Numerical investigation of a CO2 loop thermosyphon in an integrated air conditioning system for free cooling of data centers

Free cooling based on loop thermosyphon is ideal energy-saving method for data centers. Most working fluids presently used in this field will cause greenhouse effect or destroy the ozone layer. It is necessary to investigate the applicability of environment friendly fluids. A distributed-parameter model of a CO2 loop thermosyphon in an integrated air conditioning system for free cooling is built and validated. The performance is compared with traditional working fluids and the effects of some key geometric parameters are evaluated. The results show that the optimal filling ratios for CO2, R22 and R134a are 120%, 100% and 90%, respectively. The circulation flow rate of CO2 is much smaller than those of R22 and R134a. For CO2 loop thermosyphon, the heat transfer rate decreases with the increase of pipe length, while increases with the increase of riser diameter and height difference. The heat transfer rate increases with the increase of tube number and length of the evaporator, while the increasing rate decreases gradually. The optimal tube number and length are 80 and 0.6 m, respectively. The relationships between the above phenomenon and the internal flow state are also analyzed.