Investigation of Thermohydraulic Performance in a Turbulent, Large-Scale Single-Phase Natural Circulation Loop

Abstract Passive cooling reliability is crucial in nuclear and thermal systems, making single-phase natural circulation loops (SPNCLs) a vital mechanism for heat removal. However, their turbulent thermal–hydraulic behavior under strong buoyancy remains insufficiently quantified. This study experimentally assesses the thermal–hydraulic performance of a turbulent SPNCL operated under quasi-steady-state conditions. Experiments were performed using the large-scale FASSIP-02 Ver 02 facility with heat source temperatures between 50°C and 80°C. Dimensionless analysis was applied to characterize the dominant heat transfer regime and buoyancy-driven circulation. Increasing the heat source temperature intensified buoyancy forces, which enhanced circulation and improved heat removal. The system achieved a maximum temperature difference of 5.2°C and a low rate of 28.5 L/min at heat source temperatureT WHT = 80°C, with Re = 13,521– 33,850 and Gr = 2.997 × 10 12 –1.739 × 10 13 , confirming fully turbulent operation (Ra > 10 8 ). A correlation between Re and Gr/NG was obtained, with C = 0.00735 and r = 0.6534 (R 2 = 0.96), providing a quantitative model of buoyancy– low interaction. The results supply a validated dataset and correlation for predicting thermal–hydraulic behavior in support of passive cooling system designs.