Search

Flow boiling in microchannel heat sinks is capable of providing the high-heat-flux dissipation required for thermal management of next-generation wide bandgap power electronics at low pumping power and uniform surface temperatures. One of the primary issues preventing implementation of these technologies is the presence of flow boiling instabilities, which may reduce the heat transfer performance. However, the effect of individual instabilities, such as the parallel channel instability or pressure drop oscillations, on the overall heat transfer coefficient and critical heat flux in microchannel heat sinks has not been fully quantified. The primary cause of these dynamic flow boiling instabilities is the interaction between the inertia of a two-phase mixture in a heated channel and sources of compressibility located upstream of the inlet. In order to isolate the effect of pressure drop oscillations on flow boiling heat transfer performance, experiments are performed in a single-square microchannel cut into a copper heat sink, with a controlled level of upstream compressibility. The impact of pressure drop oscillations on the heat transfer coefficient and critical heat flux is characterized through analysis of both time-averaged steady-state data as well as high-frequency pressure signals synchronized with high-speed visualization. The dielectric working fluid HFE-7100 is used in all experiments with a saturation temperature of 60 °C at the channel outlet pressure. The occurrence and effect of pressure drop oscillations in 20-mm-long microchannels of three different channel widths (0.5, 0.75, and 1 mm) are related to mass flux, the degree of two-phase flow confinement, and the severity of pressure drop oscillations. [ABSTRACT FROM AUTHOR]

High-vapor-quality flow boiling on structured surfaces has the potential to enhance heat transfer performance and reduce temperature/pressure instability compared to conventional lower quality flow boiling on smooth surfaces. In this article, we discuss recent research progress on structured surface-enhanced flow boiling. We discuss lessons learned and focus on the challenges remaining. Although some degree of mechanistic understanding of the effect of surface structures on the flow boiling process has been gained, many important challenges remain to enable real utilization. Primarily, a need exists for a greater fundamental understanding of the processes if design tools are to be developed capable of guiding engineers to select and implement enhancements. The challenges discussed stem from a community-wide workshop focusing on high-exit-quality flow boiling, held in June 2020. Four key challenges were identified: 1) the need for better understanding of the mechanisms governing surface-structure-enhanced flow boiling and their effect on heat transfer coefficient, critical heat flux, pressure drop, and flow stability; 2) the need for rigorous quantification of surface durability and manufacturing scalability; 3) the need to understand effects of using working fluids other than water including refrigerants, supercritical fluids, and other dielectric fluids; and 4) the need to establish thermal resistance limits dependent on liquid film thickness. We end this article by providing conclusions detailing where we believe that the community should direct both fundamental and applied efforts in order to solve the identified challenges, which limits the implementation of high-vapor-quality flow boiling on surface structures. [ABSTRACT FROM AUTHOR]

This paper is intended to ensure adequate thermal management to extract and dissipate the heat emitted by a square Light Emitting Diode (LED) and to guarantee efficient and safe operation.Towards this aim, we developed a three-dimensional, time-dependent code that uses Comsol Multiphysics to solve the equation systems for mass, momentum, and energy. After the validation of this model, it is used to find an inexpensive solution to decrease the junction temperature of the square LED lamp.The drilling of the heat sink in square or circular form is the subject of this paper. For a precise study, the calculation of the convection heat transfer coefficient was calculated carefully according to all the necessary parameters. It is found that the junction temperature of the square LED lamp decreases by drilling the heat sink with two square or cylindrical shapes at different input powers. In addition, the role of the convection heat transfer coefficient is very important with the increase of internal exchange surface for good heat dissipation to the outside and cooling of the lamp.

In this paper, the impact of an extended surface on the passage of a microchannel using cylindrical ribs with variable sector angles on heat transfer rate is presented using computer simulation. Extended surfaces in the form of cylindrical ribs of varying sector angles in the passage of microchannel in a staggered manner have been designed. The sidewalls of a new kind of microchannel incorporating five distinct ribs with sector angles ranging from 45° to 80° have been analyzed. Ansys Fluent workbench software has been exploited to simulate this novel design of a microchannel heat sink. A three-dimensional heat transfer and fluid flow model of the microchannel heat sink (MCHS) was developed, and the fluid and solid regions were discretized in very fine meshes. All CFD simulations were performed for Reynolds numbers between 100 and 900. Nusselt numbers are varied in the following ranges: 6.93 to 13.87, 6.93 to 14.38, 6.93 to 17.80, 7.15 to 27.86, and 7.20 to 37.38 at sector angles of 45°, 50°, 60°, 70°, and 80°, respectively. It is concluded that the Nusselt number is strongly influenced by the Reynolds number. At an angle of 80°, the maximum friction factor and pumping power requirements were observed. Additionally, a 45° angle has been proven to be the minimal friction factor and pumping power requirement. It is revealed that the THPP has all values larger higher than 1. At angles of 80° and 45°, the maximum and minimum values of THPP have been discovered, respectively. In addition, thermo-hydraulic performance parameters have been evaluated, which are greater than one for all sector angles.

Effective removal of excess heat from electronics equipment is the key to its intended functionality. A Defense Advanced Research Projects Agency (DARPA) hard problem is proposed in microtechnologies for the air-cooled heat exchangers (MACE) program that posed challenges to reduce the heat sink size fourfold while using only 50% of the current power budget. While numerous heat removal techniques have been proposed over the last several decades, synthetic jet actuator (SJA) is one of the leading candidates to create a game-changing cooling performance. This is due to its inherent advantages, such as size, low power consumption, ease of use, and affordability. This review article looks at different actuation methods (electromagnetic, piston cylinder, piezoelectric, and so on) to generate synthetic jet cooling. The performances of these approaches vary and need a procedure to unify the predictive capability. The performances are evaluated based on actuation (stroke length, operating frequency, and jet-to-heater spacing) and geometric parameters (shape and size of the cavity and the orifice). The flow characteristics and jet formation criterion are also discussed methods, and a critical commentary is added to normalize previous findings. Available correlations for predicting Nu numbers are evaluated and summarized through data sets for a possible unification. Finally, an empirical correlation is proposed based on available data using nonlinear regression via computational tools. To conclude, the challenges and research gaps are enumerated covering the fundamental and applied aspects of those jets for the implementation in electronics thermal management. [ABSTRACT FROM AUTHOR]

Lateral thermoelectric devices, where the Peltier cooling and heating occur in a lateral direction, have shown promise for thermal management of on-chip hot spots, and because of the device orientation, thermoelectric materials with anisotropic properties such as holey silicon have shown even more promising cooling potential. However, the role of anisotropy in transient cooling and heating effects is little known, and thermal management optimization using a pulse has not been considered with anisotropic materials. Here, we study temporal and spatial interplays of Peltier, Joule, and Thomson effects in a holey silicon-based lateral thermoelectric device with varying pulse conditions and material properties. Our simulations show a supercooling effect, driven by Peltier cooling before Joule heating diffuses in, and that holey silicon with anisotropic thermal conductivities is favorable by delaying the diffusion in the lateral direction while allowing rapid heat dissipation in the vertical direction. Depending on the local temperature distribution, the Thomson effect is shown to strengthen the supercooling effect. A holey silicon-based thermoelectric device with a 1 kW ⋅ cm−2 hot spot shows temperature reductions from 117 °C to 102 °C in steady-state and temporarily to 100 °C with optimal pulse conditions. The transient cooling performance can be further improved by incorporating phase-change materials within holey silicon, in which their melting process delays a temperature overshoot. The anisotropy, specific heat, and latent heat play important roles in determining the transient cooling performance. Our findings show that lateral thermoelectric devices with anisotropic properties are promising for dynamic thermal management of on-chip hot spots. [ABSTRACT FROM AUTHOR]

The Fast TracKer (FTK) is an ATLAS trigger upgrade built for full-event, low-latency, high-rate tracking. The FTK core, made of 9U VME boards, performs the most demanding computational task. The associative memory board (AMB) serial link processor and the auxiliary card (AUX), plugged on the front and back sides of the same VME slot, constitute the processing unit (PU), which finds tracks using hits from eight layers of the inner detector. The PU works in pipeline with the second stage board (SSB), which finds 12-layer tracks by adding extra hits to the identified tracks. In the designed configuration, 16 PUs and four SSBs are installed in a VME crate. The high power consumption of the AMB, AUX, and SSB (respectively, of about 250, 70, and 160 W per board) required the development of a custom cooling system. Even though the expected power consumption for each VME crate of the FTK system is high compared with a common VME setup, the 8 FTK core crates will use ≈60 kW, which is just a fraction of the power and the space needed for a CPU farm performing the same task. We report on the integration of 32 PUs and eight SSBs inside the FTK system, on the infrastructures needed to run and cool them, and on the tests performed to verify the system processing rate and the temperature stability at a safe value. [ABSTRACT FROM AUTHOR]

Development of electronic devices depends on cooling techniques. An efficiency of cooling methods can be improved using more effective heat-transfer fluids and extended heat-exchange surfaces. Free convection of alumina nanoliquid in a chamber with a copper finned thermal sink and a thermally-producing unit is simulated in the present research. Basic equations written by means of non-dimensional non-primitive characteristics have been solved using the finite difference method. Influences of the Rayleigh number, fins height and nanoparticles concentration on energy transport and flow structures within the chamber have been scrutinized. It has been ascertained that a rise of the fins height and particles concentration intensifies the heat removal from the heated source. [ABSTRACT FROM AUTHOR]

In the current work, a lattice-structured mini channel heatsink is subjected to an impinging flow (IF) in which the flow enters from the top and exits from the lateral sides. An experimental setup is built, and numerical simulations are performed. The hydrothermal performance of the heatsink subjected to the IF is investigated and compared with the same heatsink when a cross flow (CF) enters from one side and exits from another. The pressure drops, overall thermal resistances, and heat transfer coefficients are scrutinized for the mass flow rates between 2 and 5 gr. s − 1. The cases with CF depict superior thermal performance at the same mass flow rates; however, the pressure drops are greater than those with the IF. The IF case leads to lower thermal resistances at the same pumping powers such that when the pumping power is approximately 0.75 W , thermal resistance for the IF is 17.1% lower than CF. The heat fluxes proportional to the maximum base temperatures of the heatsink are calculated. For the critical base temperature of 75 ° C , the allowable heat flux of the heatsink is in the range of 2.24 − 4.33 W / cm 2 for the studied mass flow rates. [ABSTRACT FROM AUTHOR]

• Machined post array with sintered wick extends maximum power. • Machined post array shows improvement over sintered post array. • Vapor vent features improve thermal resistance at low power. • Combining geometric features shows significant performance improvement. To enhance the dryout heat flux of vapor chambers over a 1 cm × 1 cm area, this study describes the testing of four different vapor chamber design architectures (each having an overall size of 50 mm × 50 mm × 5.5 mm). The designs leverage a combination of internal liquid feeding posts and evaporator vapor venting features to improve the capillary-fed boiling performance. The study provides systematic insight into increasing the dryout limit of a vapor chamber and lowering the thermal resistance by assessing the effect of each geometric feature independently. A jet impingement air cooling test facility is used to measure the total thermal resistance of the package in a representative application, while a liquid cooling test facility is used to measure the thermal resistance of the vapor chamber itself. The results show that a vapor chamber with a combination of machined posts coated with sintered wick and vapor vent features has superior performance over vapor chambers with independent features. It dissipated the highest heat flux of 589 W/cm2 and provided the lowest total package thermal resistance of 0.28 K/W for a 1 cm2 heat input area. The performance improvements are attributed to an improved liquid supply utilizing the porous-wick-coated machined posts and a reduction vapor egress pressure utilizing the vent features above the heater area. [ABSTRACT FROM AUTHOR]

• A novel method of accurately characterizing heat pipes. • Thermoelectric modules used as heat flux meters enhance characterization accuracy. • Reference measurements indicate a high degree of accuracy, ±2%. • Uncertainties within the derived thermal characteristics of heat pipes are <3.2 %. • Times to steady-state of less than 40 min are achieved. This study details an experimental method for the accurate and rapid thermal characterization of low-intermediate temperature heat pipes (<200 °C). The aim of this work was to provide a method of measuring the thermophysical characteristics of heat pipes while providing definitive uncertainty values that enable accurate performance estimations prior to implementation into electronic systems. To achieve this, the apparatus used two heat flow meters, constituting cubic copper interface blocks and thermoelectric modules that measured heat flow rates across the specimen while precise, well-calibrated (±0.01 K) thermistors measured axial temperature drops along its length. The method was first demonstrated by characterizing a cylindrical copper reference specimen of known thermal conductivity, indicating a high degree of accuracy, with measured effective thermal conductivity values within ± 2 % of the correlated value. Concerning the thermal characterization of heat pipe specimens, thermal resistances and corresponding effective thermal conductivities of 0.122–0.08 K/W and 61.8–90.8 kW/m.K were measured for heat flow rates between 5 and 30 W. Uncertainties within the derived characteristics were less than 3.2 % for low heat flow rates (5–15 W) and less than 1.3 % for higher heat flow rates (15–30 W). Moreover, a 450 % reduction in characterization time was achieved compared to methods that employ thermally massive heat flow meters. In many aspects, the presented method achieved superior performance compared to those presented within the literature, proving its effectiveness for the accurate characterization and subsequent implementation of heat pipes into electronic systems. [ABSTRACT FROM AUTHOR]

• An embedded jet-enhanced HU-type manifold microchannel heat sink is presented. • The impact of the plenum and the manifold channel inlet profile is investigated. • Cooling 2000 W cm−2 of heat with 40 kPa pressure drop and 65 K temperature rise. To efficiently remove the increasingly higher heat fluxes from electronic chips, this numerical study presents an embedded single-phase water-cooled heat sink which consists of a microchannel layer and a jet-enhanced HU-type manifold layer. The manifold features two liquid inlet plenums symmetrically positioned on two opposite sides and some outlets arranged in the middle. In order to reduce the average thermal resistance, the fluid impinging velocity is boosted by narrowing the manifold channel inlet. However, narrower inlet also leads to flow maldistribution which increases the maximum thermal resistance. To address this problem, we demonstrate two approaches: either adding a flat plate at the manifold outlet or adjusting the manifold channel walls to form a tapered pathway. Further, the impact of the plenum and manifold channel inlet profiles on fluid flow and heat transfer is investigated. Negligible difference is found between rectangular and trapezoidal plenums. As to the manifold channel inlet, although similar heat transfer performance is obtained with different shapes, the total pressure drop can be substantially different. To reduce the large pressure drop at the inlet, a semicircular or triangular inlet profile is preferred, instead of a rectangular one. Quantitatively, we show that for a 3 × 3 mm2 heated area, our heat sink can dissipate fluxes up to 2000 W cm−2 with a maximum temperature rise of 65 K and a pressure drop below 40 kPa, which represents one of the highest performances and energy efficiencies reported to date. [ABSTRACT FROM AUTHOR]

• Multi-channel heat pipe is experimentally studied with acetone as working fluid. • Optimum liquid reservoir L res = 5 mm and fluid volume of 2.5 ml dissipated 90 W. • 35.4 % enhancement in heat dissipation rate noted for axial face (α a) condition. • 24.5 % reduction in temperature fluctuation noted for lateral side inclination (α l). • Antigravity heat transport capability noted up to 15 W at α a = −15o. The heat transfer characteristics of a miniatured flat heat pipe (MFHP) with multi-channels, featuring a port diameter of 1.18 mm, is investigated experimentally. Various operating parameters are considered, including the working fluid volume (V f = 1.5, 2.5, and 3.5 ml), length of the liquid reservoir (L res = No reservoir, 5, and 10 mm), orientation such as axial face (α a) or lateral side (α l), inclination angles (α = −15 to 90o), and cooling water flow rates (ṁ i = 10, 15, and 20 LPH). Based on the experiments, the optimal values for the working fluid volume, reservoir length, and flow rate are determined as V f = 2.5 ml, L res = 5 mm, and ṁ i = 20 LPH, respectively. Further analysis reveals that, the heat dissipation rate for the axial face is significantly higher than that of the lateral side, with an average percentage increase of 35.4 %. However, the lateral side outperforms the axial face in terms of stabilizing the evaporator wall temperature, reducing fluctuations by an average of 24.5 %. Moreover, the presence of multi-channels allows the MFHP in axial face orientation to dissipate a maximum heat load of 15 W against gravity at an inclination angle of α a = −15o. Finally, the variations in MFHP operation based on the orientation and its underlying physical mechanisms that contribute to enhancing heat transfer are discussed. [ABSTRACT FROM AUTHOR]

• Development of boehmite, etched aluminum, and copper oxide structured tubes for refrigerant pool boiling enhancement. • Pool boiling experimental facility for heat transfer coefficient characterization on tubes. • Enhancements demonstrated for R-134a, R-1336mzz(E) and R-1336mzz(Z) refrigerants. • Heat transfer coefficient enhancement of 250% on etched aluminum tube with R-134a. • Not all structures enhance boiling. Structure length scale and refrigerant properties affect pool boiling performance. Pool boiling of refrigerants is a primary heat transfer mode in flooded evaporators, as well as a promising solution in electronics cooling applications. In this work, the pool boiling performance of R-134a, R-1336mzz(E), and R-1336mzz(Z) on the external side of round tubes are reported and compared to Cooper's heat transfer coefficient (HTC) model. Modification of Cooper's correlation is found to be necessary for R-1336mzz(Z) at low reduced pressure. Micro- and nanostructured tubes are tested using the three different refrigerants and are shown to enhance the boiling HTC. Specifically, three various structures consisting of: boehmite (AlO(OH)) nanostructures, etched aluminum microstructures, and copper oxide (CuO) micro-nanostructures are fabricated on the external sides of aluminum and copper tubes. The surface structures have characteristic length scales of 100 nm, 1 µm, and 10 µm for boehmite, CuO, and etched aluminum, respectively. While the boiling HTC is enhanced up to 250% (compared to smooth tubes) using R-134a with etched aluminum, it shows a 15% decrease in HTC when boehmite structures are used. For CuO, higher heat fluxes showed a 25% HTC enhancement, while lower heat fluxes showed negligible effects when compared to smooth copper tubing. Different refrigerants showed various relations between structure and HTC enhancements. To take account of both structure length scale and refrigerant properties, we introduce a normalized structure size (R n) and conclude that HTC enhancement ratio increases non-linearly with R n. Our study not only provides valuable data on refrigerant pool boiling of recently developed low global warming potential and ozone depleting potential fluids, it also provides valuable design guidelines for the development and application of micro- and nanostructured surfaces in chiller and electronics cooling applications. [ABSTRACT FROM AUTHOR]

We propose and analyze a novel two-phase microchannel cooling device that incorporates perforated side walls for potential use as an embedded thermal management solution for high heat flux semiconductor devices. A dense array of perforated side walls separate alternating liquid and vapor microchannels, allowing the vapor generated through evaporation of liquid supplied through micro-perforations to flow only in the dedicated vapor channels. By separating the liquid and vapor flows, these "perspiring" side walls enable us to circumvent flow instabilities and other challenges associated with conventional two-phase microchannel cooling while at the same time effectively take advantage of the large extended surface areas available in high-aspect-ratio microchannels. One implementation of our design is parametrically analyzed using finite element modeling, demonstrating the potential of our proposed device for handling high heat flux electronic and optoelectronic semiconductor devices. © 2013 Elsevier Ltd. All rights reserved.

Cooling electronic chips to satisfy the ever-increasing heat transfer demands of the electronics industry is a perpetual challenge. One approach to addressing this is through improving the heat rejection ability of air-cooled heat sinks, and nonlocal thermal-fluid-solid modeling based on volume averaging theory (VAT) has allowed for significant strides in this effort. A number of optimization methods for heat sink designers who model heat sinks with VAT can be envisioned due to VAT's singular ability to rapidly provide solutions, when compared to computational fluid dynamics (CFD) approaches. The particle swarm optimization (PSO) method appears to be an attractive multiparameter heat transfer device optimization tool; however, it has received very little attention in this field compared to its older population-based optimizer cousin, the genetic algorithm (GA). The PSO method is employed here to optimize smooth and scale-roughened straight-fin heat sinks modeled with VAT by minimizing heat sink thermal resistance for a specified pumping power. A new numerical design tool incorporates the PSO method with a VAT-based heat sink solver. Optimal designs are obtained with this new tool for both types of heat sinks, the performances of the heat sink types are compared, the performance of the PSO method is discussed with reference to the GA method, and it is observed that this new method yields optimal designs much quicker than traditional approaches. This study demonstrates, for the first time, the effectiveness of combining a VAT-based nonlocal thermal-fluid-solid model with population-based optimization methods, such as PSO, to design heat sinks for electronics cooling applications. The VAT-based nonlocal modeling method provides heat sink design capabilities, in terms of solution speed and model rigor, that existing modeling methods do not match. © 2014 by ASME.

We propose and analyze a novel two-phase microchannel cooling device that incorporates perforated side walls for potential use as an embedded thermal management solution for high heat flux semiconductor devices. A dense array of perforated side walls separate alternating liquid and vapor microchannels, allowing the vapor generated through evaporation of liquid supplied through micro-perforations to flow only in the dedicated vapor channels. By separating the liquid and vapor flows, these "perspiring" side walls enable us to circumvent flow instabilities and other challenges associated with conventional two-phase microchannel cooling while at the same time effectively take advantage of the large extended surface areas available in high-aspect-ratio microchannels. One implementation of our design is parametrically analyzed using finite element modeling, demonstrating the potential of our proposed device for handling high heat flux electronic and optoelectronic semiconductor devices. © 2013 Elsevier Ltd. All rights reserved.

With the development of miniaturization and integration of electronic devices, the conventional manifold microchannels (MMCs) structure has been unable to meet the heat dissipation requirements caused by the rapid growth of internal heat flux. There is an urgent need to design a new heat dissipation structure with higher heat dissipation capacity to ensure the working stability and life of electronic devices. In this paper, we designed a novel manifold dual-microchannel (MDMC) cooling system that embedded the microchannel structure into the manifold microchannel structure. The MDMC not only has good heat dissipation performance that can meet the development needs of electronic equipment to miniaturization and integration, but also has a compact structure that does not increase the overall thickness and volume compared with MMC. The high temperature uniformity and heat transfer performance of MDMC are significantly improved compared to MMC. The Tmax is reduced by 13.6% and 17.5% at the heat flux density of 300 W/cm2 and 700 W/cm2, respectively. In addition, the influence of the inlet−2 velocity and the total microchannels number on the heat transfer performance of the MDMC structure are numerically investigated. The results show that the decrease rate of Tmax and ΔT is about 6.69% and 16% with the increase of inlet−2 velocity from 1.2 m/s to 2.4 m/s and microchannels number from 10 to 48, respectively. At the same time, the best temperature uniformity is obtained when the number of microchannels is 16.

In this study, experimental tests have been carried out on the coupling thermoelectric cooling module with minichannel heatsink subjected to impinging airflow for cooling desktop central processing unit (CPU). A controlled thermoelectric-forced test system was designed for this purpose. This was designed using electronic Arduino card. The proposed hybrid cooling system was compared with the conventional forced air-cooling technique. Three power of heat source (CPU) were adopted, investigated, and compared, namely 60, 87, and 95 W. Performance of controlled thermoelectric cooling with three preset temperature were experimentally examined. The effects of air velocity and thermoelectric input current on the case temperature (Tcase), thermal resistance, and heat transfer coefficient were analyzed. Results showed that the Tcase increases with the increase of its input power. In addition, increasing air jet velocity and thermoelectric input current improve CPU cooling significantly. For a CPU power of 95 W, the recorded Tcase temperature was 57°C with the conventional system. While it was maintained below 50°C in the hybrid system. The thermoelectric cooler has had a major effect on CPU cooling, having 15% improvement over conventional forced air-cooling. However, this was accompanied by an increase in energy consumption in the range of 45 W. [ABSTRACT FROM AUTHOR]

Data center proliferation has been increasing significantly around the world attributed to growth in technologies such as the Internet of Things (IoT), bitcoin mining, and high-performance computing (HPC). A direct consequence of these developments is an enhancement in processing units and a corresponding rise in CPU and GPU power densities. Limitations of air cooling to dissipate increasing power densities in processors have compelled the researchers to move toward better and efficient liquid cooling solutions. In an earlier study, a custom-made mini-rack with liquid-cooled 2OU (open rack Unit) web servers were tested for comparison of centralized and distributed pumping with constant flow rates at the server level. The effect of higher inlet temperature in terms of IT power, cooling power consumption, and CPU temperature was reported along with a comparison of centralized versus distributed pumping. In this article, the same 2OU server is used to show the effect of variable flow rate on server thermal performance. The parameters monitored for performance quantification are the core temperatures, dual in-line memory modules (DIMM) temperatures, and platform controller hub (PCH) temperature. These parameters were reported by varying the CPU power consumption, coolant inlet temperatures, and coolant flow rates by controlling the distributed pumps. The server was fully enclosed with no outside air intake where the CPUs were cooled by cold plates and the rest of the components with internally recirculating air. The results obtained from the experiments were compared with the results obtained from a previous study where the effect of the variable flow rate was ignored. A full-factorial design of experiments (DoE) was designed using Minitab 19 to analyze the results statistically. [ABSTRACT FROM AUTHOR]

Compared to traditional continuous jets, synthetic jets (jets with oscillatory flow such that the time-average velocity is zero) have specific advantages, such as lower power requirement, simpler structure and the ability to produce an unsteady turbulent flow that is known to be effective in augmenting heat transfer. This study presents experimental and computational results that document heat transfer coefficients associated with impinging a synthetic jet flow onto the tip region of a longitudinal fin used in an electronics cooling system. The effects of different parameters, such as amplitude and frequency of diaphragm movement and jet-to-cooled-surface spacing, are recorded. The computational results show a good match with experimental results. In the experiments, an actual-scale (1 mm jet orifice) system is introduced and, for finer spatial resolution and improved control over geometric and operational conditions, a large-scale mock-up (44 mm jet orifice) is applied in a dynamically-similar way, then tested. Results of the experiments at the two scales, combined with the computational results, describe fin heat transfer coefficients on and near the jet impingement stagnation point. A linear relationship for heat transfer coefficient versus frequency of diaphragm movement is shown. Heat transfer coefficient values as high as 650 W/m2K are obtained with high-frequency diaphragm movement. Cases with different orifice shapes show how jet impingement cooling performance changes with orifice shape. [ABSTRACT FROM AUTHOR]

The present study elucidates the design and experimentation of a minichannel evaporator in an R134a vapour compression refrigeration system for electronics cooling applications. In the current study, a calculation module was developed to design a minichannel evaporator to keep the surface temperature of the chip below a certain value for reliable operation conditions in electronic cooling applications. In the calculation module, the conventional-scale heat transfer correlation was used to predict the surface temperature of the chip. On the other hand, the conventional-scale and microscale pressure drop correlations were tested to assess the pressure drop in the minichannel evaporator. The proposed calculation module was verified using experimental tests for different heat loads. It was found that the proposed calculation model predicted very well the experimental data of the surface temperature of the chip for all heat input. The calculation module with micro-scale pressure drop correlation predicted well the experimental pressure drop data in the minichannel evaporator for all heat loads. Moreover, the effects of the degree of subcooling, superheating degree and condensation temperature on the surface temperature of the chip and pressure drop in the minichannel evaporator were investigated to determine optimum operating conditions at different cooling capacities using the calculation module. The results showed that the increase in the degree of subcooling enhances the performance of the minichannel evaporator. On the other hand, the lower degree of superheating and condensation temperature yielded better performance for the minichannel evaporator. The feasibility of the results for electronic cooling applications is discussed based on the findings. [ABSTRACT FROM AUTHOR]

The ever-increasing computational power of the electronic devices accompanies a greater power dissipation. Understanding and optimization of the heat rejection systems are thus vital to ensure effective thermal management of electronic devices in an often highly confined space. This paper reports the design, development, and results of an experimental investigation of a fast responding and passive heat-rejecting copper–water-based miniature loop heat pipe (mLHP) system. The designed mLHP has a square flat-faced evaporator of 20 mm length with opposite replenishment, four extended surfaces with vapor space, a liquid reservoir or compensation chamber, fin- and tube-type condenser, and different diameter transport lines. Capillary pumping action was ensured by a composite of 100PPI copper mesh with 37% porosity and absorbent wool of 20 µm average pore size. Wick hydration was safeguarded by a 67% filling ratio of mLHP at a cold state. A decrease in thermal resistance and an increase in heat transfer coefficient for evaporation were observed. The thermal resistance of the evaporator and mLHP was 0.109–0.396 °C/W and 0.132–0.644 °C/W with an uncertainty of 3.749% and 3.744%, respectively. The heat transfer coefficient for evaporation achieved during experimentation was 6.313–23.00 kW/m2K with 0.345% uncertainty. No reverse flow of vapors and evaporator dry-out was detected. The maximum temperature sustained was 108 °C against the 205 W heat load with a 4.5-min start-up duration. These results reveal the suitability of the designed miniature loop heat pipe for the heat dissipation of electronics. [ABSTRACT FROM AUTHOR]

The cooling or thermal management issues are facing critical challenges with the continuous miniaturization and rapid increase of heat flux of electronic devices. Significant efforts have been made to develop high-efficient cooling and flexible thermal management solutions and corresponding design tools. This article reviews the latest progress and the state-of-the-art in electronic cooling, which could help inspire future research. The commonly used methods in electronic cooling, classified into direct and indirect cooling, are reviewed and discussed in detail. Direct cooling consists of air cooling, spray and jet impingement cooling, immersion cooling, and droplet electrowetting. As for indirect cooling, the most popular and hot topics of using microchannel, heat pipe, vapour chamber, thermoelectric, and PCM are overviewed. The effectiveness of the thermal management methods for different-level requirements of electronic cooling and the ways how heat transfer capability can be improved are also introduced in detail. Meanwhile, the pros and cons of these thermal management methods are discussed based on their inherent heat transfer performances/characteristics, optimisation methods, and relevant applications. In addition, the current challenges of electronic cooling and thermal management technologies are explored, along with the outlook of possible future advances.

The thermal management of electronics is essential, since their lifetime and reliability are highly dependent on their operating temperature and temperature uniformity. Regarding that, Reciprocating-Mechanism Driven Heat Loop (RMDHL) technology has been invented and shows potentiality to become an effective high heat flux cooling system. In this paper, the performance of a reciprocating cooling loop, in terms of heat transfer and temperature distribution, is studied experimentally and analytically. The experimental results showed that, as the reciprocating flow amplitude increases, the loop surface temperature decreases, and the temperature uniformity along the loop improves. However, in contrast to the amplitude effect, a higher frequency may not necessarily improve the temperature uniformity, although the condenser section temperature may be lower. Further, adiabatic section temperature appears to be insensitive to the reciprocating frequency. The experimental results were then summarized in a semi-empirical correlation that demonstrates a useful design tool for the thermal engineer community. Additionally, the analytical results provide critical design requirements that should be considered during Reciprocating-Mechanism Driven Heat Loop (RMDHL) system design.

The present numerical study uses finite volume formulation to investigate the thermal performance of cylindrical heat sinks with different minichannel profiles and working fluids, including nanofluid and pure water. The numerical simulations are carried out in the laminar flow regime. The considered minichannel profiles include straight, converging-diverging, zigzag, and wavy. First, the straight design and the nonstraight ones with equal amplitudes were compared based on their thermal performance, and it was observed that the wavy minichannel has an edge over the other profiles in the studied range of Reynolds numbers. Then, the wavy design was chosen and further investigated by changing the amplitude of the walls’ waveform for both working fluids. It was found that larger wave amplitudes raise the pressure drop and the heat transfer coefficient; however, the thermal performance parameter, which compares the relative importance of these two parameters, still follows an increasing trend. For the wavy minichannel heat sink, the thermal performance factor can increase up to 17% when the waviness is increased from 0.25 to 0.75 mm. Finally, after fitting the obtained numerical data, novel predictive correlations are derived based on different thermal and hydrodynamic parameters of the wavy heat sinks. This is achieved by the optimization of independent variables using a genetic algorithm. [ABSTRACT FROM AUTHOR]

Thermal designs for microchannel heat sinks with laminar flow are conducted numerically by combining constructal theory and entransy theory. Three types of 3-D circular disc heat sink models, i.e. without collection microchannels, with center collection microchannels, and with edge collection microchannels, are established respectively. Compared with the entransy equivalent thermal resistances of circular disc heat sink without collection microchannels and circular disc heat sink with edge collection microchannels, that of circular disc heat sink with center collection microchannels is the minimum, so the overall heat transfer performance of circular disc heat sink with center collection microchannels has obvious advantages. Furthermore, the effects of microchannel branch number on maximum thermal resistance and entransy equivalent thermal resistance of circular disc heat sink with center collection microchannels are investigated under different mass flow rates and heat fluxes. With the mass flow rate increasing, both the maximum thermal resistances and the entransy equivalent thermal resistances of heat sinks with respective fixed microchannel branch number all gradually decrease. With the heat flux increasing, the maximum thermal resistances and the entransy equivalent thermal resistances of heat sinks with respective fixed microchannel branch number remain almost unchanged. With the same mass flow rate and heat flux, the larger the microchannel branch number, the smaller the maximum thermal resistance. While the optimal microchannel branch number corresponding to minimum entransy equivalent thermal resistance is 6. [ABSTRACT FROM AUTHOR]

Natural convection inside a rectangular enclosure is investigated experimentally and numerically. One of the sidewalls is heated discretely by two flush-mounted heat sources. The other sidewall is kept at a constant temperature, while the horizontal walls are unheated. Heat dissipation rates of the heat sources are equal to each other. The aspect ratio of the enclosure (AR) is 2 and the working fluid is air (Pr=0.71). The study is focused on the validation of the two and three dimensional computations under real test conditions against experiments for various modified Rayleigh number values. Experimental study is performed for various modified Rayleigh numbers in the range of 7.7x105 and 3.1x106 while numerical part covers the values between 104 and 5x106. Temperature measurements and flow visualization studies are performed in the experimental work, and streamlines, isotherms and heatlines are presented in the numerical part of the study. From the experimental and numerical studies, it is shown that two dimensional computations reflects the general characteristics of the problem, conduction and radiation heat transfer are not negligible, surface temperatures increase with the modified Rayleigh number and heatline approach is an important tool to analyze convective heat transfer.

Experiments were performed to investigate the transient cooling characteristics of intermittent spray cooling (ISC) in single-phase and nucleate-boiling regimes. The consumption of residual liquid in the non-injection period was confirmed via observations of the dynamic behavior through visualization experiments. The time variation of the surface heat transfer is relatively small in the single-phase regime and minimal in the initial nucleate-boiling regime. Further, ISC was considered as a process where the transient mass flow rate was specified by the characteristic time. This idea was successfully evaluated in the prediction of the critical heat flux in cryogen spray cooling. Abbreviations: CHF: critical heat flux; CSC: cryogen spray cooling; DC: duty cycle; ISC: intermittent spray cooling; SFSM: sequential function specification method [ABSTRACT FROM AUTHOR]

Heat is an inevitable by-product of every electronic device and thermal management is very much essential for them to improve reliability and to prevent premature failure. Liquid coolants are one of the best switching options to achieve higher cooling rates. However, by adopting them in cooling systems such as microchannel heat sinks in which flow enters in to and out of the system may cause leakage of the coolant, which results in the complete firing of the electronic device. Hence, in the present study, this problem is avoided by adopting lid-driven square cavity as a cooling system. A 2-D numerical study has been carried out to obtain the convective heat transfer efficiency (CHTE) of gold and carbon nanofluid (NF) in a lid-driven square cavity over silicon solid block under the influence of a variety of magnetic fields. The geometrical domain consists of a square cavity containing NF that is driven by means of lid moving in one direction. This circulating NF will extract an enormous amount of heat from the solid block underneath the cavity resulting in conjugate heat transfer. A finite volume method-based conjugate homogeneous heat transfer model has been developed, validated, and used in the present study. The heat efficient Gold (Au) and single-walled carbon nanotube (SWCNT) NF pairs obtained by Nimmagadda and Venkatasubbaiah are used in the present study. Moreover, efficiently varied magnetic fields identified by Nimmagadda et al. are also implemented and compared with the uniform magnetic field. Furthermore, the magnetic field is applied over the geometrical domain along the two axial directions separately and the effective CHTE is obtained. The significant impact of broad parameters, such as magnetic field type, location and strength, nanoparticle concentration and type, and Reynolds number on the CHTE, is systematically analyzed and presented. [ABSTRACT FROM AUTHOR]

In many high-performance compact air-cooling technologies, it is desirable to adopt materials that provide combined heat dissipation with noise reduction (NR) in the same device. A device that adopts such material is termed here as a “noise-reducing heat sink” (NRHS). NRHS design would involve fundamental thermal-acoustic performance tradeoffs, thus necessitating investigation of their combined thermal-acoustic performance. In this article, the thermal-acoustic performance of NRHS is evaluated by determining their thermal-acoustic index (TAI) as a function of airflow rates (Mach number < 0.1). This index is determined by combining thermal resistance as the basis for thermal performance characterization and NR improvement to characterize the acoustic performance. Conventional parallel-plate heat sinks/slit-based geometries, stochastic metal foams, and a designed periodic foam structure are compared as NRHS candidates. The results show that, for the same material volume fraction, stochastic metal foam heat sinks have better combined thermal-acoustic performance compared with a conventional parallel-plate heat sink. A designed periodic foam NRHS is shown to have significantly improved TAI, thus leading the way for NRHS design in compact cooling technologies. For the best combined thermal and acoustic performance, laminar flow is identified as a suitable operational flow regime. [ABSTRACT FROM AUTHOR]

In this article, we describe a new approach for the compact and energy-efficient cooling of converters where multiple miniaturized microfluidic cold plates are attached to transistors providing local heat extraction. The high pressure drop associated with microchannels was minimized by connecting these cold plates in parallel using a compact three-dimensional-printed flow distribution manifold. We present the modeling, design, fabrication, and experimental evaluation of this microfluidic cooling system and provide a design strategy for achieving energy-efficient cooling with minimized pumping power. An integrated cooling system is experimentally demonstrated on a 2.5-kW switched-capacitor dc–dc converter, cooling down 20 GaN transistors. A thermal resistance of 0.2 K/W was measured at a flow rate of 1.2 mL/s and a pressure drop of 20 mbar, enabling the cooling of a total of 300 W of losses in the converter using several milliwatt of pumping power, which can be realized with small micropumps. Experimental results show a tenfold increase in power density compared with the conventional cooling, potentially up to 30 kW/L. This proposed cooling approach offers a new way of coengineering the cooling and the electronics together to achieve more compact and efficient power converters. [ABSTRACT FROM AUTHOR]

The present study deals with computational investigation of natural convection in a porous chamber having a copper finned heat sink and a thermally producing element of finite thickness and heat conductivity. The enclosure is cooled from the external vertical and upper horizontal walls, whilst the remaining borders are considered to be thermally insulated. Newtonian and heat-conducting liquid with temperature-dependent viscosity is assumed as a working medium. Governing equations are written in non-dimensional variables «stream function—vorticity—temperature» and they are worked out by the finite difference technique. The temperature distribution, streamlines and the dependences of the integral characteristics on the governing characteristics, such as the Darcy and Ostrogradsky numbers and geometric characteristics of the radiator have been illustrated. Presented results can be used for the development of passive cooling systems. [ABSTRACT FROM AUTHOR]

Passive two-phase cooling systems are an efficient and viable alternative to conventional air-cooling schemes for thermal management of electronics due to their capability of dissipating higher power densities with lower power consumption and noise levels. This article aims to assess the performance of a novel two-phase air-cooled thermosyphon for cooling shelf-level telecommunications equipment that is designed with 18 line cards. In particular, the main objective is to investigate the effect of the condenser technology on the thermal-hydraulic performance of the cooling system. The thermosyphon consists of a multimicrochannel evaporator with 18 individual microchannel zones (one per card) connected to an air-cooled condenser via riser and downcomer tubes. The total height of the loop, from midevaporator to midcondenser, is approximately 50 cm. Two different condenser designs have been tested: a single-pass louvered-fin flat-tube condenser, and a multipass wavy-fin, circular-tube condenser. An extensive experimental campaign is performed to evaluate the performance of these two air-cooled condensers in a front-to-back airflow configuration. The experiments are carried out with R134a as the working fluid, varying filling ratios from 45% to 60%, and heat loads from 102 to 1023 W under uniform and nonuniform conditions. The influence of the air-side fan configuration is also investigated by varying the fans speed from 10 548 to 15 480 min−1 and by increasing the fan-tray height from 65 to 103 mm. The results demonstrate that the single-pass louvered-fin flat-tube condenser provides lower liquid-side frictional pressure drops and consequently higher refrigerant mass flow rate in the loop. This leads to a higher critical heat flux value, which is ideal for increasing power densities and limiting dry-out occurrence. On the other hand, the multiple-pass wavy-fin, condenser offers lower thermal resistances over a larger range of dissipated heat loads. This is mainly attributed to its lower air-side pressure drop. [ABSTRACT FROM AUTHOR]

Heat pipes have applications in several areas of thermal engineering, including electronic components to spacecraft thrust chamber exchangers. Although they can transfer large amounts of heat over a spatially restricted interval, they exhibit several operating limitations. The properties of the working fluid, the capillary structure, the orientation of the pipe, the length and the diameter of the pipe, are parameters that affect these limits. The present study aims to numerically evaluate the heat transfer capacity of a heat pipe. A one-dimensional model is developed considering water, ammonia and sodium as working fluids. The capillary structure is taken to be a wire mesh wick or a square groove, for which several operating thermal ranges are investigated. A comparative study for different properties of heat pipes has been performed using the Matlab® software. [ABSTRACT FROM AUTHOR]

Summary: The rapid improvements in electronic devices have led to a high demand for effective cooling techniques. The purpose of this study was to investigate the heat transfer characteristics and performance of different aluminum heat sinks filled with aluminum foam for an Intel core i7 processor. The aluminum foam heat sinks were subjected to water flow covering the non‐Darcy flow regime (300‐600 Reynolds numbers). The bottom side of the heat sinks was heated with a heat flux between 8.5 and 13.8 W/cm2. Three different heat sinks were examined in this study. Models A, B, and C contained two, three and four channels, respectively. Each channel gap was filled with ERG aluminum foam. The distributions of the local surface temperature and the local Nusselt number were measured for each heat sink design. The experimental data were compared with the numerical results. The average Nusselt number was obtained for the range of Reynolds numbers, and an empirical correlation of the average Nusselt number as a function of the Reynolds number was derived for each heat sink. The pressure drop across the characteristics of each heat sink design was measured. The thermal performance of each aluminum foam heat sink was evaluated based on the average Nusselt number and the required pumping power. The experimental results revealed that model B achieved the highest average Nusselt number compared with models A and C. However, model C had the highest surface to volume ratio; the thermal boundary layers, which are formed on adjacent fin surfaces inside the aluminum foam, interface with each other causing a reduction in the overall heat transfer. The numerical results were in good agreement with experimental data of local Nusselt number and local temperature with maximum relative errors of 2% and 1%, respectively. [ABSTRACT FROM AUTHOR]

As part of the research in the field of thermal control of electronic components, we confined a phase change material (plastic paraffin) in a liquid and we heated it on one vertical side by a hot plate. The presence of the liquid around the phase change material (PCM) prevents the direct contact between the hot plate and the PCM (increases the lifetime of the PCM by reducing overheating zones). It improves heat transfer by increasing the thermal conductivity around the PCM (increases the thermal exchange surface) and by accelerating the convective transfer (it starts very quickly without waiting for the beginning of melting). In this work, we examine experimentally and numerically the effect of the choice of liquid on the PCM and on the heating plate. In this paper, water and Siloil M40.165/200.10 are employed around the phase change materials and a comparative study of some important results examining the melt front, total melting time, flow direction and thermal behavior is conducted to investigate which liquid is the most optimal to use. Finally, it has been found that the thermal properties of water (especially heat capacity) seem to be the most interesting for a thermal protection role. [ABSTRACT FROM AUTHOR]

A heating area of 190 mm × 90 mm large flat-plate loop heat pipe was designed for the heat dissipation problem of multi-heat sources. The design process was also briefly introduced. The evaporator was made of aluminum alloy, and heat dissipation fins were arranged on the back side of the compensation chamber to enhance the heat transfer between the compensation and the ambient. The stainless steel wire mesh worked as the porous wick, and the acetone was chosen as the working fluid. Six ceramics heating blocks were used as the heat sources. The results showed that the system could start up and work normally between 20 W–140 W, and maintained the heating surface temperature below 90 °C. The system behaved as a zigzag start below 20 W, and the condenser inlet temperature oscillated periodically, and the system could start up stably between 25 W and 140 W. When the heat load was increased, there occurred periodic temperature fluctuation in condenser outlet. The system could establish a new balance quickly during variable heat loads operation, which reflected the good reliability of the LHP. The experiment of changing the heat dissipation condition on the condenser side and the evaporator side was carried out. When the heat load was 120 W and the ambient temperature was constant, the system equilibrium temperature difference caused by the air ventilation of the condenser was changed, which was less than the heat dissipation of the evaporator under the same conditions. The evaporator thermal resistance decreased with the increase in heat load, and the minimum thermal resistance of 0.032 °C/W was achieved at the heat load of 120 W. The total thermal resistance of the LHP was distributed between 0.312 °C/W and 0.212 °C/W. It was also pointed out that it was very important to improve the thermal uniformity of the heated surface of a large-plane loop heat pipe system with multiple heat sources. • A heating area of 190 mm × 90 mm large flat-plate loop heat pipe was designed. • The LHP was tested for the heat dissipation problem under multi-heat sources. • LHP can start up and work normally between 20 and 140 W with surface temperature below 90 °C. [ABSTRACT FROM AUTHOR]

In this study, electronic devices are experimentally examined to improve the thermal performance of the plate fin heat sink. It is performed on the basis of the paraffin wax used as a phase change material (PCM) filled in a heat sink plate. The aim of the study is to select the most efficient SiO2 volume fraction in paraffin wax to be filled in a heat sink with a plate finish. The SiO2 is considered to be a nanoparticle and 1%, 3%, and 5% of SiO2 volume is selected for the preparation of nanoPCM. At the base of the plate fin heat sink, a constant heat source is applied. A plate fin heat sink is selected to quantify the effect of nanoPCM as a reference heat sink. The effect of thermal performance of heat sinks was examined using a different volume fraction of nanoPCM. The thermal performance comparison is carried out with and without PCM for the plate heat sink of Reynolds number 4000- 16000. In order to find the effect of PCM and variable Reynolds number, the investigation of the plate fin heat sink is examined. The results showed that the inclusion of PCM (paraffin wax and SiO2-based nanoPCM) in the heat sink plate provides better cooling performance and keeps the desired temperature. The results show that the heat sink based on PCM enriched with nanoparticles provided better thermal performance compared to the heat sink with a simple PCM. The thermal performance of the SiO2 based nanoPCM 3% volume fraction is better than 1% volume fraction and the heat sinks of the plate finish. [ABSTRACT FROM AUTHOR]

The impact of a capped diamond layer for enhanced cooling of multifinger AlGaN/GaN high-electron-mobility transistors (HEMTs) has been investigated under the steady-state operating condition. By depositing a capped diamond thin film onto the HEMTs, the temperature distribution around the hot spots tends to be more uniform and the junction temperature can be suppressed significantly. The capped diamond serves as a highly effective heat spreader, and its thermal spreading ability depends on the structural design patterns and working conditions. Some key parameters affecting the thermal performance of the capped diamond have been examined, including the heat dissipation power density, gate pitch distance, embedding depth of the heat source, thermal boundary resistance, substrate material, as well as the cap thickness. For the 12-finger model with 20-μm gate pitch distance and gate power density of 6 W/mm, a 20-μm layer of capped diamond could reduce the junction temperature by 12.1% for GaN-on-diamond HEMTs and by 25.3% for GaN-on-SiC HEMTs. Even with a 1-μm capped diamond layer, the reduction would be 7.6% and 9.9%, respectively. The temperature reduction for GaN-on-Si is more significant. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional numerical investigation on the thermohydrodynamic performance of a recently proposed recharging microchannel (RMC) is carried out. In this design, a straight microchannel is split into more than one smaller length channels (having individual inlet and outlet) placed end to end. This design enhances overall heat transfer and maintains temperature uniformity across the substrate length. The comparison of fluid flow and heat transfer performance of RMC, interrupted microchannel (IMC) and straight microchannel (SMC) with the same hydraulic diameter and substrate length are presented to explore the effect of geometrical configuration on heat transfer enhancement. The parametric variations include the number of channels (n), transverse wall length (Ltw), channel aspect ratio (α), and flow Reynolds number. The results reveal that recharging microchannel shows better thermal performance compared to simple and interrupted microchannel with a maximum performance factor of 1.80. The results also indicate that the performance factor of RMC increases with an increase in the number of small channels, transverse wall length, and channel aspect ratio. The outcome of this study indicates the possible use of recharging microchannel heat sinks for high heat flux removal applications such as electronic cooling. [ABSTRACT FROM AUTHOR]

Air Impinging Jet is an effective cooling method for various applications, ensuring optimal performance and reliability of heat-generation systems. In the present work, a novel technique of cooling discrete protruding heat sources mounted in a rectangular channel by using a double air jet is introduced experimentally. A constant and uniform heat flux is produced from protruding heat source surfaces where the bottom and the upper channel walls are insulated. The effects of various parameters on the heat source cooling performance such as the Nusselt number for single and double air jets are obtained. Various parameters such as the air Reynolds number, aspect ratio, double jet position ratio, and jet inclination angle are investigated. The results show that those parameters effectively affect the average temperatures and the standard deviation of all protruding heat sources. It is indicated that the maximum value of the average Nu number occurs at the heat source just underneath the impinging air jet compared to other downstream heat sources. The highest average Nu is achieved at a position ratio of 3/7 and an inclination angle of 10o. The cooling system accomplishes an acceptable Nu and temperature uniform distribution for a position ratio of 4/7 compared to the other position ratios. The improved performance and longevity of heat sources are achieved by the innovative double-air jet method, which archives a consistent temperature distribution for all projecting heat sources. [ABSTRACT FROM AUTHOR]

This paper examines the performance of a heat sink that combines an evaporator and an expansion device into a single component within a compact vapor compression refrigeration system designed for applications with high heat fluxes. The liquid refrigerant at high pressure expands through orifices, generating a spray that impacts a copper heater. The heated surface is enhanced with laser-micromachined micropillars featuring a square cross-section to improve the thermal conductance of the boiling liquid film. Qualitative evaluations of the enhanced surfaces using interferometry analyses determined the heat transfer surface area and assessed the effectiveness of the fabrication technique. The experiments involved using R-134a, a two-orifice expansion device, and enhanced surfaces with pillar edge sizes ranging from 200 to 600 μ m. This analysis encompasses the cooling system's thermodynamic performance, including compressor power and coefficient of performance, as well as steady-state heat transfer parameters such as convection coefficient and critical heat flux. The system achieved a cooling capacity of 230 W (35.9 W/cm 2), with the heater surface temperature staying below 22.9 °C and the highest average heat transfer coefficient reaching as high as 55 kW/(m 2 -K). This represents a 129% increase in thermal conductance compared to mirror polished surfaces, leading to a significant reduction in the temperature of the heated surface. Additionally, the area-corrected heat transfer coefficient is computed to verify the thermal resistance reduction unrelated to the surface area enhancement, and a 60% reduction is found. Furthermore, although this heat transfer enhancement shows minor to no influence on the thermodynamic performance of the refrigeration system, the heater cooling efficiency, defined as the ratio of the enthalpy rise delivered by the heat source to its ideal maximum, increased by 13%. The findings are supported by high-speed video sequences illustrating the boiling heat transfer features. • Compact refrigerator combines evaporator and expansion device in new spray heat sink. • Laser-micromachined surfaces provide two-fold enhancement during flow boiling. • Two-orifice expansion device produced cooling capacity of 230 W (35.9 W/cm 2). • Maximum recorded average heat transfer coefficient of 55 kW/(m 2 K). • The thermal conductance increased by 129% compared to a polished surface. [ABSTRACT FROM AUTHOR]

• Reveal the cooling performance of Z-type manifold microchannel heat sink. • Trapezoidal manifold presents better thermal and hydraulic characteristics. • Dissipate an ultra-high heat flux of 1842 W/cm2 with an ultra-low pumping power. Ultra-high heat flux (up to 1000 W/cm2) thermal management has been an urgent need for advanced power electronic devices to maintain their superior electrical performance and reliability. Embedded microchannel cooling is a promising and challenging thermal management method for high-heat-flux chips. This study presents an embedded Z-type manifold microchannel (Z-MMC) cooling design for a 5 × 5 mm2 heating zone, which can dissipate an ultra-high heat flux up to 1842 W/cm2 by using water single-phase flow. The microchannel array is etched on silicon wafer, while the manifold structure is fabricated by PDMS molding. The test chips are assembled using O 2 plasma bonding technique, which provides an observation window for visualizing the flowing state of the coolant during the heat transfer process. We experimentally test and compare four test chips with different microchannel geometries and manifold designs. The average heater temperature, temperature distribution, thermal resistance, heat transfer coefficient and pressure drop are reported for mass flow rates of 2–6 g/s. The proposed Z-MMC cooler is capable of dissipating up to 1800 W/cm2 with a total thermal resistance of only 0.075 cm2·K/W (only 0.039 cm2·K/W after subtracting the conduction thermal resistance), at a flow rate of 6 g/s and a pressure drop of 54 kPa. Compared to the rectangular manifold arrangement, the trapezoidal manifold demonstrates a lower temperature rise, a more uniform temperature distribution and a smaller pressure drop at the same heat flux and flow rate, mainly due to a more uniform fluid distribution in the microchannel. Furthermore, we compare our proposed design's thermo-hydraulic performance with those of representative manifold microchannel cooling data in the literature, and the proposed Z-MMC design demonstrates relatively low thermal resistance and pressure drop values. [ABSTRACT FROM AUTHOR]