Experimental analysis of helium bubble-driven flow for enhanced natural circulation in passive molten salt fast reactor

Abstract

A helium bubbling system is proposed for the Passive Molten salt Fast Reactor to remove insoluble fission products that could accelerate local corrosion and perturb reactivity. This process also induces bubble-driven flow, which enhances heat transfer efficiency and consequently elevates the target thermal power in naturally circulating systems. Thus, a comprehensive understanding of two-phase flow, particularly bubble-driven dynamics, is essential and requires well-designed experimental investigations. However, experimental studies on bubble-driven flow under diverse conditions remain limited, especially those utilizing helium as the dispersed phase. Thus, this study experimentally evaluated bubble-driven flow under adiabatic conditions, focusing on helium injection. The variations in key thermal-hydraulic parameters, including liquid velocity and void fraction, were analyzed with respect to hydraulic diameter, liquid viscosity, gas types, and superficial gas velocity. High-speed visualization captured bubble behavior, revealing strong bubble interactions such as coalescence and break-up. In the narrow channel, longer slugs were observed more frequently due to enhanced wake entrainment. Notably, a significant transition in liquid velocity was observed near a superficial gas velocity of 0.055 m/s at 8 mPa & sdot;s viscosity, likely due to competing viscous and inertial forces. In all cases, helium improved natural circulation performance by at least 4% compared to air, attributed to its lower density.