Experimental visualization and characterization of a single bubble rising in high-Prandtl molten salt.

Gas-liquid interactions in molten salts are important for nuclear and thermal energy systems, but detailed understanding of bubble dynamics in high-Prantl-number fluids remains limited. In this study, we experimentally investigate single bubble rising behavior in Hitec eutectic salt (NaNO₂–NaNO₃–KNO₃) using a custom-built transparent conductance-coated quartz tube, which can be heated up to 700°C. A single bubble was injected through a precision 100 μm nozzle into a stagnant Hitec. The bubble's diameter, trajectory, rise velocity, shape deformation, wake structure, and spanwise vorticity were characterized and compared between water and Hitec salt over a range of dimensionless parameters, including Eötvös, Morton, Weber, Galileo, and Taylor numbers. The estimation of drag coefficients under varying bubble sizes. Drag coefficients were estimated under varying bubble sizes. Compared to distilled water, bubbles in Hitec showed slower rise velocities, more elongated shapes, and delayed wake formation. These results provide insight into interfacial transport phenomena in high-temperature molten salt environments and support future modeling of multiphase flows in molten salt reactors and thermal energy storage systems.