# Experimental Study of a Novel Prototype of Dual-Cycle Two-Phase Cooling System for a Data Cabinet
## 2 Description of the Dual-Cycle Two-Phase Cooling System  
Figure 1 illustrates the schematic diagram of the DTCS. The system achieves the efficient cooling of all heat within the data center through the synergistic operation of the PCTCS and the PBTCS. The PCTCS, dedicated to chip cooling, consists of two pump-driven loops. The internal loop is responsible for efficiently extracting heat generated by the chips. To ensure system reliability and safety, the internal loop uses low-pressure, non-toxic, non-conductive coolant as the working medium, with pressure controlled within 0.5 MPa. Powered by two gear pumps (one primary and one backup), the internal loop evenly distributes the coolant to each layer of the server through a liquid-phase manifold. After absorbing heat from the CPU, the cold plate evaporator enters a two-phase manifold and ultimately transfers the heat to the external loop via a plate heat exchanger (PHE). The external loop, also composed of a pump-driven two-phase loop, is responsible for efficiently transferring and dissipating the heat generated by the chips to the outdoor environment. The PBTCS is responsible for cooling the heat generated by components other than the chips, such as memory modules, power supplies, motherboards, and other data center equipment. R134a was selected as the same working medium for PBTCS and PCTCS external loop. It consists of a pump-driven two-phase loop that installs an evaporator on the rear door of the cabinet to reduce the outlet air temperature and thereby maintaining a stable data center temperature. The refrigerant from the evaporator enters an outdoor finned-tube condenser (FC) and condenses into a liquid phase. A PHE is installed after the  
condenser, and a chiller unit is used to address the cooling needs of the data center during high-temperature seasons. By adjusting the three-way valve in front of the condenser, cooling modes can be switched for different seasons. Figure 2 shows the internal structure of the cabinet and a picture of the outdoor heat dissipation unit. In this research, IBM X3550 M3 servers served as the experimental subjects. Each server was fitted with two X5680 CPU chips, each possessing a thermal design power (TDP) of 130 W and characterized by a chip surface package dimension of roughly 3.4 cm \(\times\) 3.2 cm. The experiment involved a total of 25 modified servers, managed centrally via a switch situated at the base of the server rack. The principal aim of the experiment was to assess the system's feasibility and efficiency. Thus, the cold plate is configured with a long straight path, and two kinds of cold plate fins are used for testing. The parameters of the internal fins of the cold plate are shown in Table 1. The internal configurations of both the server and the evaporator are depicted in Figure 3. In the PCTCS, components such as the PHE,  
Figure 1: Schematic of the DTCS. Figure 2: Data cabinet and cooling system photos: (**a**) the cabinet internal structure; (**b**) outdoor cooling unit. gear pump, reservoir, and multiple pressure and temperature sensors were consolidated into the coolant distribution unit (CDU), placed at the top of the cabinet. The PBTCS consists of an indoor evaporator, an outdoor heat dissipation unit, and a reservoir. Utilizing a parallel flow heat exchanger (PFHE) as its evaporator, the PFHE is designed with a single-pass structure, boasting a heat transfer surface area of approximately 30 m\({}^{2}\). Following the PFHE, a strategic arrangement of five fans is implemented to modulate the airflow rate into and out of the system. The entire backplate assembly is mounted at the back of the data cabinet. The outdoor cooling unit encompasses an FC, a PHE, and a 5 HP chiller unit. When the outdoor ambient temperature is low, the FC can independently provide the required cooling effect, allowing the system to operate in natural cooling mode.