Mechanisms of Boiling in Micro-Channels: Critical Assessment

Thome, John R.; Consolini, Lorenzo

doi:10.1080/01457630903312049

Cited by 46 publications

(15 citation statements)

References 22 publications

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“…While the physical mechanism responsible for the decrease in heat transfer coefficient in regime 'b' warrants further investigation, Kandlikar [26] suggested that formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling; furthermore, he proposed that wall dryout occurs during the passage of elongated bubbles. For slug flow in microchannels, Jacobi and Thome [27] and Thome and Consolini [28] suggested that liquid thin film evaporation is the dominant heat transfer mode.…”

Section: Boiling Regime 'B'mentioning

confidence: 99%

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

Chen

Garimella

2011

International Journal of Heat and Mass Transfer

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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 ...

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Section: Boiling Regime 'B'mentioning

confidence: 99%

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

Chen

Garimella

2011

International Journal of Heat and Mass Transfer

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“…Situation seems to be a little less complex in the case of flow boiling in minichannels and microchannels. In such flows the annular flow structure is dominant for most qualities, Thome and Consolini (2008). In such case the heat transfer coefficient is primarily dependent on the convective mechanism.…”

Section: Introductionmentioning

confidence: 99%

Comparative study of flow condensation in conventional and small diameter tubes

Mikielewicz¹,

Andrzejczyk²

2012

Archives of Thermodynamics

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Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena. Their description however with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where the bubble generation is not present. Similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such case the heat transfer coefficient is primarily dependent on the convective mechanism. In the paper a method developed earlier by the first author is applied to calculations of heat transfer coefficient for inside tube condensation. The method has been verified using experimental data from literature on several fluids in different microchannels and compared to three well established correlations for calculations of heat transfer coefficient in flow condensation. It clearly stems from the results presented here that the flow condensation can be modeled in terms of appropriately devised pressure drop.

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“…In such flows the annular flow structure is dominant for most qualities [1]. Then the heat transfer coefficient is primarily dependent on the convective mechanism.…”

Section: Introductionmentioning

confidence: 99%

“…Alternatively, there is a group of modern approaches based on models that start from modeling a specific flow structure and in such a way postulate more accurate flow boiling models, usually pertinent to slug and annular flows. The most popular approach, however, to modeling flow boiling is to present the resulting heat transfer coefficient in terms of a combination of the nucleate boiling heat transfer coefficient and convective boiling heat transfer α TPB = (α cb F) n + (α PB S) n 1/n (1) where α PB is the pool boiling heat transfer coefficient and α cb is the liquid convective heat transfer coefficient, which can be evaluated using, for example, the Dittus-Boelter type of correlation. The exponent n is an experimentally fitted coefficient without recourse to any theoretical foundations.…”

Section: Introductionmentioning

confidence: 99%

A Common Method for Calculation of Flow Boiling and Flow Condensation Heat Transfer Coefficients in Minichannels With Account of Nonadiabatic Effects

Mikielewicz

2011

Heat Transfer Engineering

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Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena; however, their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. A similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such a case the heat transfer coefficient is primarily dependent on the convective mechanism. In this article a method developed earlier by the authors is applied to calculations of the heat transfer coefficient for flow condensation. The modifications of interface shear stresses between flow boiling and flow condensation are considered through incorporation of the so-called blowing parameter, which differentiates between these two modes of heat transfer. Satisfactory consistency with well-established correlations for condensation has been found, as well as with selected experimental data.

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Mechanisms of Boiling in Micro-Channels: Critical Assessment

Cited by 46 publications

References 22 publications

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

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

Comparative study of flow condensation in conventional and small diameter tubes

A Common Method for Calculation of Flow Boiling and Flow Condensation Heat Transfer Coefficients in Minichannels With Account of Nonadiabatic Effects

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