Effect of layer-by-layer assembled carbon nanotube coatings on dropwise condensation heat transfer associated with non-condensable gas effect

Abstract

The purpose of this study is to investigate the effect of layer-by-layer (LbL) assembled carbon nanotube (CNT) coatings on dropwise condensation heat transfer under steam-air mixture conditions. To promote dropwise condensation, an additional hydrophobic coating was added to the LbL-assembled CNT coatings, which resulted in a 16% enhanced condensation heat transfer coefficient (HTC) compared to that of the bare surface under saturated steam conditions. Despite the existence of the nanostructures on the CNT-layered surface, the larger droplet size was observed from the high-speed images than that of the bare surface due to flooding condensation phenomenon. Considering that the HTC was enhanced without smaller droplet size, it is evident that this enhancement was mainly attributed to the reduced constriction resistance driven by the high thermal conductivity of the CNT layers. A new HTC model for dropwise condensation incorporating constriction resistance was developed to better predict the measured condensation HTC. Although the enhancement of the condensation HTC on the LbL-assembled CNT coatings was diminished with the air mass fraction, this work reports a new perspective to enhance the dropwise condensation heat transfer performance by controlling the constriction resistance using a highly thermally conductive coating.