Operational characteristics of a JEST-type loop thermosyphon with HFE working fluid (Effect of initial liquid level)

An experiment study was conducted on the operational characteristics of the JEST-type loop thermosyphon when an initial liquid level of a working fluid was lowered. This thermosyphon was invented in 2012 by one of the authors with a jet explosion stream technology (JEST) for cooling high-heat-generation and high-heat-flux CPUs. The present experiment aims at lowering the height of the thermosyphon for rack-level thermal management in a datacenter. Hydrofluoroether (HFE)-7000 was used as the working fluid. In experiment, an evaporator section of the thermosyphon was heated with a heating block while a condenser section was water-cooled using a thermostatic bath. Temporal changes in temperatures of the thermosyphon were obtained with thermocouples. Moreover, the circulation flow rate of the working fluid in the thermosyphon was obtained with a simple measurement method. The initial liquid level of the working fluid was changed as 166, 268, 368 mm while the height of the thermosyphon was 1200 mm. Experimental results are shown regarding the effect of the initial liquid level on the circulation flow rates of the vapor and liquid phases of the working fluid as well as the heat transfer coefficient at the evaporator section. An additional experiment was also conducted when the height of the thermosyphon was lowered from 1200 mm to 480 mm. It was confirmed that the thermal performance of the thermosyphon decreased when the initial liquid level was lowered; however, the thermal performance was recovered by lowering the height of the thermosyphon. Therefore, the JEST-type loop thermosyphon can be applied to the rack-level thermal management in a datacenter. was conducted to investigate the heat transfer characteristics of the JEST-type loop thermosyphon. It was shown that the maximum heat transfer rate of the JEST-type loop thermosyphon was 901 W under the condition that the temperature of the evaporator section was less than 90  C. This maximum value of the heat transfer rate corresponds to 127 W/cm 2 at the heated surface of the evaporator section. It was also shown that the heat transfer coefficient at the evaporator section was in the range from 17500 to 46500 W/(m 2  K), implying that the cooling performance of the JEST-type loop thermosyphon is significantly higher than that of conventional thermosyphons, heat pipes and liquid cooling systems. Moreover, in the previous paper by Suzuki et al. (2017), the effect of a heat source size on the operational characteristics and cooling performance of the JEST-type loop thermosyphon was described. Recently, the demand for thermal management in a datacenter is increasing to save the energy used for cooling (Cheng et al., 2021; Ding et al., 2021; Ling et al., 2021). By lowering the height of the JEST-type loop thermosyphon, this thermosyphon could be applied to rack-level thermal management in a datacenter. Effectiveness of this application would be expected because the cooling performance of the JEST-type loop thermosyphon was already confirmed as mentioned earlier. However, to lower the height, the investigation was needed on the operational characteristics of the JEST-type loop thermosyphon when an initial liquid level of the working fluid in the thermosyphon was lowered. In the present study, therefore, the experimental investigation was conducted by changing the initial liquid level in the JEST-type loop thermosyphon. Hydrofluoroether (HFE)-7000 was used as the working fluid because its dielectric property is preferable for electronics cooling. A simple measurement method was devised to obtain