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RESEARCH PAPERS

Effects of Dissolved Air on Subcooled Flow Boiling of a Dielectric Coolant in a Microchannel Heat Sink

[+] Author and Article Information
Tailian Chen

Cooling Technologies Research Center, School of Mechanical Engineering,  Purdue University, West Lafayette, IN 47907-2088

Suresh V. Garimella1

Cooling Technologies Research Center, School of Mechanical Engineering,  Purdue University, West Lafayette, IN 47907-2088sureshg@purdue.edu

1

Author to whom correspondence should be addressed.

J. Electron. Packag 128(4), 398-404 (Feb 01, 2006) (7 pages) doi:10.1115/1.2351905 History: Received July 11, 2005; Revised February 01, 2006

The effects of dissolved air in the dielectric liquid FC-77 on flow boiling in a microchannel heat sink containing ten parallel channels, each 500μm wide and 2.5mm deep, were experimentally investigated. Experiments were conducted before and after degassing, at three flow rates in the range of 3050mlmin. The dissolved air resulted in a significant reduction in wall temperature at which bubbles were first observed in the microchannels. Analysis of the results suggests that the bubbles observed initially in the undegassed liquid were most likely air bubbles. Once the boiling process is initiated, the wall temperature continues to increase for the undegassed liquid, whereas it remains relatively unchanged in the case of the degassed liquid. Prior to the inception of boiling in the degassed liquid, the heat transfer coefficients with the undegassed liquid were 300500% higher than for degassed liquid, depending on the flow rate. The heat transfer coefficients for both cases reach similar values at high heat fluxes (>120kWm2) once the boiling process with the degassed liquid was well established. The boiling process induced a significant increase in pressure drop relative to single-phase flow; the pressure drop for undegassed liquid was measured to be higher than for degassed liquid once the boiling process became well established in both cases. Flow instabilities were induced by the boiling process, and the magnitude of the instability was quantified using the standard deviation of the measured pressure drop at a given heat flux. It was found that the magnitude of flow instability increased with increasing heat flux in both the undegassed and degassed liquids, with greater flow instability noted in the undegassed liquid.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental system

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Figure 2

Schematic of the expandable reservoir used to degas the fluid

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Figure 3

Details of (a) the microchannel heat sink, (b) the heat sink test section used in the experiments, and (c) cross section of the test section assembly in the flow direction

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Figure 4

Prediction of bubble incipience temperature for degassed and undegassed FC-77 (bulk liquid pressure is 1atm, and the bubble embryo radius is 5.0μm)

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Figure 5

Wall temperature measured at different heat fluxes for both degassed and undegassed cases

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Figure 6

Heat transfer coefficient for undegassed and degassed cases at three flow rates

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Figure 7

Pressure drop at different wall temperatures for both degassed and undegassed liquid flow

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Figure 8

Measured pressure drop at different heat fluxes for undegassed and degassed liquid flow

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Figure 9

Comparison of instability in the undegassed and degassed cases

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