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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. 

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 millimeter-scale 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 watts per square centimeter and subcooling from 2.7 - 11.5 degrees C 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. 

Passive thermal management is of interest in cooling of electronics and avionics in terrestrial and reduced gravity environments. This paper describes the use of microscale asymmetric surface patterns, or ratchets, to generate preferential fluid motion during phase change. The asymmetric patterns take the form of an array of ratchet structures. Preferentially directed bubble growth is demonstrated for boiling on surfaces with such ratchets augmented with re-entrant cavities to produce nucleation at preferred sites. During pool boiling in FC-72, the asymmetric geometry of microstructures causes bubbles to grow normal to the sloped surface rather than in a vertical direction, resulting in a net motion in a preferential direction. Bubble growth from the re-entrant cavities is studied using high-speed photography and image processing techniques. The concept of self-propulsion is extended to an open-ended channel configuration, wherein high-speed videos that document preferential motion of vapor slugs with velocities in the range of several mm/s are presented. Liquid motion is explained using a semi-empirical force balance. 

Passive thermal management is of interest in cooling of electronics and avionics in terrestrial and reduced gravity environments. This paper describes the use of microscale asymmetric surface patterns, or ratchets, to generate preferential fluid motion during phase change. The asymmetric patterns take the form of an array of ratchet structures. Preferentially directed bubble growth is demonstrated for boiling on surfaces with such ratchets augmented with re-entrant cavities to produce nucleation at preferred sites. During pool boiling in FC-72, the asymmetric geometry of microstructures causes bubbles to grow normal to the sloped surface rather than in a vertical direction, resulting in a net motion in a preferential direction. Bubble growth from the re-entrant cavities is studied using high-speed photography and image processing techniques. The concept of self-propulsion is extended to an open-ended channel configuration, wherein high-speed videos that document preferential motion of vapor slugs with velocities in the range of several mm/s are presented. Liquid motion is explained using a semi-empirical force balance.