Enhanced heat transfer with nanostructured coatings in two-phase immersion cooling
Yeo, Jie Wei
Date of Issue2017
School of Mechanical and Aerospace Engineering
In recent years, there is a significant growing trend in the amount of data stored in company servers. This results in the need for data centers to run continuously all year-round, creating overheating issues where conventional method of air cooling could no longer sustain. Single phase liquid cooling was introduced as an alternative but was still insufficient to overcome the heat generated. This led to the proposal of two phase liquid cooling, where the liquid is boiled to vapour and condensed back to liquid using available pool boiling. Using fluid with a low boiling point, the saturation point of the liquid is easily achieved by pool boiling. This helps to cool the hot surfaces with the constant vaporisation and condensation of the liquid at saturation point. Novec 7100 was selected due to its dielectric property, which is suitable for usage on servers and electronics. In this report, heat generated from the exposed surface causes the formation of bubbles in the liquid that contacts the surface. Heat is then transferred to the bubbles. The exposed surface was orientated vertically, providing a good path for the bubbles to travel. This causes the bubbles to rise rapidly which in turn cools the surface. Addition of nanotube structure to the exposed surface was also evaluated in this report. Thus, this report serves to summarize the enhancements evaluated to improve the cooling efficiency which includes using subcooled conditions and surfaces with in-built structures. Heat flux and heat transfer coefficient were obtained and compared. Results from the experiment showed that normal plain copper surface can reach a critical heat flux (CHF) of 37 W/cm2. A linear increase in CHF was being observed at subcooled conditions. At subcooled conditions of 15°C, a 46% higher heat flux as compared to saturated conditions was observed. Copper surfaces with titanium nanotubes posed an issue as the heat flux did not increase proportionally. It was suspected that the disproportionality of the heat flux increase is contributed because of using two different material. Hence, the study was repeated using a titanium block with nanotube to evaluate the effect of nanotube enhancement. The results indicated a lower heat flux as compared to the plain copper surface. This indicates that copper is far superior than titanium in terms of heat transfer. However, due to lack of time future studies can focus on nanotube as well as the effects and properties of the bubble departure.
Final Year Project (FYP)
Nanyang Technological University