Study On The Rack Cooling System Based On The Pulsating Heat Pipe In Data Centers

The cooling energy consumption in data centers are getting higher and higher nowadays, while the PUE that represents the energy efficiency remains high. In the meantime, the traditional rack cooling approaches are becoming powerless in response to the higher heat flux in servers due to the trend of integration of electronic equipment. Compared to the room-based cooling strategy in rack cooling system, the rack-based cooling strategy is more direct and higher efficient. In addition, regarding the problem of leaking in water cooled systems, it is important to develop a novel rack cooling system. In this paper, a rack cooling system based on pulsating heat pipe (PHP) is discussed and investigated by experiments and simulation.First of all, the performance of large scale flat plate PHP with refrigerant working fluid suitable for the rack cooling system in data centers is investigated by experimental study. The PHP is heated by hot wind, the influence of several factors such as the temperature of the hot wind, the heat source power, the heat source power distribution, the filling ratio and the fins are analyzed. It is found that there is a start-up threshold for the PHP, only if the heat source power exceeds the threshold, the PHP will start up. The heat resistance of the PHP decreases as the power increases. What’s more, PHP with higher filling ratio will have shorter start-up time and smaller heat resistance. The heat resistance of PHP with fins attached to condenser sections is smaller than that without fins.In addition, the inner air flow field characteristics of closed rack cooling system based on PHP is investigated. The PHP is installed on the back of the rack as the rear door and two servers are put on the bottom of the rack. The temperature and velocity distributions in the rack influenced by the load and PHP are analyzed. The results show that the heat generated by the servers tends to accumulate in the region of middle area in the rack. Therefore, "hot spots" are likely to exist in there. The start-up of PHP makes the temperature lower and the temperature distribution more uniform in the rack. There is an optimal load coupled with the filling ratio of the pulsating heat pipe when the temperature distribution in the rack is the most uniform. A lower temperature of the rack will be reached as the velocity of the chilled air increases.At last, numerical simulation study on the rack cooling system based on PHP with internal duct is investigated. In the simulation, the simplified thermal conductivity model of PHP is established and verified by experiments. The CPU temperatures are evaluated in consideration of the load, the PHP, the temperature of the chilled air and the pressure of the fans. It shows that the CPU temperatures rise as the load increases, and the start-up of PHP makes the CPU temperatures lower and the CPU temperature distribution more uniform. The CPU temperatures will be lower when the temperature of the chilled air decreases or the pressure of the fans increases. By installing an inner duct in the rack to adjust the inner structure of the rack, the temperature of the inner air is obviously decreased and will remove some "hot spots" in the rack. For rack with load within 1380W, the rack cooling system with internal duct can remain the CPU temperatures within 60℃.