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Two-phase flow regimes and heat transfer in a manifolded-microgap
Keywords: High heat flux thermal management, Embedded two-phase cooling, Centripetal acceleration effects on two-phase flow
Thermal management is becoming an increasing challenge as electronic devices—including processors, amplifiers, and laser diodes—continue to dissipate more heat per chip area. In order to maintain allowable junction temperatures, cooling systems must generate high chip-scale heat transfer coefficients for further advancements. Embedded on-chip two-phase cooling, coupled with manifold-microchannel geometry, is a thermal management system recently developed to fulfill the growing need, while maintaining manageable operating pressures (200-300 kPa) and pressure drop (<100 kPa). These systems have recently exhibited chip-scale heat transfer coefficients exceeding 200 kW/m2-K with silicon and silicon carbide chips operating at less than 120°C (with temperature increasing with heat flux), using R245fa as the heat transfer fluid. As impressive as these results are, the fluid physics occurring in the U-shaped microchannel flow paths and the associated local heat transfer coefficients on the channel walls are not well understood. Additionally, the available literature on heat transfer occurring in U-shaped two-phase flow paths is sparse, despite the several recent studies on (optically inaccessible) manifold-microchannel two-phase coolers. Therefore, little information is available to guide future design decisions and variations of similar high heat flux coolers. The research effort reported here is a visualization study designed to characterize the flow regimes, unique flow behavior, and heat transfer in a “unit cell” of a manifold-microchannel cooler. The results illustrated the effects of centripetal acceleration due to flow path curvature on two-phase flow behavior. Somewhat similar to the acceleration due to gravity, centripetal acceleration led to phase separation due to buoyancy, but—depending on mass flux and quality—buoyancy forces ranged from 5 to 10^7 times that of straight horizontal two-phase flows. At low heat fluxes and low quality flows, the centripetal acceleration buoyancy force drove bubbles toward the interior of the curve where they coalesced into larger slugs. A map of the size of the interior slugs was developed, which correlated with low local heat transfer coefficients. It was additionally found that the square corners of the manifold-microchannel type geometry led to largely stagnant recirculation zones, which then developed cycles of large slug formation in the bubble and slug flow regimes. A unique characteristic of the behaviors observed was film dryout around slugs in the low mass flux recirculation zones. The relevance of the recirculation zones appeared to decrease as dryout became suppressed at higher quality when the flow transitioned to the annular regime. Dryout was suppressed up to outlet quality of about 80% at the lowest channel mass flux. Maps of flow regime, film dryout, and heat transfer coefficient were developed. Finally, it was shown that vapor superficial velocity is a useful parameter to characterize changes in flow behavior, including flow regime and instances of local dryout, for the given channel geometry. The trends in heat transfer coefficient at the center of the channel wall in this study were in good agreement with the trends of a previous operational microcooler, thus providing insight into the evolving heat transfer mechanisms in a U-shaped microgap channel as heat flux and outlet quality increase.
David C. Deisenroth, PhD Candidate
University of Maryland
College Park, MD

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