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1. Hotspot Cooling Performance of Two-Phase Confined Jet Impingement Cooling at the Stagnation

   Chowdhury, Tanvir A.; Putnam, Shawn A. (January 2022, 2022 21st IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (iTherm))

   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. 

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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. Piezo-Actuated Enhanced Nucleate Boiling: From Saturated to Controlled Flash Regimes in Two-Phase Direct-to-Chip Cooling for Artificial Intelligence Superchips and Data Centers

   https://doi.org/10.1115/1.4072247  

   Narain, A; Bhutka, H; Naik, R R; Mohite, N; Belikoff, Y (September 2026, ASME Journal of Heat and Mass Transfer)

   Abstract Two-phase direct-to-chip cooling is emerging as an important thermal management pathway for the rapidly increasing thermal design powers of modern AI superchips while maintaining data center energy efficiency. This paper presents a combined passive–active approach that transitions high-performance saturated heterogeneous nucleate boiling to a stabilized and controlled flash-nucleate boiling regime using low-power piezoelectric actuation that produces feather-like in-plane and out-of-plane microvibrations of the heated surface. Experiments on microstructured copper cold plates (CPs) with dielectric fluids, at 100–140 kPa pressures, establish baseline enhanced nucleate boiling (ENB-1) performance in partial flow-boiling configurations typical of direct-to-chip flow loops. These results quantify cold-plate behavior and indicate that physics-guided modifications of the experimentally tested microstructures and flow-loop design, as modeled and proposed for further experimental testing, can substantially increase heat-transfer coefficients and critical heat flux (CHF). Activation of megahertz piezo-actuators with low-frequency modulation near antiresonance generates small microvibrations that modify nucleating bubble dynamics and their characteristic frequencies. This triggers a rapid transition to a controlled, stabilized high-pressure (400–500 kPa) heterogeneous flash-nucleate boiling regime (ENB-2), sustained at the boiling surface by “on–off” microlayer heating and acoustic streaming mechanisms. This transition from ENB-1 to ENB-2 accompanies a short-duration (tens of milliseconds) increase in interfacial mass transfer across bubble microlayers, producing transient heat extraction of 200–400 J from underneath the heated surface, that is, from the chip to the ENB-2 condenser with a sustained high pressure at the vapor exit of the cold plate. Three-dimensional transient conduction simulations predict chip-level temperature reductions of about 15–20 °C for low-thermal-inertia test vehicles and future chip implementations. Baseline experiments show nucleate boiling heat transfer with critical heat flux near 50–60 W/cm2 and a lower heat-transfer coefficient (HTC) of 1–2 W/cm2 °C. The tested and deployed architecture is amenable to further optimization to higher CHF (135–175 W/cm2 or more) and associated HTC (up to 150 W/cm2 °C) for ENB-1 and 10–20 °C cooler chips through ENB-2. Observed ENB-2 condenser performance suggests a shift from the ENB-1 condenser to a direct-contact tank/pool condenser for ENB-2, which would allow sustained waste-heat recovery from the vapor at, say, 70 °C and 400 kPa. That is, a stabilized flash-boiling regime supports lower junction temperatures, improved chip reliability, and more energy-efficient next-generation AI data centers. 

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4. Experimental Characterization and Server-Level Demonstration of Confined, Direct-on-Silicon Two-Phase Jet-Impingement Cooling With Additively Manufactured Porous-Wick-Assisted Phase Separation

   https://doi.org/10.1115/1.4071262  

   Yogi, Ketan; Sahu, Gopinath; Rajeev, Sidharth; Lattupalli, Harish Kumar; Wang, Keyu; Chen, Zhengwei; Yang, Yunchun; Schiffres, Scott N; Rangarajan, Srikanth; Sammakia, Bahgat; et al (June 2026, Journal of Electronic Packaging)

   Two-phase jet impingement cooling is a promising solution for high-heat-flux semiconductor thermal management, as it combines strong convective heat transfer with boiling to remove large heat loads at relatively low flow rates and pressure drops. However, practical deployment is hindered by challenges including inconsistent boiling initiation on smooth surfaces, surface dry-out, vapor-induced flow instabilities, and premature critical heat flux (CHF). Excessive vapor generation within confined geometries can disrupt flow uniformity, causing temperature oscillations and unstable operation. To address these challenges, this work presents a confined, direct-on-silicon two-phase jet impingement cooling approach incorporating a porous-wick-assisted phase separation mechanism. The engineered porous wick enhances nucleate boiling and enables in situ phase separation at the boiling surface. Integrated with a custom three-path manifold, the design routes separated liquid and vapor streams, minimizing vapor accumulation within the confined region and suppressing two-phase instabilities. The porous wick is directly printed onto the silicon substrate using advanced additive manufacturing, eliminating the need for a thermal interface material (TIM) and its associated thermal resistance. Thermal–hydraulic characterization using a low-surface-tension dielectric fluid demonstrates that wick-assisted phase separation reduces thermal resistance by 23–29% compared to configurations without phase separation. Extended testing over more than 200 h of continuous operation confirms stable thermal performance and indicates strong potential for long-term reliability. System-level validation is demonstrated in a 1 U server equipped with an NVIDIA V100 GPU (graphics processing unit) incorporating a direct-on-silicon printed wick. 

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5. Supervised and Unsupervised Learning Models for Detection of Critical Heat Flux During Pool Boiling

   https://doi.org/10.1115/ht2022-85582  

   Dunlap, Christy; Pandey, Hari; Hu, Han (July 2022, Journal of Electronic Imaging)

   Abstract The rapid growth and scaling of electronics are causing more severe thermal management challenges. For example, the high-performance computing processors are driving the data center power density to unprecedented levels, approaching the limit of conventional air cooling. In electric vehicles (EVs) and hybrid EVs, the power conversion electronics are integrated into a compact space, leading to ultra-high heat fluxes to dissipate. Among the available thermal management mechanisms, two-phase cooling that involves the phase-change process of the working fluid can maintain electronic devices at safe operating temperatures by taking advantage of the high latent heat of the fluid. Particularly, pool boiling plays a critical role in the two-phase immersion cooling of servers and other IT hardware, integrated cooling for three-dimensional electronic packaging, cooling of the core, and used fuel in nuclear reactors. Two-phase coolers are limited by instabilities such as the critical heat flux (CHF). At the critical heat flux, the temperature increases. It is important to be able to identify the CHF in order to prevent overheating. We aim to develop and compare boiling image classification models to distinguish between 2 boiling regimes. We will leverage principal component analysis (PCA) and K-means clustering to investigate the key differences between bubbles during nucleate boiling (pre-CHF) and transition boiling (post-CHF). We will also compare the results of the unsupervised learning model against popular supervised learning models that have been used for boiling regime classification in existing studies, such as convolutional neural networks, multiplayer perceptrons, and transformers. We successfully created 4 supervised and 1 unsupervised learning models to distinguish between the two types of boiling images. 

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