminum substrate is shown to sustain DWC for more than 100 h without transition to filmwise condensation (FWC), which is an excellent result among those achieved on aluminum substrates. A comparison between the different coatings is done by surface characterization (contact angles measurements and Scanning Electron Microscopy) performed before and after condensation tests. Video analyses are carried out looking at droplet departing radius, droplets population and surface time renewal using a home-made software to detect the dimensions of the droplets. The present data are used to assess the expression proposed by Le Fevre and Rose (1966) for the droplet population, the equation by Kim and Kim (2011) for the departing radius and the model proposed by Chavan et al. (2016) for the heat transfer coefficient.