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  1. null (Ed.)
  2. This experimental, terrestrial study is part of a larger effort to dissipate increased heat fluxes through enhanced pool boiling in spacecraft electronics prior to an extensive study to be conducted on the International Space Station under pristine microgravity conditions. The absence of buoyancy forces in microgravity causes vapor bubbles to grow to a very large size, leading to premature critical heat flux (CHF). Using an engineered surface modification, namely an asymmetric sawtooth ratchet, to create mobility of the vapor mass can alleviate this problem. The stainless steel (SS 316L) test surfaces were fabricated using powder bed fusion, a metal additive manufacturing process. Vapor mobility was observed in the downward-facing configuration for the asymmetric sawtooth structure explored in the study. A thin liquid film was observed underneath the vapor bubbles as they slid along the microstructure. The asymmetric nature of this liquid film is explored using high-speed imaging at the crest and trough of the sawtooth. The proposed asymmetric saw-tooth microstructure is a potential technique to induce motion of vapor bubbles across electronic components when reduced buoyancy forces do not detach vapor bubbles from the surface. 
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  3. Passive fluid pumping during boiling using the concept of asymmetry in the geometry of the heated surface is studied experimentally. The geometry consists of a channel that is located within a chamber filled with dielectric fluid. The channel ends (inlet and outlet) are exposed to the chamber, such that the ends of the channel have the same pressure prior to the addition of heat. Two types of asymmetry are introduced on the heated surface, and their effect is assessed on the net bubble growth and motion within the open-ended channel. The first is a millimeterscale asymmetry caused by contouring the vertical walls of the channel into repeating 60-30-degree ratchets. The second asymmetry consists of microscale reentrant cavities, located periodically on the shallow ratchet face of the ratchet. To assess the motion of two-phase flow within the channel, visualization at various heat fluxes ranging from 0.8 – 2.6 W/cm2 and subcooling from 2.7 - 11.5 oC is performed. Videos of bubble ebullition, mergers and slug transport are analyzed to obtain growth rates, velocities, and frequency counts of slugs emanating from either end of the open-ended channel. 
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