Enhancement of vapor condensation heat transfer on the micro- and nano-structured superhydrophobic surfaces

Abstract

Recently, micro- or nano- structured surfaces haven been developed to enhance condensation heat transfer, water harvesting and self-cleaning. However, at large subcoolings, condensate floods the subcooled substrate, thus deteriorating the heat transfer efficiency. Here, the superhydrophobic surfaces with micropillared and nanopillared structures are proposed to enhance heat transfer at large subcoolings. The influence of micropillar spacing and surface subcooling on the droplet dynamics and heat transfer performance is experimentally investigated using microscopic visualization techniques. In addition, the microscopic modeling of condensation heat transfer on the microstructured surfaces is performed using the mesoscopic lattice Boltzmann method. The results demonstrate that the droplet size distribution on the micropillared surface is significantly smaller over that of the nanostructured surface. The heat transfer coefficient decreases with the increase of micropillar spacing. As the subcooling rises, although the condensate floods the substrate, the heat transfer coefficient of the S10R30 (S10R30 represents the micropillar arrays with s = 10 μm and 2r = 60 μm) surface is enhanced by 26.4% compared to the hydrophobic nanostructured surface. This is because the height of liquid film is the same of order of magnitude as the micropillars, reducing the thermal resistance caused by the liquid layer. Combining environmental scanning electron microscope (ESEM) observations and LB simulation results, it is concluded that the droplets first nucleate at the bottom corner of micropillars. In addition, the condensate droplets merge to form a film, fill the micropillar gaps, and cover the entire micropillars, leading to a sharp decrease in heat flux. These findings provide a theoretical and experimental guidance for the development of condensing surfaces to enhance heat transfer.