Enhanced Pool Boiling Performance of Hierarchical Nanostructures with Excellent Surface Wickability

Abstract

Boiling is one of the most efficient way for thermal energy conversion and management for high power density electronics or nuclear power plants. Despite the advantage of excellent heat transfer coefficient, boiling heat transfer has limitation called the critical heat flux (CHF) which can lead to catastrophic failure of system. The critical heat flux occurs when a vapor film is formed on boiling surface and it decreases the heat transfer coefficient significantly. The vapor film on the boiling surface is generated when a bubble formation speed exceeds the detaching speed and the bubbles merge together. The failure of a boiling system can be solved by delaying the generation of vapor film on the boiling surface. The vapor film can be separated by using different thermal conductivity and different pattern of wettability of the surface. On the other hand, the vapor film can be delayed by using surface wickability to promote the detachment of bubbles generated on the boiling surfaces. In this study, we applied superhydrophilic surfaces of diverse nanostructures with excellent surface wickability to enhance the critical heat flux of boiling surfaces. The superhydrophilic surface property is characterized with measuring the surface water wickability and the selected titanium nanostructure showed 1.5 times enhanced water spreading ability. In addition, with the increased surface wickability, the nanostructures demonstrated increased critical heat flux up to 2.8 times than bare surface by delaying generation of vapor film. The visualization of the generation of a vapor film is conducted and we confirmed that the formation of vapor film is delayed on nanostructured titanium surfaces than bare titanium surface. Furthermore, we investigated the effect of macro size structure for accelerating the bubble detachment by using copper substrate and copper oxide nanowires and achieved double times increased critical heat flux.