Comparative Study of Gravity-Driven and Pump-Driven CO₂ Two-Phase Thermosyphon Loop

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.