The trade-off between efficient cooling and low power consumption is a goal that has always been very desirable in electronics cooling, especially nowadays that power densities of processing units are increasing. Conventional cooling solutions do not have the necessary cooling capacities for these power densities or require significant power consumption. In this study, a novel air-cooled thermosyphon cooling system for desktop computers is presented and experimentally tested. The thermosyphon comprises a vertical micro-channel cold plate as the evaporator and a horizontal air-cooled multiport coil as the condenser. The thermosyphon has a total height of 12 cm and operates with a fan speed of 1700 RPM. The working fluid selected for the thermosyphon loop is R1234ze(E), chosen for its advantageous thermophysical properties and nearly zero-GWP (Global Warming Potential). The test results presented in this paper aim to analyze thermosyphon’s thermal and hydraulic performance by studying the trends of thermal resistance and mass flow rate as a function of different operating conditions. The maximum heat rejection under safe conditions is 250 W, corresponding to a heat flux of about 18 W/cm 2 .

Two-phase cooling systems based on the thermosyphon operating principle exhibit excellent heat transfer performance, reliability, and flexibility, therefore can be applied to overcome thermal challenges in a wide range of electronic cooling applications and deployment scenarios. However, extremely complex nature of two-phase flow physics involving flow patterns and phase transitions has been the major challenge for technology adoption in industry. This paper demonstrates a machine learning (ML) based model for evaluating the thermal performance and refrigerant mass flow rate, of a thermosyphon cooling system for telecom equipment. Unlike conventional laboratory approach that requires numerous sensors attached to a cooling system to capture their thermal performance, the new model requires a minimum number of sensors to monitor the health of a thermal management solution. Using the proposed model, a system control module can be further developed which could identify optimal operating parameters in real-time under dynamically changing heat load conditions and actively maintain safety and thermal requirements.

Data centers housing high performance computing equipment have large and growing rack densities, which pushes the limits of traditional air cooling technologies because of limited heat transfer coefficients. Therefore, on-chip cooling using so-called cold plates is emerging as a necessary cooling option for high-density electronics. The use of mini-channels or pins fins to enhance internal heat transfer area inside cold plates requires extensive micro-machining that is relatively time consuming and expensive for mass production. As an alternative approach, inserting and bonding pre-manufactured metal foams into hollow bodies are explored as a potentially inexpensive means to enhance the interior heat transfer area of cold plates. One key aspect of the performance of metal foams in cold plates is the thermal contact resistance in the bonding between the foam and the substrate. This project predicts the contact resistance using measurements of different foam types (pure Cu and Cu with oxide), porosities (63%, 80%, 93%, and 95%) and thicknesses (4 mm, 8 mm, and 10 mm). These measurements are carried out with and without the use of thermal interface material (TIM) pads. A theory is proposed and implemented to estimate the contact and foam thermal resistances, but further work is needed to gain confidence in the results. Observations suggest that different thermal behavior is seen for the Cu foams compared to the Cu with oxide foams, and that the use of TIM pads can achieve 10x to 40x reduction in overall thermal resistance for highly porous foams bonded on Cu substrates.

With increased focus on miniature high power density electronic packages, there is a need for the development of new interface materials with lower thermal resistance. To this end, high conductivity thermal paste or similar thermal interface materials (TIMs), reinforced with superior thermal conductivity materials such as multi-walled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), graphite-derived multilayer graphene (g-MLG) offer an effective strategy to provide efficient paths for heat dissipation from heat source to heat sink. In an earlier paper, we had demonstrated that multilayer graphene derived from coal (coal-MLG) synthesized using our in-house developed one-pot process, has increased presence of phenolic groups on its surfaces, which translates into better dispersion of coal-MLG in silicone thermal paste. In this paper, we first compare the thermal conductance of a high conductivity thermal paste (k = 8.9 W/mK) using coal-MLG as an additive with that realized with other nano additives — MWCNTs, GNPs, and g-MLG. The data shows that coal-MLG as an additive outperforms all the other investigated nano additives in enhancing the thermal performance of the paste. With the coal-MLG as an additive, ∼70% increase in thermal performance was observed as compared to the base thermal paste used. This increase is about 2.5 times higher than that obtained using g-MLG as an additive. We also measured the thermal performance of coal-MLG-based TIM with its different wt.% fractions. The data confirmed our hypothesis that the optimum level of the loading fraction of the additive that can be dispersed in the matrix (paste in this case) before the onset of agglomeration is higher for the coal-MLG (3%) than for the other additives (2%). The implication is further improvement thermal performance with coal-MLG. The data shows the additional thermal enhancement to ∼2X. Finally, since coal-MLG produced by our in-house process is relatively cheaper and more environmentally friendly, we believe that these results would pave the path for enhanced thermal performance with non-silicone thermal pastes at a significantly lower cost. We also expect similar benefits for the silicone-based thermal pastes.

This paper advances the state-of-the-art in novel passive two-phase systems for more efficient cooling of datacenters and telecom central offices compared to the traditional air-based cooling solutions (e.g. aisle-based containment systems). The proposed passive two-phase technology uses numerous server-level thermosyphons to dissipate the heat generated by critical components, such as central processing units, accelerators, etc., with the flexibility of selecting the rack-level and room-level cooling elements depending on the deployment scenarios. The main goal of this paper is to experimentally investigate the thermal performance and maximum heat removal capability of a server-level thermosyphon for cooling compact servers. The experimental apparatus, built at Nokia Bell Labs, incorporates a single 7-cm high liquid-cooled thermosyphon that fits within a 2U server (smaller form factors can be achieved by a proper design that would further reduce the thermosyphon’s height). The heat source is represented by a pseudo-chip, composed of six parallel cartridge heaters installed in a copper block that incorporates local temperature measurements and is able of dissipating a total power of ≈ 500 W over a footprint area of 3.5 cm × 3.5 cm (corresponding heat flux of ≈ 41 W/cm 2 ). Steady-state experiments were carried out at various heat loads up to 240 W (corresponding heat flux of ≈ 20 W/cm 2 ), filling ratios and secondary side inlet conditions (coolant temperatures and mass flow rates), using R1234ze(E) and deionized water as the working fluids on the primary and secondary side, respectively. Test results demonstrate high heat transfer performance of the server-level thermosyphon over a wide range of conditions, and operating points are identified and classified into an operational map. Thermosyphon-based cooling systems across multiple length scales can significantly improve operation in terms of lowering energy consumption, allowing for higher hardware density, increased processing speed and reliability.

This work presents a simplified approach to optimally designing a heat sink with metallic thermal conductivity enhancers infiltrated with phase change material for electronic cooling. In present study, thermal conductivity enhancers are in the form of a honeycomb structure. A benchmarked two-dimensional computational fluid dynamics model was employed to investigate the thermal performance of the phase change material-metallic thermal conductivity enhancer composite heat sinks. Metallic thermal conductivity enhancers are often used in conjunction with phase change material to enhance the conductivity of the composite heat sink. Under constrained heat sink volume, the higher volume fraction of thermal conductivity enhancers improves the effective thermal conductivity of the composite, while it reduces the amount of latent heat storage simultaneously. The present work arrives at the optimal design of heat sink for electronic cooling by resolving the stated tradeoff. In this study, the total volume of the heat sink and the interfacial heat transfer area between the phase change material and thermal conductivity enhancers are constrained for all design points. Furthermore, assuming conduction-dominated heat transfer, an effective numerical model that solves the single energy equation with the effective properties of the phase change material- metallic thermal conductivity enhancer composite has been developed. The temperature gradient-time history is compared and matched for both the models to arrive at the accurate effective thermal conductivity value. The relationship of effective thermal conductivity as a function of metal volume fraction and thermal conductivity of metallic thermal conductivity enhancer is obtained. The figure of merit (FOM) is used to define the balance between effective thermal conductivity and energy storage capacity. The FOM is maximized to find the optimal volume fraction for the present design. The results from the study reveals that there exists an optimal metal volume fraction that maximizes the thermal performance of the composite.

Thermosyphon cooling systems represent the future of datacenter cooling, and electronics cooling in general, as they provide high thermal performance, reliability and energy efficiency, as well as capture the heat at high temperatures suitable for many heat reuse applications. On the other hand, the design of passive two-phase thermosyphons is extremely challenging because of the complex physics involved in the boiling and condensation processes; in particular, the most important challenge is to accurately predict the flow rate in the thermosyphon and thus the thermal performance. This paper presents an experimental validation to assess the predictive capabilities of JJ Cooling Innovation’s thermosyphon simulator against one independent data set that includes a wide range of operating conditions and system sizes, i.e. thermosyphon data for server-level cooling gathered at Nokia Bell Labs. Comparison between test data and simulated results show good agreement, confirming that the simulator accurately predicts heat transfer performance and pressure drops in each individual component of a thermosyphon cooling system (cold plate, riser, evaporator, downcomer (with no fitting parameters), and eventually a liquid accumulator) coupled with operational characteristics and flow regimes. In addition, the simulator is able to design a single loop thermosyphon (e.g. for cooling a single server’s processor), as shown in this study, but also able to model more complex cooling architectures, where many thermosyphons at server-level and rack-level have to operate in parallel (e.g. for cooling an entire server rack). This task will be performed as future work.

The demand for high power density, therefore high heat dissipation, power electronics modules is propelled by applications such as hybrid transportation and asynchronous power generation, among others. Besides a low thermal resistance, these applications require high thermal capacitance to manage transient operations. The Package Integrated Cyclone COoler (PICCO) is an additively manufactured, thermal energy storing cooler codesigned by GE Research (GRC) in collaboration with the US Army Research Lab (ARL). The key aspect of PICCO is its capability to swirl a two-phase coolant, i.e. liquid-gas. The centrifugal field creates a radial pressure gradient inducing buoyancy. The strong radial acceleration to which the fluid is subject forces relatively cold flow outward to reach the hot wall, thus boosting the heat transfer, while hot flow and bubbles migrate inward and the two-phase system is nearly isothermal (thermal storage). The proposed study models the swirled flow in terms of liquid film heat conductance and critical heat flux predictions. The resulting heat transfer coefficient can be applied to the walls of the cyclone and used as a boundary condition for the heat conduction problem through the cyclone wall and the module layers.

Frost formation on evaporators negatively affects the cooling performance of refrigerators. It increases the thermal resistance between the refrigerant and air leading to a reduction in the system cooling capacity. In this study, the effect of frost accumulation over a bare and finned surface on the convective thermal resistances has been experimentally investigated under impinging flow conditions. The surfaces are vertically positioned in a horizontal wind tunnel. The convective resistances have been measured with an in-house developed heat flux measurement system. Finally, the effectiveness of the finned surface was derived from the measurements for dry, condensing flow and as well as for frosting conditions. Under frosting conditions, the effectiveness of the finned surface is measured as 1.4 that is by a factor of 2X lower compared to the effectiveness of the same finned surface operating under dry conditions. It has been observed that the frost accumulation initially takes place at the tip of the fins and leads to a 45% drop in the heat transfer rate when the fin tips are completely covered with frost. Further frost accumulation on the fin base does not result in an additional drop in the heat transfer rate. In this regard, the study emphasizes the importance of the fin tip design for the heat sinks operating under frosting conditions.

The present study is focused on the experimental characterization of two-phase heat transfer performance and pressure drops within an ultra-compact heat exchanger (UCHE) suitable for electronics cooling applications. In this specific work, the UCHE prototype is anticipated to be a critical component for realizing a new passive two-phase cooling technology for high-power server racks, as it is more compact and lighter weight than conventional heat exchangers. This technology makes use of a novel combination of thermosyphon loops, at the server-level and rack-level, to passively cool an entire rack. In the proposed two-phase cooling technology, a smaller form factor UCHE is used to transfer heat from the server-level thermosyphon cooling loop to the rack-level thermosyphon cooling loop, while a larger form factor UCHE is used to reject the total heat from the server rack into the facility-level cooling loop. The UCHE is composed of a double-side-copper finned plate enclosed in a stainless steel enclosure. The geometry of the fins and channels on both sides are optimized to enhance the heat transfer performance and flow stability, while minimizing the pressure drops. These features make the UCHE the ideal component for thermosyphon cooling systems, where low pressure drops are required to achieve high passive flow circulation rates and thus achieve high critical heat flux values. The UCHE’s thermal-hydraulic performance is first evaluated in a pump-driven system at the Laboratory of Heat and Mass Transfer (LTCM-EPFL), where experiments include many configurations and operating conditions. Then, the UCHE is installed and tested as the condenser of a thermosyphon loop that rejects heat to a pumped refrigerant system at Nokia Bell Labs, in which both sides operate with refrigerants in phase change (condensation-to-boiling). Experimental results demonstrate high thermal performance with a maximum heat dissipation density of 5455 (kW/m 3 /K), which is significantly larger than conventional air-cooled heat exchangers and liquid-cooled small pressing depth brazed plate heat exchangers. Finally, a thermal performance analysis is presented that provides guidelines in terms of heat density dissipations at the server- and rack-level when using passive two-phase cooling.

The demand for high power density, therefore high heat dissipation, silicon carbide power electronics modules is propelled by applications such as hybrid transportation and renewable power generation and conversion, among others. Besides a low thermal resistance, these applications require high thermal capacitance to manage transient operations. The Package Integrated Cyclone COoler (PICCO) is an additively manufactured, thermal energy storing cooler codesigned by GE Research (GRC) in collaboration with the US Army Research Lab (ARL). The key aspect of PICCO is its capability to swirl a two-phase coolant, i.e. liquid-gas. The centrifugal field creates a radial pressure gradient inducing buoyancy. The strong radial acceleration to which the fluid is subject forces relatively cold flow outward to reach the hot wall, thus boosting the heat transfer, while hot flow and bubbles migrate inward and the two-phase system is nearly isothermal (thermal storage). In this paper, we introduce a novel power module package which brings together silicon carbide devices, Power OverLay (POL) wirebondless interconnect, and two-phase swirling flow in an additively manufactured cooler. Various embodiments of this power module structure are presented along with a discussion on their thermal behavior when subjected to a hybrid vehicle drive cycle.

The present study is an experimental investigation of a set of five additively-manufactured compact, lightweight, low-cost, air-to-water cross-media heat exchangers suitable for liquid cooling applications in desktop computers, among other applications. The heat transfer between the two fluids is facilitated by solid metallic wires arranged in a staggered tube-bank configuration, in direct contact with both fluids separated by polymer walls. Since the liquid flows externally over the wires instead of flowing inside the tubes in conventional tube-bank fin heat exchangers, smaller wires can be used in iCMHXs, resulting in lighter and more efficient units. The additively-manufactured iCMHX units are post-processed using a conformal polyurethane sealant. The units are experimentally studied in two case studies based on their post-processing techniques. The experimental studies include instrumentation calibration as well as uncertainty analysis. The first case study considers three geometrically identical iCMHX units sharing the same post-processing method. The overall iCMHX performances characterized by the thermal and hydrodynamic parameters, such as thermal resistance and pressure drop for both waterside and airside, are compared. Their experimental results are also compared to 2D CFD predictions. To provide probable reasoning behind the differences in the comparisons, a second case study is then carried out by experimentally investigating two iCMHX units but with variable post-processing approaches such as by using a thinned sealant and by using a single layer of sealant.

Thermal management of power electronics continues to be one of the limiting factors in the peak power capability of the traction inverter system and overall efficiency of the e-drive. Successful design and implementation of Embedded Microchannels with a 3D-Manifold Cooler, or EMMC, could enable higher power density that allows increase in the inverter peak power output. In the present work, we have conducted a parametric study on geometric dimensions of the EMMCs to analyze thermofluidic performance by using computational fluid dynamics (CFD) simulations. The study was conducted for a 5 mm × 5 mm cold-plate foot print, heat flux 800 W/cm 2 , and single-phase water as working fluid at inlet temperature of 25 °C. We implemented strategies such as i) symmetric distribution of manifold inlet/outlet conduits, ii) reducing the thickness of cold-plate substrate, iii) increasing fluid-solid interfacial area in cold-plate microchannels that resulted in reduction in thermal resistance of the baseline EMMC design from 0.1 to 0.04 cm 2 -K/W with pressure drop from 8 to 37 kPa.

Electronic cooling represents a major portion of a Data Centers energy consumption, thus efficient thermal management dramatically impacts energy savings. This work proposes reducing the energy consumption associated with server air-cooling by vectoring (tilting) the main upward tile flow using adjacent synthetic jets. The particular fluid dynamics generated by synthetic jets allows controlling the angle at which the tile flow emanates, directing the cooling air toward areas with higher cooling demand. Three-dimensional simulations were performed using k – ε standard turbulence model with the commercial software Ansys Fluent. In order to quantify and localize the inefficiencies of the system, we estimated the Exergy Destruction distribution in the cold aisle and servers. In previous studies, this technique proved successful in finding optimum operation conditions in Data Center cooling. As opposed to a base case without flow control, the adjacent synthetic jets directed the incoming fluid to areas with higher cooling demand, thus saving energy by avoiding over-provisioning air into servers operating under normal demand. The decrease in the overall Exergy Destruction demonstrated that vectoring improves the system’s global energy efficiency.

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.

As modern day electronics develop, electronic devices become smaller, more powerful, and are expected to operate in more diverse configurations. However, the thermal control systems that help these devices maintain stable operation must advance as well to meet the demands. One such demand is the advent of flexible electronics for wearable technology, medical applications, and biology-inspired mechanisms. This paper presents the design and performance characteristics of a proof of concept for a flexible Electrohydrodynamic (EHD) pump, based on EHD conduction pumping technology in macro- and meso-scales. Unlike mechanical pumps, EHD conduction pumps have no moving parts, can be easily adjusted to the micro-scale, and have been shown to generate and control the flow of refrigerants for electronics cooling applications. However, these pumping devices have only been previously tested in rigid configurations unsuitable for use with flexible electronics. In this work, for the first time, the net flow generated by flexible EHD conduction pumps is measured on a flat-plane and in various bending configurations. In this behavioral characteristics study, the results show that the flexible EHD conduction pumps are capable of generating significant flow velocities in all size scales considered in this study, with and without bending. This study also proves the viability of screen printing as a manufacturing method for these pumps. EHD conduction pumping technology shows potential for use in a wide range of terrestrial and space applications, including thermal control of rigid as well as flexible electronics, flow generation and control in micro-scale heat exchangers and other thermal devices, as well as cooling of high power electrical systems, soft robotic actuators, and medical devices.

Pulsating heat pipe (PHP) is a two phase highly efficient heat transfer device, due to its simple and flexible construction; it can be manufactured for a variety of applications. PHP works on thermally induced self-sustaining oscillation of liquid plugs and vapor slugs, so it does not have any moving parts either. Ease of manufacturing, potential for high efficiency at different scales and the ability to handle large heat fluxes has the PHP a suitable candidate for microscale electronics cooling or power electronics cooling. However, this technology is still in developing phase and there is at present no comprehensive model which can be used to design a PHP for a specific application. There are many parameters which affect PHP operation and a thorough understanding of the relation between all the variables is first required. The present study is an attempt to investigate experimentally the effects of various parameters on PHP startup, based on startup temperature measurements under varying heat input and carefully controlled conditions. It has been observed that the oscillations in PHP start (startup) as soon as it reaches a minimum temperature corresponding to the minimum Etvos number required for vapor bubble rise.

Gravity-driven two-phase liquid cooling systems using flow boiling within micro-scale evaporators are becoming a game-changing solution for electronics cooling. The optimization of the system’s filling ratio can however become a challenging problem for a system operating over a wide range of cooling capacities and temperature ranges. The benefits of a liquid accumulator to overcome this difficulty are evaluated in the present paper. An experimental thermosyphon cooling system was built to cool multiple electronic components up to a power dissipation of 1800 W. A double-ended cylinder with a volume of 150 cm 3 is evaluated as the liquid accumulator for two different system volumes (associated to two different condensers). Results demonstrated that the liquid accumulator provided robust thermal performance as a function of filling ratio for the entire range of heat loads tested. In addition, the present liquid accumulator was more effective for a small volume system, 599 cm 3 , than for a large volume system, 1169 cm 3 , in which the relative size of the liquid accumulator increased from 12.8 % to 25% of the total system’s volume.

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.

High heat flux from electronic devices remains a continuing challenge for cooling of electronics hardware in radar antenna applications pertaining to the defense industry. Cooling methods for such applications have varied from conduction cooling approaches for the cooling of circuit card assemblies, to advanced convection cooling using two phase flow (with pumped refrigerant) for the high heat flux devices used in transmit / receive modules. It is found that the limiting parameter in such applications is usually the heat flux from the electronic device. This paper provides an overview of the cooling techniques used for defense electronics, as well as current modeling tools and analytical methods used for thermal design during the product development phase. The role of thermal interface materials used in the material stack up for the thermal design solutions will also be touched upon. Additionally, the importance of using experimental techniques to characterize the heat transfer coefficient for the pumped refrigerant two phase flow will be discussed.