Active liquid cooling is one of the most efficient and promising strategy for extreme thermal issues. As is the power source of the active liquid cooling system, a reliable and powerful micropump is urgently needed. In this study, we numerically studied the fluid flow of a hydrodynamic levitated micropump, considering the fluid flow in the motor. We found that the load capacity of the journal bearing is not influenced by the pump fluid flow. However, the pressure distribution of the journal bearing results in the dissymmetric pressure distribution in the spiral groove bearing, leading to worse stability of the axial levitation performance. The axial suspension force is at least 1.0N with the liquid film thickness of 15μm and is sufficient for the rotor with weight of 30g to be stably levitated in the fluid. Owing to the pressure difference inside the pump, the balance point of the rotor should be lower than the theoretical design when the micropump is operating.

Fully immersion of servers in electrically nonconductive (dielectric) fluid has recently become a promising technique for minimizing cooling energy consumption in data centers. The improved thermal properties of these dielectric fluids facilitate considerable savings in both upfront and operating cost over traditional air-cooling. This technology provides an opportunity for accommodating increased power densities. It also minimizes the common operational issues of air cooling technique like overheating and temperature swing in the system, fan failures, dust, air quality, and corrosion. This paper presents various data about the thermal performance of a fully single-phase dielectric fluid immersed server over wide temperature ranges (environment temperatures) from 25°C to 55°C for prolonged periods in an environmental chamber. This work explores the effects of high temperatures on the performance of a server and other components like pump, along with potential issues associated with extreme climatic conditions. The experimental data serves as a means to determine failure criteria for the server and pump by subjecting the system to accelerated thermal aging conditions i.e. around 55°C, consequently simulating the most extreme environmental condition that the server may encounter. Connector seals are inspected for expected degradation upon temperature cycling typically at such extreme conditions. Throttling limit for the server and pump power draw for different temperatures was determined to assess pump performance. Determining the relations between component behavior and operating temperature provides an accurate measure of lifetime of a server. The scope of this paper can be expanded by reviewing the effects of low temperatures on server and component performance. Changes to various performance parameters like power draw of pump and server at the higher and the lower operating temperatures and an understanding of issues like condensation can be used to quantify upper and lower limits for pump and server performance.

The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for other applications (e.g., spacecraft and aircraft). The lack of acceptable models and correlations for orientation- and gravity-independent operation has limited the use of two-phase coolers in such applications. Previous research has revealed that gravitational acceleration plays a diminishing role in establishing flow regimes and transport rates as the channel size shrinks, but there is considerable variation among the proposed microscale criteria and limited research on two-phase flows in low aspect ratio microgap channels. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity on two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation and mass flux on near-saturated flow boiling of HFE7100 in a 1.01 mm tall by 13.0 mm wide by 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes, heat transfer coefficients, and flow regimes across evaporator orientations, was achieved for mass fluxes of 400 kg/m 2 -s and greater. The present results are compared to published criteria for achieving gravity-independence.

For improving the functionality and signal speed of electronic devices, electronic components have been miniaturized and an increasing number of elements have been packaged in the device. As a result there has been a steady rise in the amount of heat necessitated to be dissipated from the electronic device. Recently microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging, In the present study an experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel with gap depth of 0.52 mm and width of 5 mm was conducted with deionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m 2 s and imposed heat flux from 3 to 20 W/cm 2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, flow pattern and onset of nucleate boiling of subcooled flow boiling were investigated through instrumental measurements and a high speed camera. It was found that the slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, and the slope was greater for lower mass fluxes, with lower superheat required for boiling incipience. As for the visualization images, for relatively lower mass fluxes the bubbles generated were larger and not easy to depart from the vertical upward placed narrow microchannel wall, giving elongated bubbly flow and reverse backflow. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewetting occurred. Meanwhile the initiative superheat and heat flux of onset of nucleate boiling were compared with existing correlations in the literature with good agreement.

As electronic devices continue to shrink in size and increase in functionality, effective thermal management has become a critical bottleneck that hinders continued advancement. Two-phase cooling technologies are of growing interest for electronics cooling due to their high heat removal capacity and small thermal resistance (< 0.3 K-cm 2 /W) [1]. One typical example of a two-phase cooling method is droplet evaporation, which can provide a high heat transfer coefficient with low superheat. While droplet evaporation has been studied extensively and used in many practical cooling applications (e.g., spray cooling), the relevant work has been confined to spherical droplets with axisymmetric geometries. A rationally designed evaporation platform that yields asymmetric meniscus droplets can potentially achieve larger meniscus curvatures, which give rise to higher vapor concentration gradients along the contact line region and therefore yield higher evaporation rates. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on a porous micropillar structure. The equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes are explored using the Volume of Fluid (VOF) method. The evaporative mass transport at the liquid-vapor interface is modeled using a simplified Schrage model [2]. The results show highly non-uniform mass transport characteristics for asymmetrical microdroplets, where a higher local evaporation rate is observed near the locations where the meniscus has high curvature. This phenomenon is attributed to a higher local vapor concentration gradient that drives faster vapor diffusion at more curved regions, similar to a lightning rod exhibiting a strong electric field along a highly curved surface. By using contact line confinement to artificially tune the droplet into a more curved geometry, we find the total evaporation rate from a triangular-based droplet is enhanced by 13% compared to a spherical droplet with the same perimeter and liquid-vapor interfacial area. Such a finding can guide the design and optimization of geometric features to improve evaporation in high performance electronics cooling systems.

The rapid growth of the global network infrastructure has resulted in a sharp increase in the number and size of data center facilities. Total data center power consumption now represents a significant fraction of global electricity production. To conserve natural resources, and to satisfy the cooling demands of compact, powerful electronics, thermal management strategies with high heat transfer coefficients must be employed. Two-phase liquid immersion cooling is one such strategy that has been gaining momentum in commercial cooling applications over recent years. The work discussed in this paper provides information on two different flow boiling investigations performed on vertically oriented surfaces in a small form factor server model. Two different types of surfaces — bare silicon, and silicon surfaces attached with microfinned heat sinks were tested in this study. Novec 649 dielectric fluid was used as the primary working fluid. The first investigation compares the thermal performance of parallel and impinging flow distribution systems, for different subcooling and flow rate conditions. The second investigation is on nucleation suppression in flow boiling for the parallel and impinging flow distribution systems. In this study, flow rates ranging from 0 ml/min to 1650 ml/min were tested and high-speed imaging was performed to capture the change in bubble characteristics. The resulting observations, including highest heat flux values supported without nucleation activity, are reported and discussed.

This work presents the design and characterization of a two-phase, embedded manifold-microchannel (MMC) system for cooling of high heat flux electronics. The study uses a thin-Film Evaporation and Enhanced fluid Delivery System (FEEDS) MMC cooler for high heat flux cooling of electronics. The work builds upon our group’s earlier work in this area with a particular focus on the use of an improved bonding structure and implementation of uniform heat flux heaters that collectively contribute to enhanced performance of the system. In many MMC systems targeted for high heat flux applications microchannels and manifolds are fabricated separately due to different dimensions and tolerances required for each. However, assembly of the system often leaves a gap between the channels and the manifold, thus causing the working fluid to leak through the top of the microfins leading to decreased cooler performance. The effect of this gap is parametrized and analyzed with ANSYS Fluent CFD simulations and discussed in this paper. The findings show that even a few microns wide gap can cause a noticeable degradation of the MMC system performance. Imperfect assembly and the deformation of a microchannel chip due to working fluid pressure can cause gaps, indicating the necessity of uniform and hermetic bonding between the manifold and the tips of the microfins. Furthermore, this work presents the need for better heater designs to enable uniform and high heat flux to the heat transfer surface. Serpentine heaters are often used to mimic electronics in a laboratory environment, but there is a lack of study on the performance characterization of the heaters themselves. In the current work, the performance of a conventional serpentine heater is characterized using ANSYS thermo-electric modeling software. The results show that conventional serpentine heaters are insufficient at providing uniform heat flux in applications where there is a lack of heat spreading-such as in the current embedded cooler — showing deviations ranging over 200 % of the nominal value. The deviations are caused by the many bends present in a serpentine pattern where current density concentrations vary significantly. Two alternate designs are proposed, and numerical simulations show that these new heater designs are capable of providing uniform heat flux, not deviating more than 20% from the nominal heat flux value. The conventional and newly proposed heaters are fabricated, tested, and analyzed with a working FEEDS system.

In this study, we proposed a bell shape phosphor layer geometry and the corresponding dual-step phosphor coating method for enhancing the angular color uniformity (ACU) of phosphor-converted white light-emitting diodes (pcLEDs). Numerical simulation based on Volume of Fluid (VOF) model was applied to predict phosphor geometries. Based on the simulated results, experiments were conducted to realize the phosphor geometries. The simulated results show that the VOF model can predict the phosphor geometries with an acceptable geometric deviation within 5%. The experimental results show that compared with the spherical cap phosphor layer geometry, the bell shape geometry can achieve better ACU performance, an optimal bell phosphor layer geometry with equal coating volume above and around the LED chip was achieved, for the corrected color temperature (CCT) of 4000 K, the angular CCT deviation of the optimal geometry is 62 K, while it is 382 K for the spherical cap geometry.

A series of experiments was conducted to investigate the performance characteristics of a heat pipe with a hybrid wick that combined grooves and a wire screen. The heat pipe in this study was designed primarily for the cooling of high-density power electronic elements such as IGBTs, and it had tiny triangular grooves along its entire length. The container was a copper tube which had an outer diameter of 19 mm and length of 0.8 m, and the working fluid was water. To lower the thermal resistance against increased thermal loads, a higher performance was desired for the heat pipe, without changing the external dimensions. A fine mesh wire screen was partially applied to the evaporator to enhance the heat transfer performance. The hybrid wick heat pipe was tested and analyzed from the viewpoints of thermal resistance, effective thermal conductance, and operating temperature. For a 1.6 kW effective thermal load, as a typical result, the heat pipe with the hybrid wick exhibited a 70 % decrease in thermal resistance compared to that with a groove wick only. The paper includes results for various thermal loads and fluid charges. The results herein can be utilized in applications that require an intensive enhancement in heat pipe performance.

As technological advances lead to miniaturization of high power electronics, the concentration of heat generating components per area increases to the point of requiring innovative, integrated cooling solutions to maintain operational temperatures. Traditional coolant pumps have many moving parts, making them susceptible to mechanical failure and requiring periodic maintenance. Such devices are too complex to be miniaturized and embedded in small scale systems. Electrohydrodynamic (EHD) conduction pumps offer an alternative way of generating fluid flow in small scales for use in modern thermal control systems for high power electronics, both for terrestrial and aerospace applications. In EHD conduction, the interaction between an applied electrical field and the dissociation of electrolyte species in a dielectric fluid generates an accumulation of space charge near the electrodes, known as heterocharge layers. These layers apply electric body forces in the fluid, resulting in a flow in the desired direction based on the pump characteristics. EHD conduction pumps work with dielectric fluids and have simple, flexible designs with no moving parts. These pumps have very low power consumption, operate reliably for longer periods than mechanical pumps, and have the ability to operate in microgravity. EHD conduction pumps have been previously proven effective for heat transfer enhancement in multiple size scales, but were only studied in a flush ring or flush flat electrode configurations at the micro-scale. This study provides the pressure and flow rate generation performance characterization for a micro-scale pump with perforated electrodes, designed to be manufactured and assembled using innovative techniques, and incorporated into an evaporator embedded in an electronic cooling system. The performance of the pump is numerically simulated based on the fully coupled equations of the EHD conduction model, showcasing the distinctive heterocharge layer structure and subsequent force generation unique to this innovative design.

The main aim of the current paper is to demonstrate the capability of a two-phase closed thermosyphon loop system to cool down a contemporary datacenter rack, passively cooling the entire rack including its numerous servers. The effects on the performance of the entire cooling loop with respect to the server orientation, micro-evaporator design, riser and downcomer diameters, working fluid, and approach temperature difference at the condenser have been modeled and simulated. The influence of the thermosyphon height (here from 5 to 20 cm with a horizontally or vertically oriented server) on the driving force that guarantees the system operation whilst simultaneously fulfilling the critical heat flux (CHF) criterion also has been examined. In summary, the thermosyphon height was found to be the most significant design parameter. For the conditions simulated, in terms of CHF, the 10 cm-high thermosyphon was the most advantageous system design with a minimum safety factor of 1.6 relative to the imposed heat flux of 80 W cm −2 . Additionally, a case study including an overhead water-cooled heat exchanger to extract heat from the thermosyphon loop has been developed and then the entire rack cooling system evaluated in terms of cost savings, payback period, and net benefit per year. This approximate study provides a general understanding of how the datacenter cooling infrastructure directly impacts the operating budget as well as influencing the thermal/hydraulic operation, performance, and reliability of the datacenter. Finally, the study shows that the passive two-phase closed loop thermosyphon cooling system is a potentially economically sound technology to cool high heat flux servers of datacenters.

Thermal management for electric machines (motors/generators) is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. Cooling of the electric machine(s) in some electric vehicle traction drive applications is accomplished by impinging automatic transmission fluid (ATF) jets onto the machine’s copper windings. In this study, we provide the results of experiments characterizing the thermal performance of ATF jets on surfaces representative of windings, using Ford’s Mercon LV ATF. Experiments were carried out at various ATF temperatures and jet velocities to quantify the influence of these parameters on heat transfer coefficients. Fluid temperatures were varied from 50°C to 90°C to encompass potential operating temperatures within an automotive transaxle environment. The jet nozzle velocities were varied from 0.5 to 10 m/s. The experimental ATF heat transfer coefficient results provided in this report are a useful resource for understanding factors that influence the performance of ATF-based cooling systems for electric machines.

When contemplating processor module cooling, the notion of maximum cooling capability is not simple or straight forward to estimate. There are a multitude of variables and constraints to consider; some more rigid or fixed than others. This paper proposes a theoretical maximum cooling capability predicated on the treatment of the module heat sink or cold plate as a heat exchanger with infinite conductive and convective behavior. The resulting theoretical minimum heat sink thermal resistance is a function of the bulk thermal transport of the fluid dependent only on the fluid’s density, specific heat (at constant pressure) and volumetric flow rate. An ideal module internal thermal resistance will also be defined. The sum of the two resistances constitutes the theoretical minimum total module thermal resistance and defines the ideal thermal performance of the module. Finally, a module cooling effectiveness relating the actual module thermal performance to the ideal thermal performance will defined. Examples of both air and water cooled modules will be given with discussion on the relevance and utility of this methodology.

We report on experimental demonstration of multilayer molybdenum disulfide (MoS 2 ) nanomechanical resonators integrated on microchannels, with the potential for resonant operation and sensing applications in microfluidics. Microchannels with width of ∼5μm and depth of ∼1–2μm are fabricated on polydimethylsiloxane (PDMS) substrate. We transfer MoS 2 flakes with both singly-clamped cantilever and doubly-clamped membrane structure onto the PDMS channels, and measure both undriven thermomechanical resonances and optically driven responses from the devices. The devices show up to 6 thermomechanical resonances, with highest resonance frequency ( f res ) of 16.3MHz and quality ( Q ) factor ∼200. This type of MoS 2 nanomechanical resonators, when integrated with microfluidics in microchannels, would make new interesting candidates for biosensing and chemical sensing in fluids.

This study considers the optimization of a complex micro-scale cooling geometry that represents a unit-cell of a full heat sink microstructure. The configuration consists of a channel with a rectangular cross section and a hydraulic diameter of 100 μm, where the fluid flows between two cooling fins connected by rectangular crossbars (50 × 50 μm). A previous investigation showed that adding these crossbars at certain locations in the flow can increase the heat transfer in the microchannel, and in the present work we perform an optimization to determine the optimal location and number of crossbars. The optimization problem is defined using 12 discrete design parameters, which represent 12 crossbars at different locations in the channel that can either be turned off and become part of the fluid domain, or turned on and become part of the solid domain. The optimization was done using conjugate heat transfer computational fluid dynamics (CFD) simulations using Fluent 15.0. All possible 4096 configurations were simulated for one set of boundary conditions. The domain was discretized using about 1 million nodes combined for the fluid and solid domains and the computational time was around 1 CPU hour per case. The results show that further improvements in heat transfer are feasible at an optimized pressure drop. The results cover a range of pressure drops from 25 kPa to almost 90 kPa and the heat transfer coefficient varies from 60 to 120 kW/m2K. The configurations on the Pareto front show the trend that crossbars closer to the maximal fluid-solid interface result in a more optimal performance than crossbars positioned farther away. In addition to performing simulations for all possible configurations, the potential of using a genetic algorithm to identify the configurations that define the Pareto front was explored, demonstrating that a 80% reduction in computational time can be achieved. The results of this study demonstrate the significant increase in performance that can be obtained through the use of computational tools and optimization algorithms for the design of single phase cooling devices.

This paper presents a detailed approach to provide improved cooling and heat spreading in electric machine rotors using centrifugally-pumped revolving thermosiphons. Design concepts are discussed that offer the following advantages: (1) high thermal performance across a wide range of operating points; (2) low-impedance heat paths; (3) excellent opportunities for integration with electric machine design for improved electromagnetic performance and structural design, as well as practical, cost-effective manufacturing. It takes advantage of centrifugal force to provide effective inertial pumping over a wide range of operating conditions. In addition, the new thermosiphon design is compatible with existing standard electric machine manufacturing techniques and cooling needs. A condenser section fin and ramp structure provides consistently high condensation performance. Surface texture design to promote effective nucleate boiling at high speeds is discussed, and fluid fill factor is analyzed. Applications include induction and PM synchronous machines. Benefits of these thermosiphons include increased steady-state power and torque density, increased and more consistent efficiency, and reduced permanent magnet volume and cost in PM synchronous machines. Other applications may include centrifugal gas compression, chemical processes, and machine tools.

This work presents the experimental design and testing of a two-phase, embedded manifold-microchannel cooler for cooling of high flux electronics. The ultimate goal of this work is to achieve 0.025 cm 2 -K/W thermal resistance at 1 kW/cm 2 heat flux and evaporator exit vapor qualities at or exceeding 90% at less than 10% absolute pressure drop. While the ultimate goal is to obtain a working two-phase embedded cooler, the system was first tested in single-phase mode to validate system performance via comparison of experimentally measured heat transfer coefficient and pressure drop to the values predicted by CFD simulations. Upon validation, the system was tested in two phase mode using R245fa at 30°C saturation temperature and achieved in excess of 1 kW/cm 2 heat flux at 45% vapor quality. Future work will focus on increasing the exit vapor quality as well as use of SiC for the heat transfer surface upon completion of current experiments with Si.

In this paper we reported an advanced structure, the Piranha Pin Fin ( PPF ), for microchannel flow boiling. Fluid flow and heat transfer performance were evaluated in detail with HFE7000 as working fluid. Surface temperature, pressure drop, heat transfer coefficient and critical heat flux ( CHF ) were experimentally obtained and discussed. Furthermore, microchannels with different PPF geometrical configurations were investigated. At the same time, tests for different flow conditions were conducted and analyzed. It turned out that microchannel with PPF can realize high-heat flux dissipation with reasonable pressure drop. Both flow conditions and PPF configuration played important roles for both fluid flow and heat transfer performance. This study provided useful reference for further PPF design in microchannel for flow boiling.

Most methods for designing electronics cooling schemes do not offer the information on what levels of heat fluxes are maximally possible to achieve with the given material, boundary and operating conditions. Here, we offer an answer to this inverse problem posed by the question below. Given a micro pin-fin array cooling with these constraints: - given maximum allowable temperature of the material, - given inlet cooling fluid temperature, - given total pressure loss (pumping power affordable), and - given overall thickness of the entire electronic component, find out the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is completely removed by the cooling fluid. This problem was solved using multi-objective constrained optimization and metamodeling for an array of micro pin-fins with circular, airfoil and symmetric convex cross sections that is removing all the heat inputted via uniform background heat flux and by a hot spot. The goal of this effort was to identify a cooling pin-fin shape and scheme that is able to push the maximum allowable heat flux as high as possible without the maximum temperature exceeding the specified limit for the given material. Conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto frontier of best trade-off solutions. The Pareto optimized configuration indicates the maximum physically possible heat fluxes for specified material and constraints.

GaN on Diamond has been demonstrated to enable notable increases in RF power density without impacting High Electron Mobility Transistor (HEMT) peak junction temperature. However, Monolithic Microwave Integrated Circuits (MMICs) fabricated using GaN on Diamond substrates are subject to the same packaging thermal limitations as their GaN on SiC counterparts. Therefore, efforts to exploit GaN on Diamond to achieve substantial increases in MMIC power are stymied by external packaging thermal resistances that characterize the current “remote cooling” paradigm. This paper explores an intra-chip cooling alternative to the “remote cooling” paradigm, eliminating various heat spreader, heat sink and thermal interface layers in favor of integral microfluidic cooling in close proximity to the device junction. We describe an intra-chip cooling structure comprised of GaN on Diamond with integral micro-channels fed using a Si fluid distribution manifold. This structure exploits GaN on Diamond substrate technology to support increased HEMT areal power density while employing diamond microfluidics to affect scalable, low thermal resistance die-level heat removal. Thermal-electrical-mechanical co-design of integrated circuit (IC) features is performed to optimize conjugate heat transfer performance and manage the electrical and mechanical impacts associated with the presence of fluidic cooling near the electrically active region of the device. Through this, MMICs with significantly greater RF output than typical of the current state-of-the-art (SoA), dissipating die and HEMT heat fluxes in excess of 1 kW/cm 2 and 30 kW/cm 2 , respectively, can be operated with junction temperatures that support reliable operation. The modeling, simulation and micro-fabrication results presented here demonstrate the potential of diamond microfluidics-based intra-chip cooling as a means to alleviate thermal impediments to exploitation of the full electromagnetic potential of GaN.