This study experimentally investigates the impact of flow resistance in risers and downcomers on the heat transfer performance of two-phase thermosyphon loops (TPTLs) with CO₂, R134a, and R410A refrigerants. Variations in the heat transfer limit and thermal resistance of the TPTLs were analyzed. The results show that the three TPTLs respond differently to changes in flow resistance. For the CO₂ TPTL, the effects of the riser and condenser resistances on thermal performance are similar. When the opening angle of the riser or condenser valve decreases from 90° to 30°, the heat transfer limit of the CO₂ TPTL decreases from 1200 W to 700 W. For the R134a and R410A TPTLs, when the opening angle of the riser valve decreases from 90° to 30°, the heat transfer limit of the R410A TPTL decreases from 1300 W to 700 W, and the R134a TPTL does not reach normal operating conditions, resulting in substantial superheating and subcooling inside the pipes and a substantial increase in thermal resistance for both types. An increase in the condenser resistance has little effect on the thermal performances of the R134a and R410A TPTLs. For practical design considerations, the same or similar diameters should be used for the riser and downcomer of a CO₂ TPTL. However, for R134a and R410A TPTLs, the riser diameter should be significantly larger than the downcomer diameter to achieve material cost savings.

Pump-driven two-phase thermosyphon loops (TPTLs) have garnered widespread attention to improve the performance and applicability of TPTLs under various conditions. This study compared the heat-transfer performance and operational states of a TPTL using two parallel downcomer branches to switch between gravity- and pump-driven TPTL operation modes. Significant differences were observed in the normal working load range of the TPTL when the working fluid flowed through downcomer branches 1 and 2 (pump off), or only through branch 2 (pump on). The presence of a liquid pump increased the resistance to the fluid flow within the loop. When the pump was turned off, the heat-transfer limit of the TPTL decreased, and it increased when the pump was on. During the oscillatory operational stage, the reservoir was unable to effectively separate the vapor and liquid. When the working fluid flowed through downcomer branch 2 (pump off), the TPTL exhibited a longer fluctuation cycle and greater amplitude compared to those during the flow through branch 1. In the stable operational stage, the reservoir provided better vapor-liquid separation.