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During the operation of a data center, servers are gradually installed in racks, causing most racks to work under a low heating load for a long time and affecting the cooling efficiency of the loop thermosyphon system (LTS). Thus, the effects of operating parameters on the thermal performance should be investigated. In this study, a water-cooled LTS was experimentally investigated under different airflow rates and heating loads. The results show that the additional liquid refrigerant reduced the heat transfer performance and aggravated a drop in cooling capacity when the airflow rate and heating load were decreased. To further reveal the effects of the operating parameters on the thermal performance and cooling efficiency, a steady-state distributed-parameter model was developed and validated based on the experimental data. The results show that the excessive cooling capacity was reduced by decreasing the airflow rate according to the upper limit of the server exhaust air temperature under partial server utilization. The excessive cooling capacity was reduced by 14.5–52.1% under 5–56.5% server utilization. To further reduce the excessive cooling capacity while ensuring thermal security, the water side operating parameters (including the supply chilled water temperature and water flow rate) were adjusted according to the upper limit of the rack's average outlet air temperature, which reduced the excessive cooling capacity by more than 23.8% under partial server utilization. [ABSTRACT FROM AUTHOR]

To better analyzing the temperature oscillation and the two-phase behavior inside a flat loop heat pipe, visual studies were conducted. Under the 20°C water cooling and horizontal orientation, the effects of the filling ratio and heat loads on the temperature oscillation were analyzed. Based on the experimental data, the results indicate that owing to the increased system pressure, the temperature oscillation decays as the filling ratio increases from 34% to 58%. Meanwhile, during the startup process, temperature oscillation tends to occur during the boiling and steady stages due to the more violent two-phase behavior, while the temperature curves are smooth during the slow evaporation stage. Moreover, as the heat load increases, the evaporation becomes more intense at the active zone of evaporator, leading to a faster startup process and a higher oscillation frequency. Besides, owing to the synergistic effect of two-phase flow in the compensation chamber caused by heat leak and subcooled liquid backflowing, a "breathing" oscillation behavior of the vapor-liquid interface is observed at the compensation chamber, which further leads to the unstable operation behavior of the loop heat pipe system. [ABSTRACT FROM AUTHOR]

An experimental investigation on condensation heat transfer of R290 with inside diameter of 6.3 mm in horizontal micro-fin tube was presented. Effects of each case on the condensation heat transfer coefficient of R290 were analyzed at saturation temperatures of 40°C, 50°C, and 55°C with heat flux ranging of 3–8 kW/m2, mass flux ranging of 100–250 kg/(m2·s) and vapor quality up to 1.0. The results show that the condensation heat transfer coefficients increase with mass flux and heat flux, however decrease with saturation temperature. In addition, compared with the smooth tube, the condensation heat transfer coefficients of R290 in the micro-fin tube increase by about 25%. Comparing the experimental data, the condensation heat transfer correlations of Yu et al. has high accuracy. [ABSTRACT FROM AUTHOR]