Effect of the surface wettability changes on nanostructured polymer film for heat exchanger applications

Abstract

Polymer heat exchangers have drawn attention due to their special characteristics such as flexibility, low weight, corrosion, and bio-fouling resistance, as well as their ease of manufacturing. However, since their thermal conductivity is low, they require a way to increase their heat transfer rate. We investigated the effect of modifying the surface wettability of polymer films on the heat transfer rate during the condensation process, both theoretically and experimentally. Condensed water formation on the film surface depends on the surface wettability control, and thus, hydrophilic or hydrophobic surfaces can induce film-wise or drop-wise condensation, respectively. Generally, the polymer surface shows a hydrophobic property due to the intrinsic C-C and C-H bonding of polymer frames. Here, we used four different types of polymer films, which have different thermal conductivity and controlled wettability properties, including polyimide film, polyimide film containing aluminum nanoparticles, and both films with super-hydrophobic treatment. The results show that nanostructured polymers with hydrophobic treatment have a 25% lower wetted area fraction than those without hydrophobic treatment, independent of the thermal conductivity of the polymer films. To compare the heat transfer rates, we designed a thermal resistance model considering vapor convection, vapor-water interfaces and curvatures, and conductions of the droplet, nanostructure, and polymer. We found that the convective thermal resistance dominantly affects the heat transfer rate and treated polymers showed over 200% higher total heat transfer than bare polymer.