Comparative Study On Heat Transfer Characteristics Of CO₂,R134a And R410A Two-phase Thermosyphon Loops

Two-phase thermosyphon loop（TPTL）is a heat transfer device that does not require additional power input.Its advantages of high heat transfer efficiency,simple structure and long-distance transportation make it practical in data center cooling systems.In the context of“carbon neutrality”,CO₂ is more environmentally friendly than existing working fluids.Although some scholars have studied CO₂ TPTLs,the CO₂ TPTLs have not been applied in data center cooling systems yet.Therefore,this paper compares CO₂TPTL with R134a and R410A TPTLs commonly used in data centers through local visualization experiments,analyzes the differences between different working fluid TPTLs in operation mechanism and heat transfer performance,and provides theoretical basis for the application of CO₂ TPTL in practical engineering.Firstly,this paper comparatively studied the operating mechanisms of CO₂ TPTL,R134a TPTL and R410A TPTL.By observing the flow patterns inside the tubes and collecting real-time operating parameters of the TPTLs,we analyzed the different operating states of these three types of TPTLs.According to their characteristics during operation,we divided them into three stages:pre-start operation stage,oscillatory operation stage and stable operation stage.It also compared and analyzed their characteristics in each stage.The results showed that in the pre-start operation stage,none of the three TPTLs had a regular circulation flow,but there was a significant difference in flow patterns inside each pipe;in the oscillatory operation stage,due to different causes of oscillations,these three types of TPTLs had significantly different oscillation characteristics;in the stable operation stage,all three types of TPTLs had different flow patterns and passive regulation mechanisms.Secondly,this paper compares the heat transfer performance of three types of TPTLs.The heat transfer limits of the three TPTLs were analyzed and compared,and the optimal filling ratios of the three TPTLs were obtained by taking the maximum heat transfer limit as the optimization objective.Then,the driving temperature difference and thermal resistance of the three TPTLs under their optimal filling ratios were compared,as well as their normal working load ranges.The results show that there is no significant difference in the heat transfer limit among the three types of TPTLs,but the optimal filling ratio of CO₂ TPTL is higher,which is 45%,while the optimal filling ratios of R134a TPTL and R410A TPTL are both 35%.Under the optimal filling ratio,CO₂ TPTL has the smallest thermal resistance and driving temperature difference,and a larger normal operation heat load range.Finally,the heat transfer performance of the three TPTLs was compared by simulating the diameter change with the resistance change of the downcomer.How the downcomer diameter reduction or resistance increase affected the heat transfer limit,driving temperature difference and thermal resistance of the three TPTLs was compared.The results show that the downcomer diameter reduction or resistance increase leads to a decrease in the heat transfer limit and an increase in the driving temperature difference and thermal resistance of the CO₂ TPTL,but has little effect on the R134a TPTL and R410A TPTL.