This paper focuses on two-phase flow boiling of dielectric coolant HFE 7000 inside a copper multi-microchannel heat sink for high heat flux chip applications. The heat sink is
composed of parallel microchannels, 200 μm wide, 2500 μm high, and 20 mm long, with 200-μm-thick fins separating the channels. The copper heat sink consists of almost 100 channels connected by a longitude groove with a nearly trapezoidal cross section. Coolant
impinges down to the base at the groove and then goes along the microchannels. A copper block heater arrangement was used to mimic a computer chip with a footprint of 1”x1” (6.45 cm2). The base heat flux was varied from 7.75 W/cm2 to 96.1 W/cm2 and the
mass flux from 547.6 to 958.4 kg/m2s, at a nominal saturation temperature of 54 °C. Heat transfer coefficients as high as 57.5 kW/m2K were reached, keeping the base temperature under 66 °C with a maximum of 21.9 kPa of pressure drop, for inlet subcooling of 5 degree and a coolant flow rate of 958.4 kg/m2. Effects of inner diameter of tubing on thermal performance and pressure drop are also discussed. It was observed that an increase of tubing inner diameter by 60 % can result in increase of heat transfer coefficient by 47.8 % and reduction in pressure drop by 63 %.
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Critical Heat Flux Enhancement on Cylindrical Tubes With Circumferential Micro-Channels During Saturated Pool Boiling of Water
This work presents the experimental characterization of pool boiling heat transfer enhancement on cylindrical tubes with circumferential micro-channels using saturated water at atmospheric pressure as the working fluid. Three engineered copper tubes with 300 μm, 600 μm and 900 μm fin width and a fixed 400 μm channel width with 410 μm channel depth were machined using CNC. To compare the boiling enhancement on engineered tubes, one plain copper tube was used as the reference heater. The active heating area on the cylindrical tubes had a dimension of 9.5 mm outer diameter and 10.5 mm length. A custom-built cylindrical heater was designed using a nichrome wire coil of 30 AWG with a resistance of 19.57 Ω/inch of coil to provide joule heating to the cylindrical tubes. The electrical wire was insulated from the copper heater using a thin layer of alumina paste. The saturated pool boiling tests up to critical heat flux (CHF) were conducted at atmospheric pressure. While an approximate CHF of ∼70 W/cm2 was achieved for the plain copper tube, the cylindrical tube with microchannel geometry showed a CHF range of 131–144 W/cm2 that corresponds to 87%–100% enhancement as compared to plain cylindrical tube.
more » « less- Award ID(s):
- 2018995
- NSF-PAR ID:
- 10487933
- Publisher / Repository:
- American Society of Mechanical Engineers
- Date Published:
- Format(s):
- Medium: X
- Location:
- Columbus, Ohio, USA
- Sponsoring Org:
- National Science Foundation
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Herein, the efficacy of graphene nanoplatelets–copper (GNP–Cu) composite coatings formed by ball milling and sintering methods in enhancing the pool boiling, i.e., phase‐change heat transfer tested for distilled water, is presented. The pool boiling performance is quantified by an increase in critical heat flux (CHF) and heat transfer coefficient (HTC) at the lower wall superheat temperature. The ball milling facilitates the draping of highly thermally conductive GNP around individual copper particles and subsequent sintering results in morphological features that further promote high wicking rates of the coatings. A variety of coatings are formed with varying GNP concentrations and copper particle sizes. Highest pool boiling performance is achieved for the 2 wt% GNP and 20 μm diameter Cu coating, with a CHF of 239 W cm−2and HTC of 285 kW m−2 °C−1, which indicate an increment of ≈91% and ≈438%, respectively, compared with a pristine copper surface. Stretched or elongated pores observed for certain compositions of the coatings, termed as “pore pockets” in this work, contribute in higher pool boiling heat transfer due to their ability to contain and supply liquid, while acting as nucleation sites for vapor generation and escape.
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Abstract Jet impingement can be particularly effective for removing high heat fluxes from local hotspots. Two-phase jet impingement cooling combines the advantage of both the nucleate boiling heat transfer with the single-phase sensible cooling. This study investigates two-phase submerged jet impingement cooling of local hotspots generated by a diode laser in a 100 nm thick Hafnium (Hf) thin-film on glass. The jet/nozzle diameter is ∼1.2 mm and the normal distance between the nozzle outlet and the heated surface is ∼3.2 mm. Novec 7100 is used as the coolant and the Reynolds numbers at the jet nozzle outlet range from 250 to 5000. The hotspot area is ∼ 0.06 mm2 and the applied hotspot-to-jet heat flux ranges from 20 W/cm2 to 220 W/cm2. This heat flux range facilitates studies of both the single-phase and two-phase heat transport mechanisms for heat fluxes up to critical heat flux (CHF). The temporal evolution of the temperature distribution of the laser heated surface is measured using infrared (IR) thermometry. This study also investigates the nucleate boiling regime as a function of the distance between the hotspot center and the jet stagnation point. For example, when the hotspot center and the jet are co-aligned (x/D = 0), the CHF is found to be ∼ 177 W/cm2 at Re ∼ 5000 with a corresponding heat transfer coefficient of ∼58 kW/m2.K. While the CHF is ∼ 130 W/cm2 at Re ∼ 5000 with a jet-to-hotspot offset of x/D ≈ 4.2.
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Jet impingement can be particularly effective for removing high heat fluxes from local hotspots. Two-phase jet impingement cooling combines the advantages of both the nucleate boiling heat transfer with the single-phase sensible cooling. This study investigates two-phase confined jet impingement cooling of local, laser-generated hotspots in a 100 nm thick Hafnium (Hf) thin film on glass. The jet/nozzle diameter is ∼1.2 mm and the normal distance between the nozzle outlet and the heated surface is ∼3.2 mm. The jet coolants studied are FC 72, Novec 7200, and Ethanol with jet nozzle outlet Reynolds numbers ranging from 250 to 5000. The hotspot area is ∼0.06 mm2 and the applied hotspot-to-jet heat fluxes range from 20 W/cm2 to 350 W/cm2. This heat flux range facilitates studies of both the single-phase and two-phase heat transport mechanisms for heat fluxes up to critical heat flux (CHF). The temporal evolution of the temperature distribution of the laser-heated surface is measured using infrared (IR) thermometry. This study focuses on the stagnation point heat transfer - i.e., the jet potential core is co-aligned with the hotspot center. For ethanol, the CHF is ∼315 W/cm2 at Re ∼ 1338 with a corresponding heat transfer coefficient of h ∼ 102 kW/m2·K. For FC 72, the CHF is ∼94 W/cm2 at Re ∼ 5000 with a corresponding h ∼ 56 kW/m2·K. And for Novec 7200, the CHF is ∼108 W/cm2 at Re ∼ 4600 with a corresponding h ∼ 50 kW/m2·K.more » « less
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