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Research Papers

Thermal Response of Multi-Microchannel Evaporators During Flow Boiling of Refrigerants Under Transient Heat Loads With Flow Visualization

[+] Author and Article Information
Houxue Huang

Laboratory of Heat and Mass Transfer,
École Polytechnique Fédérale de Lausanne,
EPFL-STI-IGM-LTCM, Station 9,
Lausanne CH-1015, Switzerland
e-mail: houxue.huang@epfl.ch

Navid Borhani

Laboratory of Heat and Mass Transfer,
École Polytechnique Fédérale de Lausanne,
EPFL-STI-IGM-LTCM, Station 9,
Lausanne CH-1015, Switzerland

John Richard Thome

Professor
Laboratory of Heat and Mass Transfer,
École Polytechnique Fédérale de Lausanne,
EPFL-STI-IGM-LTCM, Station 9,
Lausanne CH-1015, Switzerland

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received February 23, 2016; final manuscript received April 22, 2016; published online May 17, 2016. Assoc. Editor: Mehdi Asheghi.

J. Electron. Packag 138(3), 031004 (May 17, 2016) (13 pages) Paper No: EP-16-1037; doi: 10.1115/1.4033487 History: Received February 23, 2016; Revised April 22, 2016

Multi-microchannel evaporators with flow boiling, used for cooling high heat flux devices, usually experience transient heat loads in practical applications. These transient processes may cause failure of devices due to a thermal excursion or poor local cooling or dryout. However, experimental studies on such transient thermal behavior of multi-microchannel evaporators during flow boiling are few. Thus, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of flow conditions. The effects on the base temperature behavior of the test section, heat flux magnitude, mass flux, inlet subcooling, outlet saturation temperature, and fluid were investigated. The transient base temperature response, monitored by an infrared (IR) camera, was recorded simultaneously with the flow regime acquired by a high-speed video camera. For cold startups, it was found that reducing the inlet orifice width, heat flux magnitude, inlet subcooling, and outlet saturation temperature but increasing the mass flux decreased the maximum base temperature. Meanwhile, the time required to initiate boiling increased with the inlet orifice width, mass flux, inlet subcooling, and outlet saturation temperature but decreased with the heat flux magnitude. For periodic variations in heat flux, the resulting base temperature was found to oscillate and then damp out along the flow direction. Furthermore, the effects of mass flux and heat flux pulsation period were insignificant.

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Figures

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Fig. 1

Schematic diagram of the facility

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Fig. 2

Test section: (a) schematic of the microchannel evaporator, (b) photo of the microchannels with inlet orifices [11], and (c) photo of the microheaters [11]

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Fig. 3

Wall temperature variation versus time at y = 5.0 and z = 7.5 mm of test section 1 with heat load from 0 to 30 W cm−2 at an initial mass flux of 1500 kg m−2s−1

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Fig. 4

Transient flow regime response in test section 1 after the heat load is suddenly increased from 0 to 30 W cm−2 at initial mass flux of 1500 kg m−2s−1, inlet subcooling of 5.5 K, and outlet saturation temperature of 31.5 °C: (a) superheated single-phase liquid—t = 5.250 s (moment A-B), (b) chaotic two-phase flow—t = 7.350 s (moment B), (c) pure vapor phase in channels and two-phase in inlet and outlet slits—t = 7.417 s (moment B-C), (d) two-phase flow followed by vapor phase—t = 7.550 s (moment C), (e) the vapor phase nearly replaced by the two-phase—t = 7.750 s (moment C-D), and (f) subcooled liquid at channel inlet followed by the two-phase flow—t = 8.117 s (moment D). (The time datum is 0 s, as shown in Fig. 3. The net flow direction is from left to right, same to all the figures in this paper.)

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Fig. 5

Thermal maps of test section 1 after the heat load is suddenly increased from 0 to 30 W cm−2, at G = 1500 kg m−2s−1, ΔTsub  = 5.5 K, and Tsat = 31.5 °C for six times: (a) t = 5.250 s (moment A-B), (b) t = 7.350 s (moment B), (c) t = 7.417 s (moment B-C), (d) t = 7.550 s (moment C), (e) t = 7.750 s (moment C-D), and (f) t = 8.117 s (moment D)

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Fig. 6

Evolution of centerline base temperature along the flow direction at seven typical moments versus time for test section 1 with heat load from 0 to 30 W cm−2 at G = 1500 kg m−2s−1

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Fig. 7

Effect of test section inlet orifice ratio on the thermal response of test section 2 with heat load from 0 to 30 W cm−2

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Fig. 8

Process of triggering two-phase flow during a cold startup in test section 2 for G = 1500 kg m−2s−1, ΔTsub  = 5.5 K, Tsat = 31.5 °C, and q = 0–30 W cm−2: (a) bubbles traveling back to the outlet plenum—t = t0 s, (b) small stream of vapor phase starting moving upstream from the channel exit—t = t0 + 0.008 s, (c) vapor phase arriving at the channel middle with large part of channels occupied by the liquid phase—t = t0 + 0.016 s, and (d) vapor phase arriving at the channel entrance with two-phase flow between the vapor phase and the two test section boundaries moving downstream—t = t0 + 0.030 s (the net flow direction is from left to right)

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Fig. 9

Effect of heat flux magnitudes on the thermal response of micro-evaporators: (a) test section 1 and (b) test section 2

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Fig. 10

Effect of mass flux on the thermal response of test section 2 with heat load from 0 to 30 W cm−2

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Fig. 11

Process of triggering two-phase flow during cold startup in test section 2 at G = 1000 kg m−2s−1: (a) onset of boiling by a vapor jet with liquid phase in microchannels—t = t0 s, (b) starting to explode from the channel exit upstream and downstream with still liquid phase in microchannels—t = t0 + 0.002 s, (c) liquid was pushed back by the two-phase mixture creating a vapor phase behind—t = t0 + 0.008 s, and (d) two-phase in the inlet plenum with vapor phase in channels—t = t0 + 0.024 s (the net flow direction is from left to right)

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Fig. 12

Time strips at three streamwise locations during a cold startup at G = 1000 kg m−2s−1 in test section 2: (a) z = 1.0 mm, (b) z = 5.0 mm, and (c) z = 9.0 mm (position z was marked with vertical lines in Fig. 11(d))

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Fig. 13

Effect of inlet subcooling on the thermal response of test section 2 with heat load from 0 to 30 W cm−2

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Fig. 14

Photo of slug bubbles due to flash after the inlet orifices for test section 2 at ΔTsub  = 2 K for G = 1500 kg m2s−1 and Tsat = 31.5 °C under adiabatic condition

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Fig. 15

Effect of outlet saturation temperature on the thermal response of test section 2 with heat load from 0 to 30 W cm−2

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Fig. 16

Effect of fluid property on the thermal response of test section 2 with heat load from 0 to 30 W cm−2

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Fig. 17

Effects of heat flux pulse period and distance downstream on the base temperature response of test section 2 under periodic variations in heat flux: (a) y = 5.0 and z = 0.25 mm, (b) y = 5.0 and z = 2.5 mm, (c) y = 5.0 and z = 5.0 mm, (d) y = 5.0 and z = 7.5 mm, and (e) y = 5.0 and z = 9.5 mm

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Fig. 18

Effects of mass flux on the base temperature response for periodic variations in heat flux: (a) y = 5.0 and z = 0.25 mm, (b) y = 5.0 and z = 2.5 mm, (c) y = 5.0 and z = 5.0 mm, (d) y = 5.0 and z = 7.5 mm, and (e) y = 5.0 and z = 9.5 mm

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