With the application and popularization of high-density cabinet-level servers, the use of traditional room-level precision air conditioning refrigeration systems will cause cooling capacity loss, resulting in high PUE in data centers. This paper proposes a direct expansion evaporative cooling backplane refrigeration system for data center cabinet level to reduce the cooling capacity loss of the computer room refrigeration system and improve the energy efficiency of the data center. This paper conducts an experimental study on an evaporative cold plate of a cabinet-level refrigeration system. The test environment temperature is 30℃, the simulated heat dissipation is 5-7kW, and the compressor speed adjustment range is 3000-5000r/min. The test is carried out under steady state, and the stable part of the system performance parameters is taken for data processing and test result analysis. The results show that the average temperature of the evaporative cold plate is stable at 18.5℃, and the temperature difference is controlled within 4℃, which can provide continuous and stable cooling to the cooling cabinet.
The Power Usage Effectiveness (PUE) restrictions for newly built data centers are becoming increasingly stringent. In the energy consumption structure of data centers, the energy consumption of equipment used to cool servers and dissipate heat accounts for about 40% of the total energy consumption, which is a major factor affecting its PUE. With the development of computer technology and society, users' demand for high-power servers is increasing, and data center cabinets have higher and higher requirements for cooling systems and equipment. The application of new technologies such as cloud computing and big data has increased the power density of a single cabinet from less than 5kW to no less than 7kW, or even no less than 10kW, and the demand for heat dissipation in data centers has increased dramatically.
Compared with traditional precision air conditioners, the cabinet-level evaporative cooling system has the advantages of no large fans, low noise, and low energy consumption. It is one of the important technical forms to achieve efficient cooling in data centers cooling cabinet.
The solenoid valve is connected to the condenser and the heat exchanger to realize the connection and disconnection function of the condenser and the heat exchanger. The switching between the non-humidification and dehumidification mode (the research content of the article), the dehumidification mode and the humidification mode can be realized by controlling the shutter air valve, the three-way valve and the solenoid valve.
In the system, the direct expansion evaporative cold plate uses a silicone heating plate as a simulated heat source to simulate the load. The silicone heating plate is connected to a single-phase voltage regulator. The power of the heating plate is adjusted by adjusting the voltage of the heating plate to simulate the test of the evaporative cold plate under different load conditions. One evaporative cold plate uses four silicone rubber heating plates to realize the load simulation test. As shown in Figure 5, for each evaporative cold plate, 8 K-type thermocouples are arranged, and the thermocouples are embedded in the slotted thermal grease sheet. The gap is filled with thermal grease. In this way, the upper surface temperature of the evaporative cold plate is measured to examine its temperature uniformity.
Under the same simulated heat source power, as the compressor speed increases, the maximum temperature difference in the evaporative cold plate shows a downward trend, and the average temperature also shows a downward trend. As the compressor speed increases, the evaporation pressure in the system decreases, and the corresponding heat exchange temperature in the evaporative cold plate decreases, which makes the temperature of each measuring point also decrease, and the maximum temperature difference also shows a downward trend. Therefore, in order to ensure better temperature uniformity of the evaporative cold plate, the compressor speed can be appropriately increased.
The Power Usage Effectiveness (PUE) restrictions for newly built data centers are becoming increasingly stringent. In the energy consumption structure of data centers, the energy consumption of equipment used to cool servers and dissipate heat accounts for about 40% of the total energy consumption, which is a major factor affecting its PUE. With the development of computer technology and society, users' demand for high-power servers is increasing, and data center cabinets have higher and higher requirements for cooling systems and equipment. The application of new technologies such as cloud computing and big data has increased the power density of a single cabinet from less than 5kW to no less than 7kW, or even no less than 10kW, and the demand for heat dissipation in data centers has increased dramatically.
Compared with traditional precision air conditioners, the cabinet-level evaporative cooling system has the advantages of no large fans, low noise, and low energy consumption. It is one of the important technical forms to achieve efficient cooling in data centers cooling cabinet.
The solenoid valve is connected to the condenser and the heat exchanger to realize the connection and disconnection function of the condenser and the heat exchanger. The switching between the non-humidification and dehumidification mode (the research content of the article), the dehumidification mode and the humidification mode can be realized by controlling the shutter air valve, the three-way valve and the solenoid valve.
2 Simulation Analysis
Since the working fluid flows into the evaporative cold plate in a two-phase state, the traditional serpentine flow channel has the disadvantages of difficult flow diversion and small heat transfer area, and the uneven distribution of working fluid in each flow channel will lead to a large temperature difference on the surface of the evaporative cold plate. Based on the above defects, it is proposed to optimize the design of the evaporative cold plate flow channel.
3 Experimental test
Based on the above reasons, a honeycomb flow channel direct expansion evaporative cooling plate as shown in Figure 3 was produced. By optimizing the structural parameters of the honeycomb flow channel, the problem of two-phase working fluid diversion in the evaporative cooling plate can be solved; combined with the simulation results of the honeycomb flow channel solid domain, the evaporative cooling plate with this flow channel structure has better temperature uniformity performance in theory. The flow channel width of the evaporative cooling plate is 10mm, the internal flow channel height is 3mm, and the overall thickness is 5mm.
In the system, the direct expansion evaporative cold plate uses a silicone heating plate as a simulated heat source to simulate the load. The silicone heating plate is connected to a single-phase voltage regulator. The power of the heating plate is adjusted by adjusting the voltage of the heating plate to simulate the test of the evaporative cold plate under different load conditions. One evaporative cold plate uses four silicone rubber heating plates to realize the load simulation test. As shown in Figure 5, for each evaporative cold plate, 8 K-type thermocouples are arranged, and the thermocouples are embedded in the slotted thermal grease sheet. The gap is filled with thermal grease. In this way, the upper surface temperature of the evaporative cold plate is measured to examine its temperature uniformity.
4 Results and Analysis
Figure 6 is a curve showing the surface temperature distribution of the evaporative cold plate over time under the conditions of simulating a heat source power of 5kW and a compressor speed of 4500r/min. The average temperature of the evaporative cold plate is 18.5℃; the highest temperature among the 8 temperature measurement points is 19.9℃, and the lowest temperature is 17.2℃. The temperature difference inside the evaporative cold plate is controlled within 4℃. The temperature of the evaporative cold plate starts to drop from the inlet T1. Due to the large pressure drop of the evaporative cold plate, the plate temperature drops to the T6 measurement point, and then rises to the outlet T8. Starting from the T6 measurement point, due to the increase in the working fluid dryness, the heat exchange coefficient between the working fluid and the evaporative cold plate decreases, the convective heat exchange decreases, and the temperature gradually increases.
Under the same simulated heat source power, as the compressor speed increases, the maximum temperature difference in the evaporative cold plate shows a downward trend, and the average temperature also shows a downward trend. As the compressor speed increases, the evaporation pressure in the system decreases, and the corresponding heat exchange temperature in the evaporative cold plate decreases, which makes the temperature of each measuring point also decrease, and the maximum temperature difference also shows a downward trend. Therefore, in order to ensure better temperature uniformity of the evaporative cold plate, the compressor speed can be appropriately increased.