Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Abstract

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 m width and 389 m depth.Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients.The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20 to 80 ml/min with the inlet subcooling held at 26 K in all the tests.At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5 -62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 m wide and 389 m deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels.While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal † The experiments of this work were performed when the lead author was a Post-doctoral Researcher at Purdue University. BackgroundTwo-phase flow in microchannels provides higher performance than in single-phase operation while maintaining the wall temperature relatively uniform, and offers an attractive alternative for the cooling of high-power electronics. Although microchannel heat sinks have been extensively studied [ 1 , 2 ], understanding of two-phase thermal transport phenomena in microchannels is still limited due to their complexity.A number of studies have discussed the differences in flow boiling between small and large channels; a quantitative criterion was recently proposed for delineating micro-and macroscale behavior in twophase flow through microchannels [3]. For flow boiling in large channels, both convective and nucleate boiling heat transfer are considered important [4] and the overall heat transfer coefficient is a function of mass flow ...

A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

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Study on morphology of high-aspect-ratio grooves fabricated by using femtosecond laser irradiation and wet etching

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“…Many devices based on microgrooves such as microchemical reactors [5,6], microsensors [7], and so on have been demonstrated. In addition, heat sinks with microgroove array on back side show excellent thermal control capability [8]. Silicon microgroove grating is also an important element for terahertz radiation [9].…”

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