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1. Hotspot Cooling Performance for Submerged Confined Two-Phase Jet Impingement Cooling

   https://doi.org/10.1115/IPACK2021-69317  

   Chowdhury, Tanvir Ahmed; Putnam, Shawn A. (October 2021, ASME 2021 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems)

   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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2. Hotspot Cooling Performance of a Submerged Water Jet via Infrared Thermometry

   https://doi.org/10.1109/ITherm45881.2020.9190595  

   Chowdhury, Tanvir Ahmed; Brewer, Chance; Putnam, Shawn A. (July 2020, 2020 19th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm))

   Liquid jet impingement is one of the most effective methods for dissipating local hotpot heat fluxes in microelectronics. Due to its normal incident flow-field, jet impingement cooling can achieve heat transfer coefficients (HTCs) approaching ≈1 MW/m 2 ·K due to its ability to thin the local thermal boundary layer in the stagnation region. This experimental study presents HTC data for water jet impingement cooling of a laser heated Hafnium (Hf) thin-film on glass. A laser diode induces local hotspots for either a steady- or pulsed-laser operation mode. The hotspots have areas ranging within 0.04 mm 2 to 0.2 mm 2 and heat fluxes up to ≈3.5 MW/m 2 . A submerged jet impingement configuration is pursued with an inlet jet diameter of ~1.2 mm, jet nozzle to hotspot/surface distance of ~3.2 mm, and the jet Reynolds Number of ~2004. The HTCs are measured using infrared (IR) thermometry using a 1.5-5 μm spectral resolution FLIR camera. Also investigated is the spatial dependence of the HTC relative to the offset between jet/wall stagnation point and the center of the local hotspot. For example, for impinging jets that are co-aligned with the hotspot center, HTCs of ~650 kW/m 2 ·K and ~470 kW/m 2 ·K are measured for steady and pulsed-modulated laser heating (respectively), whereas, for offsets beyond ~6 mm (x/D >5), the measured HTCs are <; 100 kW/m 2 ·K. 

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3. Validation and Application of a Finned Tube Heat Exchanger Model for Rack-Level Cooling

   https://doi.org/10.1115/1.4063252  

   Khalid, R; Youssef, E; Amalfi, R L; Ortega, A; Wemhoff, A P (June 2024, Journal of Electronic Packaging)

   Abstract A thermosyphon-based modular cooling approach offers an energy efficient cooling solution with an increased potential for waste heat recovery. Central to the cooling system is an air-refrigerant finned tube heat exchanger (HX), where air is cooled by evaporating refrigerant. This work builds on a previously published two-dimensional (2D) model for the finned-tube HX by updating and validating the model using in-house experimental data collected from the proposed system using R1233zd(E) as the working fluid. The results show that key system variables such as refrigerant outlet quality, air and refrigerant outlet temperatures, and exchanger duty agree within 20% of their experimental counterparts. The validated model is then used to predict the mean heat transfer coefficient on the refrigerant side for each tube in the direction of airflow, indicating a maximum heat transfer coefficient of nearly 1200 W/(m2 K) for a HX duty of 5.3 kW among the tested cases. The validated model therefore enables accurate predictions of HX performance and provides insights into improving the heat exchange efficiency and the corresponding system performance. 

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4. Two-phase Impingement Cooling using a Trapezoidal Groove Microchannel Heat Sink and Dielectric Coolant HFE 7000

   https://doi.org/10.1109/ITherm51669.2021.9503283  

   Hoang, Cong Hiep; Rangarajan, Srikanth; Radmard, Vahideh; Fallahtafti, Najmeh; Tradat, Mohammad; Arvin, Charles; Schiffres, Scott; Sammakia, Bahgat (June 2021, 2021 20th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (iTherm))

   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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5. Thermal Development of an Impinging Jet Using Planar Laser Induced Fluorescence (PLIF)

   Wright, L.; Seitz, S. (June 2019, ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition)

   Planar Laser Induced Fluorescence (PLIF) has been demonstrated to investigate a round jet impinging on a flat surface. Detailed thermal field distributions have been obtained near the flat target surface to characterize the wall jet development ensuing from the stagnation point. While PLIF has been demonstrated for combustion applications to measure concentration gradients within a mixture, its application for temperature field measurements is less established. Therefore, the technique was applied to a simple, cylindrical impinging jet. The jet Reynolds number varied with Rejet = 5,000 – 15,000 while the jet – to – target surface spacing varied from H / D = 4 – 10. The cooling jet (Tjet ~ 300 K) impinged on a flat, heated surface. The PLIF technique was able to capture the free jet structure and jet development along the target surface. With a short impingement length (H / D = 4), the potential core of the jet strikes the target surface. The thermal gradients captured during the experiments demonstrate the fully turbulent nature of the impinging jet with H / D = 10. The thermal boundary development along the target surface is clearly captured using this fluorescence method. The near wall temperature gradients acquired with the PLIF method have been used to calculate heat transfer coefficients on the heated surface, and these values compare favorably to those measured using a well-established steady state, heat transfer method. The PLIF technique has been demonstrated for this fundamental impingement setup, and it has proven to be applicable to more complex heat transfer and cooling applications. 

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