OpenFOAM Simulations of Passive Bubble Separations Analogous to Capillary Nucleate Boiling in Microgravity*

Steady nucleate boiling is readily achieved on Earth as lighter vapor bubbles are removed from the heated surface by buoyancy forces due to gravity. Can steady nucleate boiling take place in the low-gravity environment of space? In 2022, NASA astronaut Kjell Lindgren injected air into the vertex of an expanding partially-liquid-filled rhombic channel aboard the International Space Station. The wetting liquid is driven toward the vertex along the interior corners of the container by capillary forces. The liquid flows break up the gas into asymmetric ullages that are driven passively away from the vertex by the displacing liquid. The bubbles 'rise' to near-inscribed locations where they coalesce, flowing further into new inscribed locations, eventually merging with the liquid free surface. Analogous to bubbles rising during the boiling process due to gravity, these bubbles 'rise' in microgravity due to the combined effects of surface tension, wetting, and conduit geometry. The maximum stable gas flow rate might be considered as analogous to the maximum vapor production rate for steady nucleate boiling in low-g environments. Our research aims to develop a numerical tool to predict such large-scale low-g bubble migrations using the open-source CFD software OpenFOAM with a multi-phase incompressible volume of fluid solver. These isothermal simulations are benchmarked against the rare experimental results catalogued by bubble volumes, overall gas flow rate, conduit geometry, and fluid properties, and the liquid flux (liquid co-flow) around the bubble is measured as a function of bubble height.