• The pump’s operation enhances the stability of CO 2 TPTL. • The R134a TPTL exhibited greater oscillation amplitudes with increased pump power. • Pump raised TPTLs’ heat transfer limits with minimal impact on thermal resistance. • The reservoir exhibited different vapor–liquid separation effects for the two TPTLs. The two-phase thermosyphon loop (TPTL) is an efficient heat transfer device used in data center cooling. To enhance its performance, a pump is integrated to facilitate fluid circulation, leading to the development of pump-driven TPTLs. This paper compares the pump-driven CO 2 and R134a TPTLs, focusing on their operating states and heat transfer performance. During the oscillatory stage, activating the pump caused the CO 2 TPTL to transition from an oscillatory to a stable state, while the R134a TPTL showed an increasing amplitude of oscillations with higher pump power. In normal operation stage, increasing pump power raised the actual mass flow rate in both TPTLs. The pump operation significantly rose the heat transfer limits (HTLs) of both TPTLs, but had a slight influence on the thermal resistance (TR). With the pump off, the HTLs for the CO 2 and R134a TPTLs were 1350 W and 1280 W, respectively. At a pump power of 11.6 W, these limits rose to 2410 W and 2500 W. Under low heat loads, the CO 2 TPTL achieves a higher Energy Efficiency Ratio (EER) than the R134a TPTL, while the latter shows better EER performance at high heat loads. However, the TR of the CO 2 TPTL remained consistently about 11% lower than that of the R134a TPTL across different pump powers. The results establish an experimental foundation for energy consumption analysis of pump-driven TPTL systems using different working fluids.
Tong et al. (2026) studied this question.