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Research Papers: Evaporation, Boiling, and Condensation

Surface Structure Enhanced Microchannel Flow Boiling

[+] Author and Article Information
Yangying Zhu

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: yyzhu@mit.edu

Dion S. Antao

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: dantao@mit.edu

Kuang-Han Chu

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: flyjohn@gmail.com

Siyu Chen

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: chensiyu@mit.edu

Terry J. Hendricks

NASA/Jet Propulsion Laboratory,
California Institute of Technology,
4800 Oak Grove Drive,
Pasadena, CA 91109
e-mail: terry.j.hendricks@jpl.nasa.gov

Tiejun Zhang

Department of Mechanical and
Materials Engineering,
Masdar Institute of Science and Technology,
Building 1A, P.O. Box 54224,
Abu Dhabi, UAE
e-mail: tjzhang@masdar.ac.ae

Evelyn N. Wang

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: enwang@mit.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 9, 2015; final manuscript received April 14, 2016; published online May 17, 2016. Editor: Portonovo S. Ayyaswamy.

J. Heat Transfer 138(9), 091501 (May 17, 2016) (13 pages) Paper No: HT-15-1411; doi: 10.1115/1.4033497 History: Received June 09, 2015; Revised April 14, 2016

We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.

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Figures

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Fig. 1

Schematic of the microchannel heat sink design with micropillars on the heated surface. (a) Side view, (b) cross section view, and (c) magnified view of the liquid film forming menisci which create the capillary pressure gradient, dP/dx, that helps drive the liquid flow. The equation that describes the liquid pressure below the meniscus is the Young–Laplace equation where σ is the surface tension of the liquid, r is the radius of curvature of the local meniscus, and Pliquid and Pvapor are the local pressure of the liquid and vapor, respectively.

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Fig. 2

Design and fabrication process of the microchannel device. (a) Schematic (to scale) of the heater and RTDs on the backside of the microchannel device. The dotted sections are the electrical connection lines to the contact pads. (b) Micropillars of 25 μm height were etched in Si using DRIE. (c) A Si wafer was etched through using DRIE to define the channel. (d) Inlet and outlet ports were laser-drilled on a Pyrex glass wafer. (e) The Si layers were bonded using direct Si–Si bonding. A silicon dioxide (SiO2) layer was thermally grown on the Si surface. The Pyrex layer was bonded to the top Si layer using anodic bonding. (f) A platinum (Pt) layer was deposited on the backside of the microchannel using electron-beam evaporation and patterned to form the heater and RTDs.

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Fig. 3

Images of a representative fabricated microchannel with micropillar arrays. Optical images of the (a) front and (b) backside of a device. (c) Optical microscope image of the heater and RTD4 on the backside of the microchannel. (d) SEM image of the cross section (A–A plane in Fig. 3(a)) of a microchannel with magnified view of the micropillars (left inset) and a sidewall at the bottom corner (right inset).

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Fig. 4

Schematic of the custom flow boiling loop used in the study. The loop consists of a liquid reservoir, a pump to provide a constant flow rate, a valve for flow stabilization, preheaters to minimize subcooling, a test fixture to interface with the test device, and various sensors. The components “P,” “T,” and “M” indicate locations of pressure transducers, thermocouples and the liquid flow meter, respectively.

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Fig. 5

Temporally resolved temperature and pressure drop, and flow visualization at G = 300 kg/m2 · s. (a) Midpoint backside surface temperature T3 and pressure drop across a smooth surface microchannel and a structured surface microchannel S4 at q″ = 430 W/cm2. Insets are optical images of a smooth bottom channel surface and a structured bottom channel surface (S4). Midpoint backside surface temperature T3 and pressure drop of a smooth surface microchannel and a structured surface microchannel S4 at (b) q″ = 520 W/cm2 and (c) q″ = 615 W/cm2. The uncertainties of the temperature and pressure drop measurement were approximately ±2 °C and ±300 Pa.

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Fig. 6

Midpoint backside surface temperature T3 and pressure drop ΔP fluctuations of the structured surface microchannels at CHF (the highest heat flux beyond which dry-out occurred). (a) Device S1 at q″ = 655 W/cm2, (b) device S2 at q″ = 763 W/cm2, (c) device S3 at q″ = 819 W/cm2, and (d) device S4 at q″ = 969 W/cm2. The mass flux G = 300 kg/m2 · s. The uncertainties of the temperature and pressure drop measurement were approximately ±2 °C and ±300 Pa.

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Fig. 7

Time-lapse images of the dynamic dry-out process on a smooth surface and on a structured surface (S4) captured by a high speed camera. q″ = 430 W/cm2 and G = 300 kg/m2 · s. The structured surface showed less dry-out spatially and temporally compared to the smooth surface due to wicking. Dry patches formed at the center of the channel which indicated wicking in the transverse direction (from the sidewalls inward). Wicking along the channel direction also existed since the dry patches formed earlier at downstream locations of the channel.

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Fig. 8

The heat transfer performance characteristics of the microchannel. (a) The boiling curve (heat flux q″ versus heater temperature rise ΔT). ΔT and q″ were calculated by Eqs. (2) and (4), respectively. The arrows indicate the CHF. (b) The HTC (calculated by Eq. (8)) as a function of q″. The error bars for q″ were approximately ±1%. The error bars for ΔT were approximately ±3.5 °C for the structured devices (shown for S4) and grew with the heat flux due to the increasing temperature oscillations (±3.5 °C to ±11 °C) for the smooth surface.

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Fig. 9

Pressure drop across the microchannel as a function of heat flux for the devices investigated. The data were plotted until CHF. Error bars in pressure were approximately ±430 Pa (shown for the smooth surface microchannel), which were calculated from the STD of the temporal pressure measurement and the accuracy of the pressure transducers.

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Fig. 10

The liquid wicking velocity uave as a function of the diameters d and pitches l of the micropillars, when the height h is fixed (h = 25 μm). uave is calculated by Eq. (10), and the magnitude of uave is proportional to the flow rate of the wicking liquid film in the pillar arrays. The symbols on the curves mark the locations of the geometries of the micropillars investigated in this study.

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Fig. 11

Midpoint microchannel backside temperature T3 for sample S2. At t = 45 s the heat flux was increased to 341 W/cm2.

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Fig. 12

(a) Measured backside temperature at the inlet (T1, T2), the midpoint (T3), and the outlet (T4) of the microchannel (Sample S4, q = 618 W/cm2). (b) The inlet and outlet pressure, and the corresponding fluid saturation temperature.

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