Asymmetric Sawtooth Microstructure Induced Vapor Mobility for Suppressed Buoyancy Conditions: Terrestrial Experiment and Design for ISS Experiments

The lack of buoyancy forces in microgravity stagnates vapor bubbles on heated electronic surfaces, raising local surface temperatures above device operating limits. Though flow boiling can help dissipate higher heat fluxes, the addition of pumps and regulated flow loops increases design complexity for electronic systems. Passive directional motion of the fluid using an asymmetric sawtooth microstructure has demonstrated motion along the surface due to viscous forces. The current study explores the sawtooth microstructure in the nucleate boiling regime for a horizontal upward- and downward-facing orientation with varying sawtooth profiles (60°–30° and 75°–15°) in terrestrial gravity conditions as a precursor to the International Space Station (ISS) experiments. A laser powder bed fusion metal additive manufacturing technique was used to fabricate the test surfaces with 250- $\mu \text{m}$ nucleation sites located on the long slopes. For the test surface with cavities spaced 2-mm apart, the wall superheat was ~4 K lower compared to cavities spaced 1 mm apart in the upward-facing orientation. The downward-facing surface induced passive vapor mobility in the direction of the long slope, and a thin liquid film was observed between the vapor mass and microstructure. This liquid film thickness was predicted using a simplified force balance model modified from a prior parabolic flight experiment. The model predicted a uniform liquid film thickness of $49~\mu \text{m}$ for a vapor slug moving at 13.7 mm/s with the input heat flux at 1.25 W/cm2. These terrestrial tests in the adverse orientation suggest that the sawtooth microstructure will successfully induce vapor mobility in upcoming ISS microgravity experiments. Elements of the final flight ampoule, including the cooling zone, the two-stage pressure relief system, and the high-speed imagery setup, are discussed.