Development of Microgravity Boiling Experiments aboard the International Space Station from Terrestrial Adverse Gravity Outcomes for a Ratcheted Microstructure with Engineered Nucleation Sites

A ratcheted microstructure has demonstrated motion of vapor bubbles in adverse gravity conditions with distinct stages including nucleation of vapor bubbles from engineered nucleation sites and bubble coalescence which leads to vapor masses sliding across the microstructure. In contrast, a baseline surface with no ratchets but containing engineered nucleation sites showed stagnant vapor bubbles that grew large enough to surpass the test surface footprint. This terrestrial experimental setup was designed to recreate key aspects of the available International Space Station (ISS) hardware, including geometrical constraints and high-speed imaging through a square cross-sectioned transparent borosilicate glass ampoule. A simplified force balance model predicts the liquid film thickness between the crest and trough of the sawtooth for the downwardfacing surface by modifying a microgravity model from a prior parabolic flight experiment. At a heat flux of 1.78 W/cm
²
, the model estimated a liquid film thickness of 116 μm for a vapor slug moving at a velocity of 21.8 mm/s across the downward-facing microstructure. The design of the flight elements for the upcoming ISS experiments including the proposed two-stage pressure relief system, high-speed imaging requirements, and heater and cold zone details are discussed. Thus, the outcomes from the terrestrial downward-facing runs are considered to anticipate vapor bubble dynamics in the upcoming microgravity experiments to design the final flight ampoule elements.