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• Numerical investigation of droplet impact and heat transfer in the presence of an external electric field has been presented. • Two new modes of droplet bouncing have been identified. • The dynamic bouncing modes observed at different Weber and electric capillary numbers have been mapped. • Mechanism of droplet stretching behaviour has been revealed. • Heat transfer enhancement during droplet stretching has been quantified and explained. A numerical investigation of droplet bouncing and heat transfer over a heated surface in the presence of an external electric field has been reported. The coupled set of governing equations including the electrostatic and incompressible Navier-Stokes equations, have been implemented in the finite-volume framework of OpenFOAM®. The coupled level set and volume of fluid (CLSVOF) approach has been used to capture the droplet interface. The influence of the electric field over the droplet dynamics and bouncing modes are systematically studied. Different bouncing modes are observed and they have been mapped with respect to the electric capillary number, C a E and Weber number, W e. Two new modes have been identified at C a E = 0.1. The Coulombic attraction at the solid-liquid contact line extends the total contact time of the droplet. The local electric field distribution depends on the droplet's local curvature. The stronger distribution of electrostatic pressure near the tip of the droplet induces a stretching behavior. The stretched droplet, extended contact time and enhanced fluid circulation increase the heat transfer. This work provides a deeper understanding of droplet impact, bouncing dynamics and related heat transfer characteristics influenced by an external electric field. [ABSTRACT FROM AUTHOR]

[Display omitted] • A silicon carbide (SiC) heat sink was fabricated using laser structuring. • Laser structuring enhanced the critical heat flux of SiC by 114 %. • Laser-induced crevices and fins improved the heat transfer coefficient by up to 393 %. • Small bubbles (diameter: ∼0.3 mm) were formed at a high frequency (976 Hz). Silicon carbide (SiC), which has a wide bandgap and superior material properties, offers higher efficiency than conventional silicon in high-power and high-frequency semiconductor applications. In this study, the thermal management of SiC devices was explored through direct liquid cooling with laser-structured heat sinks. Pyramid-structured fin arrays were fabricated directly onto SiC substrates via laser structuring, and their boiling heat transfer performance was investigated through pool boiling experiments in a 5 °C subcooled condition. Laser structuring not only enhanced wettability due to oxidation but also facilitated nucleation through the laser-induced crevices. The oxidized surface created by the laser exhibited increased hydrophilicity, with a significant decrease in contact angle from 57° to 0°. In the pool boiling experiments, these crevices played a crucial role in enhancing the heat-transfer coefficient (HTC) at low heat fluxes (5–40 W/cm2), which promoted nucleation, resulting in the formation of very small and rapid bubbles. At heat fluxes above 180 W/cm2, the large surface area provided by the height of the fin structures further contributed to enhancing the HTC. The sample with the highest performance enhancement exhibited an 114 % increase in critical heat flux and a remarkable 393 % increase in the HTC compared to before laser structuring. [ABSTRACT FROM AUTHOR]

• MMC with CuIO structure efficiently cools high-heat electronics. • CuIOs-MMC dissipates up to 1147 W/cm2 while having pressure drop of 32 kPa. • CuIOs-MMC maintains temperature uniformity below 6 K. • CuIOs-MMC achieves a coefficient of performance (COP) of 4.2 × 104. The utilization of a hierarchical microstructure and three-dimensional (3D) manifold for liquid delivery and liquid/vapor extraction could potentially improve the single-phase/two-phase thermal performance of microcoolers for high-heat-flux microelectronics applications exceeding 1 kW cm−2. In this work, we utilize a conformal coating of 8-μm-thick copper inverse opal (CuIO) films with ∼ 5-μm pore size on silicon microchannels in combination with a polydimethylsiloxane 3D manifold to remove heat fluxes up to 1147 W cm−2 under a water inlet temperature and flow rate of 20 °C and 200 g min−1, respectively. We achieved a convective thermal resistance of 0.068 cm2 K W−1 and total pressure drop of 32 kPa. Moreover, owing to copper micropores, a better hot-spot temperature uniformity (<6 K) with the aid of improved boiling nucleation was achieved. The interchip microchannel with a functional porous material and 3D manifold offers a disruptive thermal-management solution for high-performance electronic devices such as data centers, defense weapons, and power electronics for electric vehicles. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a two-phase capillary-fed boiling micro-cooler that consists of a ≈ 25-μm-thick copper inverse opal (CIO) porous wicking structure for high-heat-flux boiling and a silicon 3D-manifold for distributed liquid delivery and vapor extraction across a 0.5 cm × 0.5 cm heated area. At low inlet water mass flow rates of 1.5 to 1.9 g(min)−1, the micro-cooler displays nearly two-phase boiling with exit vapor quality ≈ 1 and a high critical heat flux (CHF) of 253 to 320 W cm−2 with low superheat of ≈ 10 °C resulting in a thermal resistance of boiling ≈ 0.025 cm2 °C W−1 or heat transfer coefficient of 0.4 MW m−2 °C−1. For higher flow rates of 5, 10, and 15 g(min)−1, the micro-cooler exhibits a hybrid single-phase and two-phase cooling regime where the contribution of the sensible heat (single-phase) cooling is linearly added to that of the two-phase cooling. For the highest flow rate of 15 g(min)−1, the CHF is increased to ≈ 500 W cm−2 resulting in an overall thermal resistance of ≈ 0.18 cm2 °C W−1. However, the two-phase heat transfer effectiveness, which estimates the utilization level of the inlet mass flow rate for two-phase boiling, is reduced to ≈ 0.11. To achieve the best cooling system performances, the micro-cooler must operate entirely within the two-phase boiling regime (exit vapor quality or two-phase heat transfer effectiveness ≈ 1). Ideally, the "coolant" should be delivered near its saturation temperature (≈ 100 °C for water), which provides significant advantages for the energy-efficient operation of data centers and power electronics. We present detailed analysis with Infrared and high speed camera images at various inlet flow rates and heat fluxes to understand complex heat transfer in the micro-cooler. Furthermore, a conjugate thermofluidic simulation model, which incorporates the physics of capillary-fed boiling in a porous copper layer, agrees well with the experimental data. [ABSTRACT FROM AUTHOR]

Thermal management in power electronics relies on active and passive cooling devices that use porous copper structures. However, these critical components, such as sintered copper and copper inverse opals (CuIOs), degrade rapidly when exposed to working fluids like water and oxygen. This study investigates the thermal reliability of CuIO structures subjected to harsh boiling environments using pool-boiling tests. 10–30 μm-thick porous CuIO structures with a constant 5 μm pore diameter and neck openings of 0.6–1.4 μm are fabricated using electrodeposition on silicon through a polystyrene-sintered template. Pool-boiling heat transfer is mainly performed under a constant heat flux of ∼115 W cm−2 at a saturation temperature of 100 °C. As a result, severe degradation of CuIO structures occurs over 72 h. Therefore, A 50 nm-thick direct gold immersion coating is considered as a mitigation strategy. The gold-coated CuIO surface exhibits structural stability, maintaining a stable temperature and heat transfer coefficient compared to pristine CuIOs. Moreover, a reliability test in HFE-7100 is also conducted for applications with lower heat flux and operating temperature requirements. Extensive reliability tests on CuIOs with and without an Au protective coating reveal no noticeable degradation over 11 days at 50% and 67% of critical heat flux. This study provides essential reliability data for failure modeling and offers valuable insights into mitigation strategies and predictive tools for improving the reliability and life expectancy of CuIOs. • CuIO degradation increases thermal resistance by 82% after 72 h boiling in water. • Thermal degradation starts near edge due to bubble ebullition and liquid replenishment. • Gold-plated CuIOs show excellent non-oxidant traits during long-term water boiling. • Stable thermal resistance achieved by gold-plated CuIOs during 72 h boiling in water. [ABSTRACT FROM AUTHOR]

• The single-phase heat transfer and pressure drop of micro-pin fin arrays are studied. • The heat transfer coefficient varies with the mass flow rate and pin spacing. • The heat transfer coefficient achieves a maximum value of 18.2 kW/(m2·K). • The experimental results are validated with simulations using ANSYS Fluent. • MAEs of 9.1% for heat transfer and 14.3% for the pressure drop are obtained. Aggressive thermal management strategies such as liquid cooling have become essential for high-performance three-dimensional (3D) integrated circuit (IC) chips. Micro-pin fin arrays integrated between stacks can provide superior thermal performance with relatively less pumping power compared to microchannel cooling. In this work, we experimentally studied the single-phase heat transfer and pressure drop characteristics of micro-pin fin arrays. Three different samples consisting of 31–131 rows of cylindrical micro-pin fins with pin diameters D h = 45–100 μm, center-to-center pin spacings S = 74–298 μm, and pin height H f ∼ 200 μm were tested. Dielectric fluid R245fa was used as the working fluid with mass flow rates m ̇ = 14.7–181.6 g/min and corresponding Reynolds numbers Re = 35–481.3. The heat fluxes ranged from 2.5 W/cm2 to 48.7 W/cm2, and the inlet fluid temperature was maintained at ambient temperature in the range of 22.2–25.3 °C. The local heater temperature distributions, average heat transfer characteristics, and pressure drops for various geometries of the embedded microfluid pin–fin arrays were determined. The experimentally determined heat transfer coefficient varied with both the mass flow rate and pin spacing with an averaged heat transfer coefficient of up to 18.2 kW/(m2·K). Full-scale conjugate simulations with a turbulence model were conducted using ANSYS Fluent to validate the experimental results for the three cases. A comparison with the numerical model showed mean absolute errors of 9.1% for the heat transfer and 14.3% for the pressure drop. [ABSTRACT FROM AUTHOR]

• Manifold microchannel heat sink (MMCHS) for high-heat flux cooling is fabricated monolithically via laser powder bed fusion (LPBF) process using AlSi10Mg. • Experiments on the single-phase cooling of MMCHS are undertaken in the range of mass flow rate of 100–400 g/min and heat flux of 0–240 W/cm2, after which the streamlines and distribution of channel velocity are numerically-analyzed. • As a result of integrating the manifold layer, the experimental results suggest a total thermal resistance as low as 0.21 K/W with an incredibly low-pressure drop of 1.7 kPa and equivalent pumping power of 11.4 mW. • Compared to a traditional microchannel heat sink, the reported MMCHS reduces thermal resistance and pressure drop by up to 44 and 70%, respectively. Active liquid cooling technique with great efficiency not only reduces power consumption but also effectively dissipates high heat flux. In this study, a manifold-microchannel heat sink (MMCHS) was monolithically fabricated by additive manufacturing, and the thermal and hydraulic performance was investigated in a closed loop. Utilizing AlSi10Mg powder, the laser powder bed fusion process was used to fabricate the complex heat sink structure by directly putting a 3D liquid routing manifold structure on a typical microchannel. The MMCHS, with an overall size of 30 × 15 × 9 mm3, can support a heated area of 10 × 10 mm2 and features a tapered structure to facilitate uniform coolant flow. This system contains microchannels with a width and height of 0.2 mm and 2 mm, respectively, with an aspect ratio of AR = 21. Our results show that the MMCHS can dissipate effective heat flux up to 240 W/cm2 with a mass flow rate of 395 g/min with a considerably low-pressure drop of 1.7 kPa and low heated surface temperature of 100 °C. The corresponding total thermal resistance is as low as 0.21 K/W. In addition, numerical simulations showed detailed flow information as well as good agreement with experimental data. Finally, methods for structural improvement of the manifold microchannel were suggested based on the experimental and numerical results. [Display omitted] [ABSTRACT FROM AUTHOR]

• Energy-efficient embedded cooling in micro-pin fin heat sink is achieved. • Thermo-hydraulic performance of micro-pin fins is analyzed by the thermal performance index for various geometries and operating conditions. • The experimental results are verified via CFD simulations. • Regression models for Nusselt number and fanning friction factor in micro-pin fin arrays based on a consolidated database are proposed. Three-dimensional stacked electronics have substantially improved the electrical performance of integrated circuits. However, given the geometrical complexity and high pressure drop they entail, thermal management difficulties and energy requirements are exacerbated owing to the inapplicability of thermal management schemes. In this study, the thermal and hydrodynamic characteristics of various micro-pin fin arrays were investigated to maximize heat dissipation while minimizing the energy consumption. Specifically, a 10 × 10 mm2 micro-pin fin array was fabricated on an eight-inch silicon wafer via microfabrication. A Pyrex cover was bonded anodically with the top side of the micro-pin fins to prevent leakage, and a titanium/gold thin film serpentine heater was used to supply uniform heat flux on the backside of the micro-pin fin array. Subsequently, the heat transfer and pressure drop in the micro-pin fin heat sinks were obtained experimentally with various micro-pin fin geometries having pin diameter D f = 38–100 µm, transverse pin spacing S T = 74–301 µm, longitudinal pin spacing S L = 74–301 µm and pin height H f = 90–207 µm. Thereafter, the geometrical and operational effects on heat transfer and pressure drop were investigated based on a consolidated database cumulated from the literature. Altogether, 256 data points from 21 geometrical combinations were explored from existing relevant studies to obtain optimized geometric and operating conditions in the micro-pin fin arrays over a wide range of geometrical and operating conditions: Reynolds number Re = 35–491.3, heat flux q" = 0–114 W/cm2, pin diameter D f = 38–559 µm, pin spacing S = 74–800 µm, and pin height H f = 90–845 µm. Subsequently, new empirical correlations based on the consolidated database were formulated to describe the Nusselt number and fanning friction factor in the micro-pin fin arrays. These correlations provide suitable predictions in comparison with those based on extant correlations. [ABSTRACT FROM AUTHOR]

Electrospray (ES) cooling is a promising technique for dissipating high heat fluxes due to its excellent heat transfer characteristics and high energy efficiency. Conventional sprays exhibit a low efficiency due to droplet rebound and require a high pumping power. These issues are resolved with electrospraying, in which fine droplets are dispersed and accelerated with an electric field, thereby improving the cooling efficiency with a relatively lower pumping power and a low overall system weight. In this study, we investigate the thermal performance of ES cooling with water as the working fluid using a single stainless-steel nozzle with an inner diameter D i = 410 μm. We use three different flow rates, Q = 200, 400, and 600 μL/min, with a wide range of applied ES potentials, V 1 = 0–7 kV, to investigate various ES modes. The results show that an increase in the applied ES potential can improve the heat transfer performance by 12.58% and 6.65% in the single-phase and transition regions, respectively, while the improvement is insignificant in the nucleate boiling region and at the critical heat flux. The variations in the ES mode are examined in detail using sequential optical images captured by a high-speed camera. Dimensionless correlations for each cooling regime are proposed using the Weber, electric Weber, and modified boiling numbers. These correlations provide good predictions of the heat transfer performance for all applied ES potentials and flow rates with an overall mean absolute error of 2.96%. [ABSTRACT FROM AUTHOR]