The card retainer mechanism is mostly the preferred tool to simply fasten a PCB card to the chassis to have a thermal conduction path. Depending on the card retainer design, thermal contact resistance between the retainer and chassis wall can differ. For a 150W heat-load PCB card, the contact resistance may cause more than a 10°C temperature increment on the electronic components regardless of the location on the card. This temperature rise in the card may not be affordable as the available cooling air temperature is 71°C or higher for an aircraft application. Considering the aviation platform and the abnormal cases, intermittent operation temperature can reach the value of 71°C for a 30-minute duration. In this study, a novel packaging approach is developed for a high-power card to get rid of the thermal contact resistance thanks to eliminating the card retainer mechanism. The card with its cold plate is driven to the chassis from one of the side walls through the slot and it stops at the opposite wall. The cooling fins are on the cold plate rather than on the chassis, therefore there is no contact resistance between the fins and the cold plate. Gaskets are used to have an airtight design between the cold plate and the wall. All studies are performed both numerically and experimentally to evaluate the novel approach. In the end, thermally promising results are obtained for harsh environments.
In this work, pool boiling results using functionalized 6061 aluminum surfaces with PF-5060 as the working fluid are presented. Femtosecond laser surface processing (FLSP) is used as a surface modification technique to create self-organized, multiscale surface features which lead to additional nucleation sites and augmented surface area available for heat transfer. Four functionalized surfaces were fabricated by varying the laser fluence at a fixed pulse count to examine the impact of surface morphology on the heat transfer performance. Each surface was tested twice for repeatability up to critical heat flux (CHF) and compared to a polished baseline sample. Results reveal drastically enhanced performance compared to the polished surface. The enhancement in the maximum heat transfer coefficient over the reference surface ranged from 390% to 680% for the FLSP surfaces tested, which is attributed to the abundance of potential nucleation sites formed during laser processing. Each functionalized surface kept the surface temperature below the recommended limit of 85°C despite the temperature overshoots observed and yielded nearly isothermal behavior within the nucleate boiling regime, with most superheats remaining between 5 and 6°C. Superheat at the onset of nucleate boiling was reduced for all functionalized surfaces with reductions ranging from 35% to 53% compared to the baseline surface. Additionally, all FLSP surfaces exhibited an increase in the CHF, ranging from 9% to 49%. This work demonstrates the potential of FLSP-functionalized aluminum surfaces as a viable means of achieving significant two-phase heat transfer enhancement with dielectric fluids.
Ongoing research in flexible electronics has led to the development of various conductible inks and technologies for printing on flexible substrates. The demand for light and compact printed electric circuits has been augmented with the advancement of flexible printing technology. Using environmentally friendly solvents and low-impact waste provides a pathway for creating more sustainable solutions. Recent advances in material development have led to the development of water-based conductive inks for the aerosol-jet platform. In this paper, a Silver Nanoparticle water-based sustainable ink having low viscosity has been studied, and performance has been compared with volatile solvent-based inks. This paper aims to find the process recipe for sustainable ink using the aerosol-jet deposition process with performance comparable to that achieved with incumbent inks. The process parameter study includes investigating different printing and sintering parameters for obtaining desirable electrical and mechanical properties of printed traces. The printing parameters include ultrasonic atomizer mass flow control (UAMFC), sheath flow control (SMFC), and stage speed, and the sintering parameters include temperature and time of sintering. Optical microscopy and white light interferometry are used to characterize print quality. A regression model has been built for the line resistance for different printing parameters. The experiment found the optimal printing parameters as 35 SCCM UAMFC, 50 SCCM sheath, and 3.5 mm/s stage speed. Similarly, after considering the effect on both resistivity and SLF, the best sintering condition has been chosen as 275°C, 10 minutes, giving resistivity and SLF of $\mathbf{5.98}\boldsymbol{E}-\mathbf{06}\quad \mathbf{\Omega}. \boldsymbol{cm}$ and SLF 54.92 gF, respectively. The resistivity of sustainable silver ink is found to be 2 times greater than the non-sustainable silver ink and 3.79 times the bulk silver.
Cooling of electronics devices while targeting the localized hotspots on them is one of the important challenges in developing modern computational systems. In this work, the thermal fluid characteristics of combined air jet impingement and cross flow cooling are numerically investigated and compared with thermal fluid characteristics of pure jet impingement and pure cross flow through a channel. The comparisons are performed at similar overall coolant mass flow rate and similar jet velocity. The cooling channel is subject to the applied heat flux on its side, representing the heat dissipated form an electronics device. There is a hotspot zone on the heated surface subject to a higher heat flux value. For data analysis, non-dimensional temperature distribution on the heated surface is presented and discussed for six studied cases. Also, a standard deviation term is used to quantify the temperature uniformity of the heated surface for each case. The grid independence study and validation study are conducted to ensure the accuracy of the results. It is found that jet-in-crossflow setup significantly decreases the surface temperature and temperature standard deviation as compared to pure crossflow cases at the same mass flow rate or similar velocity. In comparing jet-in-crossflow cases with pure jet impingement cases, it is found that a proper selection of jet location and jet angle in jet-in-crossflow setup yields a lower temperature standard deviation value and required pumping power than a pure jet impingement case, operating at the same jet velocity. In addition, at the same mass flow rate, the pressure drop and required pumping power for jet-in-cross flow cases are considerably less than those of pure jet impingement. In comparing temperature standard deviation values, a proper selection of jet location and jet angle in jet-in-crossflow setup produces a very close temperature standard deviation value to that of a pure jet impingement case operating at the same mass flow rate.
Thermal management of mobile computing devices are critical to enhance user experience. Two vectors for user experience are-laptop surface temperature control in steady state workload and transient chip temperature control in bursty workload. Thermal solutions that enhance spreading in lateral direction and at the same time carry large quantities of heat from chip are needed to satisfy above two vectors. Vapor chambers are solutions that have traditionally been used in laptop industry for some time. However, their thicknesses are in the range of 0.5 mm and above, below which performance is strained. In this paper, we study an ultra-thin 0.3 mm vapor chamber that can be used in super slim competitive form factors such as tablets and premium laptops. Detailed component level experimental study is conducted using a standard characterization methodology. Figures of merit include steady state in-plane resistance, maximum steady state heat transfer capability and maximum transient heat transfer capability. We also specifically study the performance using both a uniform heater and a high-power density chip replicating actual processor considering different flow conditions expected in notebook application. Performance results indicate that ultra-thin vapor chambers have very low spreading resistance and maximum heat transfer in can be up to 8W in steady state and 15W in transient. Ultra-thin VC in thickness lower than 0.5 mm can be a potential thermal solution in next generation of notebook computers.
Direct contact liquid immersion cooling is receiving increased attention as a potential battery thermal management method. This method offers greater cell thermal homogenization and increased pack performance through enhanced rates of heat transfer. However, limited published literature exists on this thermal management method, particularly on module arrangements. This study presents an experimental investigation of a battery module consisting of four LiFePO 4 cylindrical cells connected electrically in parallel and completely immersed in the dielectric fluid Novec 7000. The module is subjected to increasing constant current discharge rates to a maximum of 3C, examining the thermal performance of the proposed method and its capability to minimize cell-to-cell temperature difference, particularly under two-phase conditions.
Mobile phone functions are evolving towards high-end cameras, AI functions, and high-performance gaming environments. To implement these functions, the System on Chip (SoC) Application Processor (AP) of mobile places the system blocks with more functions in a limited space. The problem is that when thermal runaway occurs due to overlapping of heat between system blocks on chip, the performance by temperature control may be lowered. When placing the system blocks in the chip, the level of heat generation of each system is predicted and the floor-planning is arranged to have the lowest temperature during scenario operation. After analyzing the temperature map that is generated from the thermal simulation result, the location of the system block that becomes a hotspot is moved to another location. the process of waiting for the result by requesting the simulation again is repeated to determine the final temperature-optimized floorplan. In this process, there is a problem that it takes a long time while repeating the simulation after design. If the designer can check the temperature map in real time, it is possible to design in consideration of the temperature-optimized layout before the detailed design comes out. Also, it is possible to find the optimal arrangement case with the lowest temperature. This paper explains a method to calculate the temperature distribution within the chip in real-time according to the floorplan arrangement in the initial stage of chip design. A method of reducing the time to make the thermal resistance matrix and describes the optimize method to find the lower optimal arrangement.
The ever-increasing need for power consumption in electronic devices, coupled with the requirement for thinner size, calls for the development of efficient heat spreading components. Vapor chambers (VCs), because of their ability to effectively spread heat over a large area by two-phase heat transfer, seem ideal for such applications. However, creating thin and efficient vapor chambers that work over a wide range of power inputs is a persisting challenge. VCs that use wicks for circulating the phase changing media, suffer from capillary restrictions, dry-out, clogging, increase in size and weight, and can often be costly. Recent developments in wick-free wettability patterned vapor chambers replace traditional wicks with laser-fabricated wickless components. An experimental setup allows for fast testing and experimental evaluation of water-charged VCs with liquid-assisted cooling. The sealed chamber can maintain vacuum for long durations, and can be used for testing of very thin wick-free VCs. This work extends our previous study by decreasing overall thickness of the wick-free VC down to 3 mm and evaluates its performance. Furthermore, the impact of wettability patterns on VC performance is investigated, by carrying out experiments both in non-patterned and patterned VCs. Experiments are first carried out on a wick-free VC with no wettability patterns and comprising of an entirely superhydrophilic evaporator coupled with a hydrophobic condenser. Thereafter, wettability patterns that aid the rapid return of water to the heated site on the evaporator and improve condensation on the condenser of the vapor chamber are implemented. The thermal characteristics show that the patterned VCs outperform the non-patterned VCs under all scenarios. The patterned VCs exhibit low thermal resistance independent of fluid charging ratio withstanding higher power inputs without thermal dry-outs.
Efficient thermal management in high-power electronics cooling can be achieved using phase-change heat transfer devices, such as vapor chambers. Traditional vapor chambers use wicks to transport condensate for efficient thermal exchange and to prevent “dry-out” of the evaporator. However, wicks in vapor chambers present significant design challenges arising out of large pressure drops across the wicking material, which slows down condensate transport rates and increases the chances for dry-out. Thicker wicks add to overall thermal resistance, while deterring the development of thinner devices by limiting the total thickness of the vapor chamber. Wickless vapor chambers eliminate the use of metal wicks entirely, by incorporating complementary wettability-patterned flat plates on both the evaporator and the condenser side. Such surface modifications enhance fluid transport on the evaporator side, while allowing the chambers to be virtually as thin as imaginable, thereby permitting design of thermally efficient thin electronic cooling devices. While wick-free vapor chambers have been studied and efficient design strategies have been suggested, we delve into real-life applications of wick-free vapor chambers in forced air cooling of high-power electronics. An experimental setup is developed wherein two Si-based MOSFETs of TO-247-3 packaging having high conduction resistance, are connected in parallel and switched at 100 kHz, to emulate high frequency power electronics operations. A rectangular copper wick-free vapor chamber spreads heat laterally over a surface 13 times larger than the heating area. This chamber is cooled externally by a fan that circulates air at room temperature. The present experimental setup extends our previous work on wick-free vapor chambers, while demonstrating the effectiveness of low-cost air cooling in vapor-chamber enhanced high-power electronics applications.
Nowadays, one of the important considerations for the hyper Scale data center is the carbon emission indicator which reflect to the power efficiency. Power efficiency used to be focused on CPU/GPU components' level, thermal design, fan powers, such as silicon vendors evolve their production generation to generation to pursue higher performance/W. However, from data center perspective, system power supply optimization is another key ingredient for better power efficiency and carbon emission reduction, especially for the high density rack server system. 12V input power supply is most common power solution in rack design, while the power loss is increasing dramatically along with rising system power consumption. Thus, a novel 48V system power supply solution for high density rack design is introduced in this paper. The 48V system power solution has up to 36KW power capacity with dual power input and 2% efficiency improvement compared to 12V power supply. The power loss on rack bus bar can reduce up to 93%, which results in the total number of rack power supply units reduce 37.5%. The 48V power supply system has volume deployed in Baidu's data center. In the paper, TCO calculation with volume deployment data for carbon emission reduction is also introduced.
Several forces exert an influence on two-phase bubble dynamics under terrestrial conditions, chief among these being those due to buoyancy and surface tension. Under microgravity conditions, the absence of buoyancy forces disrupts bubble dynamics preventing bubbles from detaching from surfaces. The stagnant bubbles form a large vapor mass attached to the surface, leading to a considerable rise in surface temperature. In this study, a mesoscale-engineered surface in the form of saw-toothed structures has been produced that can provide access to liquid pockets across the troughs of the microstructure in microgravity and terrestrial adverse-gravity orientation. The surface is built with intentional nucleation sites that support consistent vapor germination in both environments. Following terrestrial studies on various surface morphologies and under varying degrees of sub cooling, experiments were conducted onboard the International Space Station. The test chambers were square cross-sectioned glass ampoules with deposited thin film nichrome heaters. NASA's implementation partner developed the test and instrumentation hardware to conform to stringent flight requirements. The experimental investigation is titled Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) and was conducted in the Pore Formation and Mobility Investigation (PFMI) furnace. The paper discusses high- speed data obtained on vapor motion in microgravity and terrestrial downward-facing environments, providing insight into the differences between a flat surface and the microstructure. The images suggest the existence of a mobility diameter in microgravity, which enables favorable motion across the microstructure in both lateral directions. This mobility contrasts the slug mobility actuated by the sawtooth in the downward-facing terrestrial heater, where changes in the interfacial radius of curvature produce a net lateral motion in the direction of the long slope.
Embedded microfluidic cooling improves cooling efficiency by avoiding interfacial thermal resistance. However, the temperature rise of the fluid along the flow direction leads to an uneven axial temperature of the straight parallel microchannels (SPMCs), which then causes the degradation of microfluidic cooling uniformity, especially at the outlet position with the greatest thermal resistance. Counterflow microchannels (CFMCs) heat exchangers can be realized by adjusting the flow direction of adjacent channels to the opposite direction. In this paper, a one-dimensional thermal resistance model of CFMCs heat exchangers is established, and the surface temperature distribution characteristics of heat exchangers are studied. Under the assumption of a constant convective heat transfer coefficient, the equivalent neutral plane and the equivalent adiabatic line are defined, and the temperature distribution of the heating surface is analyzed by using the temperature rise characteristics of the working fluid after heat absorption. Both theoretical and numerical results show a quadratic distribution of the surface temperature in CFMCs. Compared with the SPMCs, CFMCs result in a higher average surface temperature but a lower maximum surface temperature, while the temperature standard deviation is reduced by more than 60%.
Thermal modelling of magnetic components in high-frequency power electronic systems is not trivial. This can be attributed to the complex non-uniform losses, heterogenous construction of magnetic components, and the temperature dependence of electrical and magnetic properties. Accurate thermal modelling of such magnetic components relies on the use of bi-directionally coupled electromagnetic-thermal numerical analysis. Although such bi-directionally coupled numerical models provide accurate results, the computational cost of such models can be restrictive. Hence, there is a need for low computational cost thermal models of magnetic components. In this paper, we develop a low computational cost thermal model of a power transformer using the admittance matrix approach. First, a bi-directionally coupled multiphysics model of a power transformer is developed and validated using experimental test results. Using the numerical model, low-cost thermal models are evaluated for surface heat transfer coefficients varying between 1 to $\mathbf{200}\ \boldsymbol{W}/\boldsymbol{m}^{\mathbf{2}}\cdot \boldsymbol{K}$ , covering the typical thermal operating range of magnetic components housed in conventional power electronic systems. The final low-cost thermal model's surface temperature and heat flux predictions were within $\pm \mathbf{8}.\mathbf{2}\%$ of the numerical results, while the junction temperature error was $\pm \mathbf{6}.\mathbf{18}\ \%$ . The simplified thermal model developed shows close to Boundary Condition Independence (BCI) behaviour and is a low computational cost alternative to the numerical model.
Sustainability has become a top priority of ecosystem as 90% of the S&P 500 index published sustainability reports in 2019, while that figure was only 20% ten years ago. Computing at the era of sustainability is re-focusing on benchmark of higher computing performance with fewer energy consumption along with circular economy system architecture from data center, rack, server platform, to silicon and software aspects. This paper introduces an advanced 54V high power density rack engineering practice and deployment in JD.COM air cooling and liquid cooling data center for sustainable computing, with all advanced technologies integrated in rack level, optimized for air cooling data center, and effectively explored higher power density for liquid cooling data center. Compared with conventional 12V rack design, the detailed energy efficiency contribution to Total Cost Ownership (TCO) of carbon emission reduction is calculated and analyzed with data center deployment results. The advantages for data center sustainability of high-power density rack evolution with 54V is also summarized in the paper.
As component and IT equipment level power and power densities continue to increase, traditional air-based cooling technologies are reaching their limits of thermal management in high performance data center servers and Edge applications. To meet these increasing performance demands and reduce total cost of ownership, more cost effective, capable, and efficient liquid cooling technologies such as cold plates and immersion are being actively investigated and implemented in many deployments. Single-phase immersion cooling is one such technology that involves immersing the entire IT equipment in a tank filled with a dielectric liquid. The dielectric liquid flows through the IT equipment, providing improved cooling compared to air via natural or forced convection. This technology has been seeing increasing interest in the industry in recent years due to its capability to cool high power servers at low cost and high efficiency as well as provide a holistic cooling solution for the entire IT equipment/server. Most installations of single-phase immersion tanks in the industry use pumps that provide circulation of the dielectric liquid through the heat exchanger to maintain tank liquid inlet temperature. However, these tanks have unique manifolds distributing the flow into the tank, which generates a unique flow inlet boundary condition to the servers for each tank design. On the market today, there are tanks that span from using only natural convection for cooling the servers, while most of the tanks use a mix between natural and forced convection. Furthermore, there is no means to measure the flowrates or understand the flow boundary conditions for each server or the flow going through the CPU heatsink. Hence, the design of efficient thermal solutions or performance prediction capabilities for such systems becomes challenging. The current paper describes thermal characterization of immersion optimized heatsinks under known and controlled boundary conditions. The results described here show the performance of the optimized heatsink in a liquid tunnel under natural convection and under forced convection regimes at a wide range of flowrates for an Intel CPU. Under natural convection, a large dependency of the heatsink cooling capability is observed on the operating power level of the CPU. As the flow rate increases, this dependence is reduced while the cooling capability improves significantly. The results show comparison of the thermal performance for air and immersion optimized heatsinks used in immersion in different flow regimes for known and controlled thermal and flow boundary conditions.
Computational fluid dynamics (CFD) is widely used for data-center-cooling applications. Most contemporary CFD codes employ SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) or one of its variants for pressure- velocity coupling. Recently, the time splitting method has seen renewed attention as it is a key element of Fast Fluid Dynamics (FFD) - which promises faster fluid simulations at the cost of some accuracy. We wondered how time splitting by itself - without the other simplifications associated with FFD - compares to SIMPLE. We compared time splitting to transient and steady-state versions of SIMPLE over 68 data center examples while utilizing the same codebase, computing hardware, physical models, computational grid, solution- control parameters, etc. We observed no statistical difference in speed or accuracy between the three methods. SIMPLE transient is slightly more robust than time splitting and both are significantly more robust than a purely steady-state analysis utilizing SIMPLE. We conclude that previously reported speed and accuracy differences between FFD and traditional CFD are not due to time splitting. Time splitting on its own offers no obvious advantage over SIMPLE.
In the era of cloud computing, data center infrastructure has evolved with high-performance computing with high thermal design power (TDP) processors. The traditional air-cooling solution is facing severe heat dissipation challenges of high TDP and is becoming the bottleneck of system heat dissipation solution. One of the main directions to address this challenge is to implement Liquid Assisted Air-Cooling (LAAC) solutions. In this paper, a novel LAAC solution is introduced. The innovative series radiator connection design is adopted to effectively improve the heat dissipation performance of radiator. The reliability and maximum mass flow rate values of the LAAC design were evaluated and simulated. Furthermore, an accelerated test model is proposed and verified in laboratory experiments and engineering deployment. Engineering results show that the new Liquid Assisted Air Cooling solution delivers better cooling capability comparing to legacy EVAC solution with identical thermal boundary condition while offering reliability for deployment. Moreover, the cost of proposed LAAC design is calculated at the rack level, and has been proved to be 70% less comparing to traditional open-loop liquid cooling design with same performance. The best-known engineering design methods and practices are also summarized in the paper that contribute to enhanced radiator heat dissipation efficiency, maximum mass flow rate values, and better life cycle and reliability.
A synthetic jet (SJ) is a microfluidic device that uses the “zero-net-mass-flux” concept to create a compact cooling solution and provide a net positive momentum flux to the local environment. SJs have been studied extensively for natural convection heat transfer, but there is a limited data available for SJs in cross flow regimes. This paper presents results based on direct numerical simulation of a SJ in a confined heat transfer channel with and without cross flow. Studied SJ had a deforming boundary that oscillated at 1000 Hz and was placed at a high orifice-to-plate distance ratio of 20. The flow field inside the device with a moving boundary was modeled in a coupled manner to the flow field outside of the device for 80 oscillation cycles. The coupled study of the flow fields inside and outside of the cavity revealed their interaction towards an unstable flow field. Moreover, comparison between SJ's and continuous jet's (CJ) cooling performance was performed with the same net mass flow rate and identical jet outlet temperatures. Without cross flow, CJ, and with cross flow, SJ outperformed in terms of heat removal. The remarkable difference in spatial evolution of CJ and SJ explains the better performance of SJ in cross flow regime. In the studied high orifice-to-plate distance, CJ stream was unable to penetrate effectively through the crossflow, while the vortical structures created by SJ were able to do so and impinge on the target surface with heat transfer augmentation at upstream. Furthermore, the SJ's cavity heating was found to be a limiting factor in its capability to achieve high heat transfer coefficients in confined channels, which needs to be addressed to maintain its reliable heat removal performance.
Thermal-aware design with limited resources essentially requires the accurate Thermal Resistance Matrix (TRM) that represents the relationship between temperature and power with adequate heat transfer structure that include package. The TRM has been widely used for temperature sensor placement and floor-plan optimization in board and on-die design. However, the large size of TRM usually causes the computational burden by consuming a lot of time for design optimization. In this paper, the systematic scheme that reduces the TRM size is being proposed. By using Singular Value Decomposition (SVD), it is presented that the size of TRM can be reduced with K dominant singular values, which can save computational resources to achieve a steady-state temperature map. Thermal resistance matrix, R, can be defined as a rectangular matrix of M x N, where M and N represent monitoring points and power, respectively. The size of [R] can be reduced using [R‘] derived by SVD. While M x N multiplications are required for the original TRM, the reduced matrix achieved from this procedure requires (N+M) x K multiplications when K is the dominant singular values. Since K is usually much smaller than N, (N+M) x K multiplications required by the modified TRM, [R‘], are also much smaller than a number of N x M multiplications required by original [R]. By using this method, several case studies are presented to demonstrate that the proposed systematic scheme is effective and useful for time-efficient optimization of on-die floor-plan and temperature placement when designing System on-Chip (SoC) that include multiple heterogeneous cores.
The primary means of heat rejection for space vehicles are limited to radiation due to the absence of the earth's atmosphere for convective heat transfer. Thermal radiators, solely, cannot regulate the temperature and often require supplemental heaters, heat pipes, and control systems to maintain the spacecraft temperature within a desirable range. A thermochromic multilayer structure with reversible optical properties, (e.g., emittance and reflectance) based on environmental temperature, may be exploited to provide a passive thermal control system for space vehicles. The current study aims to design and develop a smart multilayer structure that contains thermochromic material that allows for positive emittance switching at room temperature-based phase-transition temperature. The materials, thickness, and sequence for each layer is determined such that the structure can provide a wide dynamic range of emissivity with temperature in the mid-infrared wavelength region to ensure the effectiveness of the proposed passive thermal control system for the space vehicles. The thermochromic layer considered for the present study is Vanadium Dioxide whose phase-transition temperature is 68°C. To achieve an emittance switching temperature close to the room temperature, this study also strives to reduce the phase-transition temperature of the thermochromic system using doping. The proposed multilayer structure is grown using thin film deposition techniques consisting mainly of Magnetron sputtering with optimal operating conditions. After fabricating the proposed structure, an experimental apparatus is designed and developed to directly measure the emitted power from the coating under different operating conditions. Using the proposed test setup based on calorimetric approach, the total hemispherical emittance can directly be explored.
