A novel liquid-cooling and free-cooling combined cooling scheme is proposed for a typical data center (DC) unit in Xi’an. The cooling system consists of air-cooling and liquid-cooling combined servers (ALCS), air handling units (AHU), water pipes systems, water pumps, middle plate heat exchangers (MPHE), and air-cooled chillers. For the studied DC, the effects of fans rotation speed (FRS) on IT room cooling performance are first investigated. When FRS is constant for all servers, under the inlet air temperature of 45 ℃, the minimum FRS is 850 rad·s^-1 with the maximum HDDs, memories, and power units temperature of 59.93 ℃, 84.39 ℃, and 70.39 ℃ and a total rack fan power of 83.68 W. By adjusting the FRS in different servers, a modified FRS condition (“850-case-5”) is obtained with lower maximum HDD temperature (57.67 ℃) and rack fan power (81.36 W). Then, the free-cooling performance of twelve AHU designs is tested for the DC in Xi’an. All AHU designs can meet the cooling requirement of IT room air circulation when the environment temperature is 15 ℃, and the highest COP of AHU reaches 28 for “H12-basic”. But when the environment temperature is as high as 40 ℃, the highest COP_AHU of 3.94 is achieved by “H7.5R10h3”. For the ten-year average free-cooling performance, the optimized AHU design is “H7.5-basic”, which achieves a high COP_AHU of 17.54. The studied DC exhibits effective cooling and energy consumption performance. For the typical hot day, the highest PUE is 1.35 under the high environment temperature of 38.7 ℃, and the average PUE is 1.25. For the typical cold day, the average PUE is only 1.15. The month-averaged PUE ranges from 1.15 to 1.21 in 2023. For the ten-year period, the PUE and COP of DC are respectively 1.18 and 7.64. The COP of the free-cooling part has reached 2.2 times the COP of the water-cooling part, which indicates that the application of AHU can effectively improve the cooling efficiency.

Data center,Free cooling,Liquid cooling,Numerical simulation,Air handling unit