Interface capturing simulations of bubble injection dynamics: Effects of surface tension and orifice shape in molten salt coolants

Abstract

Advanced reactor designs, like Molten Salt Reactors (MSR) are outfitted with a fission gas removal mechanism, such as Xe gas. In this mechanism, bubbles of helium gas are injected into the molten fuel salt to remove the Xe gas. It is important to demonstrate a robust methodology capable of predicting the bubble’s behavior and void fraction within the nuclear reactor to evaluate the efficiency of such systems. However, experimental investigations of bubble dynamics in high-temperature molten salts such as FLiNaK are challenging due to extreme operating conditions and different coolant opacity levels. The present study applies interface capturing simulations (ICS) to model the bubble formation and rise dynamics when injected through oval and circular-shaped orifices with an equivalent diameter of 0.5 mm into quiescent fluid. A finite element method-based flow solver and level set approach is applied for capturing the air/water interfaces. The bubble departure diameter and time, bubble shapes, and deformability factors are evaluated for both the circular and oval-shaped orifices along with different surface tension values. The bubble departure diameter and the temporal evolution of the bubble shapes obtained from the numerical simulation are validated against experimental data. The results indicate that surface tension governs bubble departure: lower surface tension leads to earlier detachment and elongated trajectories. Detachment length is inversely related to surface tension. Moreover, for helium bubbles in FLiNaK, higher surface tension yields stable, smoother trajectories, and less wake instability compared to air bubbles in water. System-level codes as well as multiphase CFD approach rely on closure laws based on average bubble size and other local parameters. The demonstrated and validated ICS approach can help quantify those required parameters based on fluid conditions, properties as well as injector designs.