Spray cooling and condensation heat transfer in a liquid cooled server system

• Main Author: Liu, Pengfei
• Other Authors: Wong Teck Neng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/152470
• Institution: Nanyang Technological University

Development of electronics is heading towards further integration and miniaturization, accompanied with higher power densities and the requirement of more efficient thermal management. Spray cooling is well-known for its ability to produce high heat flux for heat removal, and hence a promising alternative for thermal management of electronics. The implementation of a two-phase spray cooling system inevitably requires the vaporized coolant to be efficiently condensed to maintain the system in continuous operation. Therefore, the understanding of condensation heat transfer mechanisms and the development of enhanced condensers are equally important. In the present work, a spray cooled server system was established and its thermal performances were examined under various operating conditions. Fundamental study was also conducted for better understanding of the spray cooling and condensation heat transfer mechanisms. A lab-scale spray cooled server system was successfully developed by integrating an efficient spray cooling strategy along with a newly developed condenser. It is found that the spray cooled server system leads to better thermal performance of the chips compared to a conventional air-cooling scheme. The current spray-cooled system works reasonably well under high ambient temperature conditions. Thermal response of the system was also studied by varying the spray nozzle flow rate, condenser fan power, condenser flow rate, and the heater power. It is found that the spray cooling performance is mainly affected by the spray nozzle flow rate. The condenser fan power and condenser cooling water flow rate strongly affect the vapour recovery speed and are therefore critical parameters for system pressure control. The influence of air on spray cooling was experimentally investigated. Two dielectric fluids (PF-5060 and FC-3284) were studied due to their compatibility with electronics. The results show that in the current experimental range, the spray cooling is affected by the liquid subcooling and surface superheat when air is present. The chamber total pressure appears to have a negligible influence on the spray cooling heat transfer, which differs from the spray cooling behaviour without air. A correlation was developed which provides good prediction of the heat transfer rate for spray cooling in the presence of air with an accuracy of ±15%. An experimental study was also conducted on pulsed spray cooling on a vertical surface under controlled nozzle pressure conditions. The pulsed spray cooling is found to be more efficient in fluid usage than continuous spray cooling. An artificial neural network (ANN) model has been successfully developed for pulsed spray cooling on a vertical surface. The ANN model shows excellent comparison with the experimental results with an accuracy of ±5.3% as compared with the empirical correlations (more than ±30%). A theoretical study was conducted on laminar film condensation inside and outside the diverging/converging channels. The condensation heat transfer coefficient decreases with an increase in the diverging angle. The condensation heat transfer coefficient on both inside and outside channels can be substantially enhanced by using converging channels. However, it deteriorates significantly when diverging channels are utilized. The surface tension and the liquid subcooling effects were also investigated. It was found that the effect of liquid subcooling is generally insignificant on condensation heat transfer. The effect of surface tension is also small in most of the cases. However, when the diverging angle approaches the critical angle, its influence becomes significant. A theoretical analysis was also performed on laminar filmwise condensation on the outer surface of a vertical tube in the presence of a flowing vapour. The presence of vapour shear stress acting on the liquid-vapour interface was found to enhance the condensation heat transfer by accelerating the condensate film flow. The shearing enhancement factor increases almost linearly with an increase in vapour velocity. The combined effect of surface tension and vapour velocity on the condensation heat transfer characteristics of the vertical tube was also investigated. It was determined that the surface tension has a negligible effect on the condensation performance of mini- and macro-size tubes. However, the influence of the surface tension becomes pronounced for condensation on micro-size tubes. The Kelvin effect is insignificant for condensation on micro-size tubes or tubes of larger sizes. A conjugate heat transfer analysis was also performed and a good comparison between the theoretical model and experimental results was achieved. Filmwise condensation of PF-5060 on plain and finned tubes in the presence and the absence of air was experimentally studied in a condensation chamber. Based on the present experimental results, it was found that a slight amount of air in the system can lead to a large decrease in the heat transfer coefficient. The influence of air is more significant for condensation on finned tubes due to the flooding effect of the diffusion layer. In order for the finned tubes to have a significant advantage in condensation heat transfer performance over plain tubes, the system has to operate below 0.51 wt% of air. If the system is required to operate above 0.51 wt% of air, the use of plain tubes is sufficient as the finned and plain tubes show similar condensation heat transfer performance. A new correlation was developed which is able to predict all the present results with an accuracy of ±25%.