Next-Gen Thermal Management for Electronics in Space – Asymmetric Sawtooth and Cavity-Enhanced Nucleation-Driven Transport (ASCENT)

Abstract

Nucleate pool boiling in microgravity is characterized by stagnant vapor bubble dynamics, leading to early dryout of the heated surface compared to terrestrial conditions. The Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) investigation studied an engineered microstructure in a nucleate pool boiling setup aboard the International Space Station (ISS) from November 2022-January 2023 to address this longstanding problem. The experiment was housed in the Microgravity Science Glovebox (MSG) and used degassed FC-72 as the test liquid. The engineered microstructure consisted of repeating mm-sized 60°-30° sawtooth structures located within a hermetically sealed square ampoule as the test chamber. Experimental investigations were conducted to explore a range of input heat fluxes, ranging from 0.5 to 2.3 W/cm². The resulting vapor dynamics and heat transfer were compared against a flat baseline surface. Both surfaces contained 250 µm square cavities spaced 1-mm apart.

The constrained dimensions of the 8-mm square chamber severely influenced vapor bubble dynamics across the heated surface. The presence of a liquid layer beneath vapor slugs on the microstructure was observed, in contrast to the baseline surface, where no liquid layer was visually detected. The transient heat transfer coefficient from the microstructure was higher than the baseline surface as the vapor bubbles grew larger at constant heat flux due to liquid film access provided by the asymmetric ridges of the microstructure. A significant increase in the heat transfer coefficient was observed at 1.8 W/cm², potentially due to vigorous nucleation in the observed liquid pockets along the microstructure, increasing the heat transfer coefficient from 2900 to 6200 W/m²K. Nucleation was observed at heat fluxes as low as 0.3 W/cm² from the engineered cavities, and a quasi-steady heat transfer coefficient of 2200 W/m²K was obtained at 2.3 W/cm² from the microstructured surface. The quasi-steady analysis also indicated that both surfaces performed similarly in microgravity and terrestrial gravity in the same experimental setup. The results demonstrate that the liquid film dynamics underneath vapor slugs influence heat transfer to a large extent in microgravity conditions.

Another surface was fabricated using wire electric discharge machining, with hammerhead slots (250 µm mouth) featuring on every third sawtooth (4.5 mm apart). The slots acted as engineered nucleation sites, ejecting vapor at an angle perpendicular to the long slope of the microstructure. Vapor bubbles nucleated from the engineered sites at heat fluxes as low as 0.8 W/cm², and the number of active slots increased as the heat input increased. The departure frequency also increased with increasing heat flux; each slot consisted of multiple nucleation sites. Vapor slugs moved toward the pressure relief membrane in the sealed ampoule at velocities as high as 20.4 mm/s and interacted with liquid pockets between the crest and trough of the microstructure. Individual slug departures increased the cold zone temperature by 19℃ at 2.4 W/cm². The observed vapor bubble ejection from the surface and slug mobility suggests that the surface enhancement can facilitate vapor mobility and heat transfer enhancement in reduced gravity environments.

Similar to the microgravity environments, stagnant vapor growth on a downward-facing flat surface leads to increased surface temperature and dryout at relatively low heat fluxes. The current study investigated the 60°-30° asymmetric sawtooth microstructure to mechanistically improve outcomes in adverse gravity by mobilizing vapor due to a pressure difference between the crest and trough of the surface enhancement following the Young-Laplace relationship. Bubble dynamics were experimentally investigated in an 8-mm square borosilicate glass tube and visualized using a high-speed camera to measure vapor bubble morphology and velocity in the FC-72 dielectric liquid. The vapor bubble's shape changed based on how many sawteeth it covered. This change manifested as a decrease in the curvature ratio between the crest and trough of the microstructure. The pattern of vapor slugs nucleating from intended engineered sites, coalescing to form larger slugs that slid across the microstructure, was repeated at different frequencies at different heat fluxes. The microstructure supported increased vapor volume at higher heat fluxes, suggesting that a passive and self-regulating thermal management solution for adverse gravity applications was feasible. The study proposed an empirical model to recognize the dominant forces driving the observed motion in an adverse gravity configuration. The interplay between the pressure differential force and the buoyancy force was analyzed at different liquid film thicknesses, and the force balance was compared with a 75°-15° sawtooth structure. For a vapor slug spanning four sawteeth, the 75°-15° structure supported a 54% increase in the feasible liquid film thickness range due to the increased long slope area. The outcomes and analysis allow for the creation of numerical frameworks for the microstructure. These models can be based on quantitative image processing, serving as a reference point for future development.

ASCENT investigated the effects of a surface microstructure on boiling in microgravity and terrestrial adverse gravity using degassed FC-72. The results showed that liquid film dynamics played a crucial role in heat transfer, with the microstructure producing a higher transfer coefficient than the baseline surface in microgravity. The microstructured surface offers the promise of enhanced thermal management solutions for electronics in suppressed buoyancy conditions without the complexity of flow loops while maintaining the versatility of being design-inclusive for silicon devices or a bolt-on solution on existing components.