Numerical Investigation of Surface Tension and Orifice Shape Effects on Bubble Injection Dynamics using Direct Numerical Simulation

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 capably of predicting the bubble’s behavior and void fraction within the nuclear reactor to evaluate the efficiency of such systems. The present study applies direct numerical simulation (DNS) with interface capturing approach 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. The computational domain was filled with tetrahedral mesh elements varying the size depending on the proximity to the orifices. 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 as well. The bubble departure diameter and the temporal evolution of the bubble shapes obtained from the numerical simulation are validated against the experimental data. 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 DNS approach can help quantify those required parameters based on fluid conditions, properties as well as injector designs.