Impact of Cooler Temperature Variations on Flow Boiling Phenomena in a Natural Circulation Rectangular Loop

Abstract The passive cooling system that utilizes natural circulation is effective in removing decay heat from nuclear reactors, with the direct passive residual heat removal system being the most efficient. This study employs the Rectangular-TP FASSIP-04 Ver.2 Loop with a direct heating model to investigate two-phase natural circulation flow. The coolant water temperature is set at 10°C, 15°C, and 20°C with a constant heating power of 5600 watts, to measure temperature characteristics, flow rate, flow regimes, and heat removal rate. The results show that higher coolant temperatures accelerate the achievement of steady-state conditions, reduce the average steady-state temperature, and increase the flow rate and Reynolds number. In contrast, lower coolant temperatures accelerate temperature drop and improve heat removal effectiveness during the transient cooling phase, as well as help maintain flow stability. These findings highlight the importance of controlling coolant temperature in influencing the thermal dynamics and flow stability of the system.