PTV Analysis of Passive Microgravity Bubble Separations Analogous to Steady Capillary Nucleate Boiling in Space

On earth, heat transfer away from a heated surface can easily be achieved by nucleate boiling, as buoyancy due to gravity ensures that the gas bubbles that form rise to the liquid surface. In the microgravity environments found aboard spacecraft, this seemingly simple task becomes non-trivial as bubbles do not rise with buoyancy. In cases of high heat flux, wall-bound vapor bubbles coalesce, blanketing the heated surface and preventing heat removal. In 2022, a series of demonstrations were performed by astronaut Kjell Lindgren aboard the International Space Station exploiting conduit geometry to mimic steady nucleate boiling. The 'boiling flask' container is capable of passive bubble separations in a manner that replaces the role of gravity during nucleate boiling on earth. The analogous isothermal demonstrations introduce air bubbles into the flask vertex at a near constant rate. The partially asymmetrically confined bubbles are driven downstream where they merge with the free surface. In this work, the migration and distribution of the bubbles and liquid films are analyzed using particle tracking velocimetry (PTV). It is observed how bubble trajectories, velocities, and coalescence are dependent on gas flow rate, bubble volume, conduit liquid fill level, and conduit geometry. The phenomena is found to be dependent on the inscribed bubble diameters within the channel geometry.