Superhydrophobic Si nanowires for enhanced condensation heat transfer

Abstract

Condensation is an essential process in various industrial systems. Enhancing condensation by employing superhydrophobic (SHB) surfaces had drawn significant attention in recent years because of the emerging technology for surface engineering. However, the efficacy of SHB surfaces in condensation is controversy in the literature. The observed deteriorated heat transfer on SHB in condensation is presumably a result of highly pinned Wenzel droplets or flooding formed on the SHB surfaces. Si nanowire (SiNW) array-coated surface which can simultaneously provide a large number of nucleation sites and prevent condensate from penetrating into the nano-structure is a promising candidate for enhancing condensation. In this work, heat transfer on the SHB SiNW surface was investigated. At low subcooling, jumping of liquid droplets accompanied with a high droplet departure frequency resulted in a large heat transfer coefficient (HTC) of 88±16kW/m2K on the SHB surface. This value is one of the highest reported condensation HTCs in the literature. It was 155% and 87% higher than those on the plain hydrophilic and hydrophobic surfaces, respectively. Heat transfer decreased with the rise of subcooling due to an increased condensate surface coverage ratio. However, condensate can still be rapidly shed away from the SHB SiNW surface at high subcooling, which render the comparatively larger HTC of 18.6±2kW/m2K on the SHB SiNW surface as opposed to plain hydrophobic and hydrophilic surfaces. It was evidenced that SHB surface could have a superior heat and mass transfer performance than hydrophobic surface provided that the liquid droplets on the SHB could be shed away efficiently.