Design of biphilic surfaces with nanostructures for pool boiling CHF enhancement

Abstract

To enhance the performance of a critical heat flux (CHF) and a heat transfer coefficient (HTC) which are the measurement of the limitation and the efficiency of heat transfer performances, the wettability and the microstructure of the surface are considered mainly. Previous studies considering only the wettability have reached the limit of the enhancement of CHF. Considering the combination of two or more materials having different wettability: the biphilic surface consisting of the hydrophilic surface and hydrophobic patterns has been considered as one of the most significant performance enhancement strategies. Mainly, the bubble dynamics around the hydrophobic pattern has been analyzed to figure out the mechanism of enhanced boiling performance for the biphilic surface. However, this description is not enough to fully understand the mechanism without the information of temperature distribution due to the bubble dynamics around the hydrophobic pattern. In addition, the nanostructure with mixed wettability surfaces can be adopted for further CHF enhancement. In this study, the mixed wettability surface with the nanostructure is designed to figure out the possibility and the mechanism of the enhancement of heat transfer performances of the super-biphilic surface. The super-hydrophilic surface of the nanowire and its pattern are expected to delay the CHF and enhance the HTC by capillary pumping from the vertical nanostructure and accelerating the bubble generation and release rates. The influence of mixed wettability and bubble dynamics can be confirmed by observing the heat transfer performances according to the pattern size and pitch. Pool boiling experiment is planned to investigate the heat transfer enhancement of the super-biphilic surface using super-hydrophilic surface vertically oriented by Si nanowires and super-hydrophobic pattern with carbon nanotubes (CNTs). The size and pitch of the hydrophobic patterns will be determined based on the bubble dynamics such as bubble departure diameter and frequency. The heating surface is composed of the Si substrate adhered to a transparent indium tin oxide (ITO) surface, and Si nanowires and CNTs are vertically stacked on the substrate. Bubble dynamics and temperature distribution on the surface will be measured by the high-speed camera and IR thermometry.