Research Progress of Submerged Phase Change Cooling in Data Center

         With the improvement of computer performance and the integration of computing equipment, the heat flow density of data centers has increased, and the requirements for heat dissipation systems are also increasing. Due to the hot spots and high energy consumption problems of traditional water cooling and air cooling, it is difficult to meet emerging issues 

         The heat dissipation requirements of electronic devices have led to the development of immersed phase change cooling technology. The immersed phase change cooling system is mainly composed of an evaporation section, a condensation section and a circulation system.

         The heating equipment is immersed in an insulating inert Direct contact with the agent for heat dissipation. 

         This method has the advantages of good heat dissipation performance and low energy consumption. At present, immersion phase change cooling technology is in its infancy. In the next few years, with the rapid development of technologies such as smart industry and cloud computing, new heat dissipation technologies represented by immersion phase change cooling will surely become the mainstream heat dissipation technology for data centers. 

         This article takes the main structure and principle of the immersed phase change cooling system as the starting point, analyzes the current research focus and development status from the selection of cooling materials and the boiling heat transfer mechanism, and prospects for the immersed phase change cooling technology.  

         Scholars have conducted a lot of research on the heat dissipation methods of data centers: traditional air-cooled heat dissipation has been optimized for airflow organization, water-cooled heat dissipation servers have been installed with backplanes, heat pipe heat dissipation technology, submerged phase change cooling, and waste heat recovery have been carried out. 

         Research on a variety of technologies to reduce energy consumption. At the same time, large server manufacturers at home and abroad are also actively exploring efficient heat dissipation methods. China's Sugon Company once launched an immersed cooling server, as shown in Figure 2.

         This server can be used in data centers The PUE value of the computer room is reduced to 1.05, and the energy consumption is reduced by more than 30% compared with the traditional air-cooled data center. The economic benefits are significant. The "Kirin" server launched by Alibaba does not require air-conditioning, fans and other accessories and can be deployed anywhere. It saves more than 75% of space, and its high degree of airtightness keeps electronic components away from dust, humidity and other factors, and is protected from external interference. In addition to heat dissipation equipment, some companies are committed to the research and development of cooling media, developed by 3M in the United States A Novec coolant, which has the most basic insulating properties, has a boiling point lower than that of common cooling media (such as pure water, fluorinated liquid, and mineral oil). 

          In various research directions, immersion Due to its advantages such as high heat dissipation performance, convenient maintenance, and low noise, the phase change cooling technology has become the focus of current research.

          Selection of cooling medium The immersed phase change cooling system device requires high sealing performance and higher requirements for hardware facilities; at the same time, the choice of cooling medium is also the key to the immersed phase change cooling technology. Qualified cooling medium should have the following characteristics. 

          (1) It has high latent heat of vaporization, that is, the same amount of heat dissipation uses less cooling medium, which can improve the heat dissipation efficiency while reducing the additional energy consumption of pumps, cooling towers and other equipment. 

          (2) It satisfies the insulation requirements in its liquid, gaseous and gas-liquid mixed state. 

          (3) It should have a low boiling point. When the temperature of the server is not very high, the coolant can also undergo a stable phase change to take away the heat to ensure the stable temperature of the server and the normal operation of the data center. 

          (4) Non-corrosive to server materials. 

          (5) It is environmentally friendly, non-toxic and harmless, and easy to handle. 

          (6) High economy. At present, aromatic substances, silicates and fluorocarbons are included in the research scope of the cooling medium of submerged phase change cooling system. Mineral oil and fluorinated liquid are the more widely used direct contact cooling medium. Mineral oil is cheap ､Environmentally friendly, non-toxic and harmless, but easily decomposes and is a combustible substance, which is more dangerous. Compared with mineral oil, the price of fluoride is slightly higher, but its properties are stable, flame retardant and suitable dielectric constant. The most popular cooling medium at present. Xu Yongsheng and others used needle-plate electrodes to simulate the common extremely uneven electric field environment, and built a partial discharge detection platform based on a 50Hz high voltage AC withstand voltage test system to explore the effect of refrigerant boiling on partial discharge The influence of the threshold value. The research results show that the boiling of the refrigerant will reduce the insulation threshold value of the insulation environment. Mo Shenyang and others used an insulation test device to conduct a systematic study on the breakdown characteristics of the FC-72 refrigerant in liquid, gas and two-phase states. , The dielectric strength of FC-72 vapor was obtained through the power frequency voltage breakdown test, and the multi-pressure dielectric strength test of the low boiling point refrigerant vapor was realized. Wu Xilei and others simulated 4 commonly used fluorinated liquids (FC-72, The influence of Novec649, HFE-7100 and D-1) on the heat dissipation effect of the submerged phase change cooling system. The simulation results show that the heat flux density required for D-1 boiling heat transfer is the smallest, but its maximum heat dissipation capacity is close to Novec649, slightly Lower than FC-72 and HFE-7100, it can replace the existing electronic fluorinated liquid Novec649 in immersed phase change cooling. Dong Jinxi and others analyzed synthetic hydrocarbon-based coolant (PAO) and #65 ethylene glycol through experiments The physical and thermodynamic properties of the similar coolant (hereinafter referred to as #65), #65 has a higher heat dissipation capacity than PAO, but the physical stability and non-conductivity of PAO are higher than #65. Now there are fewer researches on cooling media and need Further research was carried out and found a cooling medium with stable physical properties and high heat dissipation capacity.

           Further optimization of the submerged phase change cooling technology can improve its cooling efficiency. The next research directions are as follows. 

           (1) Prepare the cooling medium with stable properties, study the influence of the uneven electric field environment on the properties of the cooling liquid, and carry out the dielectric strength test of the cooling medium under high field strength. 

           (2) Fluorinated liquid has strong volatility, research and development of matching heat sink sealing materials. 

           (3) Through experiments, further explore the influence of different factors on the formation of bubbles in the microchannel, study the mechanism of boiling heat transfer on the surface of the server, and establish a more representative boiling heat transfer model. 

           (4) Time-test the packaging polymer materials of electronic devices to ensure their service life, reduce the maintenance cost during the use of the server and decrease the reliability caused by frequent maintenance.