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Research Papers: Evaporation, Boiling, and Condensation

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

[+] Author and Article Information
Tailian Chen

Department of Mechanical Engineering,  Gonzaga University, Spokane, WA 99258-0026chent@gonzaga.edu

Suresh V. Garimella

Cooling Technologies Research Center, an NSF IUCRC, School of Mechanical Engineering and Birck Nanotechnology Center,  Purdue University, West Lafayette, IN 47907-2088sureshg@purdue.edu

J. Heat Transfer 134(1), 011504 (Nov 18, 2011) (9 pages) doi:10.1115/1.4004715 History: Received December 03, 2010; Revised July 18, 2011; Published November 18, 2011; Online November 18, 2011

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the heat flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

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Figures

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Figure 1

Experimental test loop

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Figure 2

(a) Photograph of the printed circuit board (PCB) with the silicon die (12.7 mm × 12.7 mm × 0.373 mm) at the center and (b) mounting details of the silicon die on the PCB

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Figure 3

The arrangement of 25 heating elements U1-U25 (not shown in the figure are 25 diode temperature sensors, each located at the center of each heating element)

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Figure 4

(a) Wall temperature measurements along the flow direction at the flow rate of 60 ml/min (sharp increases are noted as the heat flux is increased from the point marked B) and (b) heat transfer coefficients, h, obtained just prior to the exit of the microchannels for the five flow rates considered in the present study, and (c) wall temperature distribution when the CHF occurs (heat flux of 86.7 W/cm2 ) at the flow rate of 60 ml/min (Included in the image are wall temperatures measured from the five diode sensors along the midline); heat flux is based on the substrate base area Ab and h is defined in Eq. 5

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Figure 5

Pressure drop measurements over the entire heat flux range including both single-phase and two-phase flow at all five flow rates; the heat flux is based on the substrate base area Ab

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Figure 6

(a) Critical heat flux variation with respect to the substrate base area, Ab , for FC-77 obtained in two independent studies and (b) corresponding critical heat input rates

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Figure 7

Critical heat input rates as a function of mass flow rate for three fluids (water, R-113, and FC-77)

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Figure 8

Critical heat input rate as a function of m·hfg for the three different fluids (water, R-113, FC-77)

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Figure 9

Thermodynamic exit quality when CHF occurs in the microchannels

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Figure 10

(a) Comparison of predictions from Eqs. 2,3 against measured CHF for FC-77 obtained in the present work and a previous study by the authors [21]; (b) comparison of predictions from Eq. 10 against the database compiled in this work (Table 1); and (c) comparison of predictions from Eq. 11 against the database (Table 1)

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