Liquid Cooled High Power Aerospace Electronics Fluid Channel Layout Design Strategy Based on Analytical Calculation

This article is about the application of liquid cooling in high power aerospace electronics. This article discusses liquid cooled cold plate fluid channel layout design strategy based on hand or analytical calculations before the three dimensional thermal model is constructed. Everybody knows how to perform thermal analysis using CFD software; however, CFD software cannot generate three dimension models automatically. Thermal engineers still need to design preliminary models to analyze. CFD software cannot solve the problems for the designer. Computers cannot substitute the human in design. In the end, it is the thermal engineer’s education, experience, knowledge, strategic thinking, and know-how that determine the outcome of the design. To simplify the task, sections of the cooling channel are suggested to be designed individually to meet the cooling needs of each individual component that segment of the fluid channel is cooling. The sections of the fluid channel routed directly underneath the heat dissipating components can be connected in parallel or in series. This article will discuss the pros and cons of both design approaches. Pressure drops versus heat transfer are the tradeoffs of the fluid layout design. An example of the analytical pressure drop calculation is provided. This article also provides guidance on calculating the flow rate of each of the cooling sections and the strategy of determining the linear velocity of the liquid. In the discussion, a brief trade study of machined or casted cold plate versus tube-in-plate cold plate design is presented. Positioning of the cold plate to control the internal ambient temperature is also briefly discussed.

Direct Liquid Cooling of High Flux LED Systems: Hot Spot Abatement

With the recent advances in wide band gap device technology, solid-state lighting (SSL) has become favorable for many lighting applications due to energy savings, long life, green nature for environment, and exceptional color performance. Light emitting diodes (LED) as SSL devices have recently offered unique advantages for a wide range of commercial and residential applications. However, LED operation is strictly limited by temperature as its preferred chip junction temperature is below 100 °C. This is very similar to advanced electronics components with continuously increasing heat fluxes due to the expanding microprocessor power dissipation coupled with reduction in feature sizes. While in some of the applications standard cooling techniques cannot achieve an effective cooling performance due to physical limitations or poor heat transfer capabilities, development of novel cooling techniques is necessary. The emergence of LED hot spots has also turned attention to the cooling with dielectric liquids intimately in contact with the heat and photon dissipating surfaces, where elevated LED temperatures will adversely affect light extraction and reliability. In the interest of highly effective heat removal from LEDs with direct liquid cooling, the current paper starts with explaining the increasing thermal problems in electronics and also in lighting technologies followed by a brief overview of the state of the art for liquid cooling technologies. Then, attention will be turned into thermal consideration of approximately a 60W replacement LED light engine. A conjugate CFD model is deployed to determine local hot spots and to optimize the thermal resistance by varying multiple design parameters, boundary conditions, and the type of fluid. Detailed system level simulations also point out possible abatement techniques for local hot spots while keeping light extraction at maximum.

Experimental Investigation of the Effect of Synthetic Jets in Minichannels With Subcooled Boiling Flow

Microchannels and minichannels have been shown to have many potential applications for cooling high-heat-flux electronics over the past 3 decades. Synthetic jets can enhance minichannel performance by adding net momentum flux into a stream without adding mass flux. These jets are produced because of different flow patterns that emerge during the induction and expulsion stroke of a diaphragm, and when incorporated into minichannels can disrupt boundary layers and impinge on the far wall, leading to high heat transfer coefficients. Many researchers have examined the effects of synthetic jets in microchannels and minichannels with single-phase flows. The use of synthetic jets has been shown to augment local heat transfer coefficients by 2–3 times the value of steady flow conditions. In this investigation, local heat transfer coefficients and pressure loss in various operating regimes were experimentally measured. Experiments were conducted with a minichannel array containing embedded thermocouples to directly measure local wall temperatures. The experimental range extends from transitional to turbulent flows. Local wall temperature measurements indicate that increases of heat transfer coefficient of over 20% can occur directly below the synthetic jet with low exit qualities. In this study, the heat transfer augmentation by using synthetic jets was dictated by the momentum ratio of the synthetic jet to the bulk fluid flow. As local quality was increased, the heat transfer augmentation dropped from 23% to 10%. Surface tension variations had a large effect on the Nusselt number, while variations in inertial forces had a small effect on Nusselt number in this operating region.

Combining Computational Fluid Dynamics (CFD) and Flow Network Modeling (FNM) for Design of a Multi-Chip Module (MCM) Cold Plate

The objective of the study is to improve on performance of the current liquid cooling solution for a Multi-Chip Module (MCM) through design of a chip-scale cold plate with quick and accurate thermal analysis. This can be achieved through application of Flow Network Modeling (FNM) and Computational Fluid Dynamics (CFD) in an interactive manner. Thermal analysis of the baseline cold plate design is performed using CFD to determine initial improvement in performance as compared to the original solution, in terms of thermal resistance and pumping power. Fluid flow through the solution is modeled using FNM and verified with results from the CFD analysis. In addition, CFD is employed to generate flow impedance curves of non-standard components within the cold plate, which are used as input for the Hardy Cross method in FNM. Using the verified flow network model, design parameters of different components in the cold plate are modified to promote uniform flow distribution to each active region in the chip-scale solution. Analysis of the resultant design using CFD determines additional improvement in performance over the original solution, if available. Thus, through complementary application of FNM and CFD, a robust cold plate can be designed without requiring expensive fabrication of prototypes and with minimal computational time and resources.

Prediction of Hot Aisle Partition Airflow Boundary Conditions

The integration of a simulation-based Artificial Neural Network (ANN) with a Genetic Algorithm (GA) has been explored as a real-time design tool for data center thermal management. The computation time for the ANN-GA approach is significantly smaller compared to a fully CFD-based optimization methodology for predicting data center operating conditions. However, difficulties remain when applying the ANN model for predicting operating conditions for configurations outside of the geometry used for the training set. One potential remedy is to partition the room layout into a finite number of characteristic zones, for which the ANN-GA model readily applies. Here, a multiple hot aisle/cold aisle data center configuration was analyzed using the commercial software FloTHERM. The CFD results are used to characterize the flow rates at the inter-zonal partitions. Based on specific reduced subsets of desired treatment quantities from the CFD results, such as CRAC and server rack air flow rates, the approach was applied for two different CRAC configurations and various levels of CRAC and server rack flow rates. Utilizing the compact inter-zonal boundary conditions, good agreement for the airflow and temperature distributions is achieved between predictions from the CFD computations for the entire room configuration and the reduced order zone-level model for different operating conditions and room layouts.

Experimental Study of the Convective Heat Transfer From a Geometrically Scaled Up 2D Impinging Synthetic Jet

Synthetic jets are created by periodically ejecting and injecting fluid from an orifice or channel. Despite delivering no net mass flow per cycle, a synthetic jet delivers flow with net positive momentum. Small, compact synthetic jet actuators can be fabricated to operate in the subaudible acoustic range and can be packaged in orientations that allow them to deliver cooling air flow to electronic devices. The most promising orientation is one that delivers the jet flow in a direction normal to the heated surface such that it impinges on the surface as a periodic jet. In previous studies, numerical simulations have been performed by the authors, utilizing a canonical geometry, with the purpose of eliminating actuator artifacts from the fundamental physics that drive the problem. The present paper reports on laboratory experiments that have been performed in order to nearly replicate the idealized synthetic jet geometry and thus allow comparison to the previous numerical investigations. The periodic volume change in an upstream plenum required to produce the synthetic jet is accomplished with an acoustic speaker operated at low frequencies. The amplitude and the frequency at which the jet is actuated determine the Reynolds and Strouhal numbers, which are the dominant non-dimensional groups that control the behavior of the impinging synthetic jet. By maintaining the Re and the St in the laboratory experiments to match those of the small scale actuators, the laboratory experiments have been geometrically scaled up to allow highly resolved measurements of the unsteady velocity field and the local time-dependent Nusselt number on the target heated surface. Experiments were performed at variable jet Re, frequencies, and height from the target surface. The dependence of the surface averaged Nu to jet parameters generally agrees with the computational results. However, discrepancies found between numerical and empirical local data are under revision.

TIM Selection for an IGBT Cold Plate Using Experimental and Numerical Modeling

Experimental measurements were used in conjunction with a numerical model to perform an in situ analysis of an IGBT cooling solution with a cold plate utilizing an 85–90°C ethylene glycol-water mixture as the cooling fluid. This process was used to aid in the selection of an appropriate thermal interface material (TIM) for the application. The effects of elevated temperature and thermal cycling on the performance of the TIM were investigated during the selection procedure. Applying the thermal grease with the cold plate at 70°C rather than 30°C caused a reduction in the junction to case resistance of 74% and 78% for the two thermal greases tested.

Compact Modeling of Data Center Air Containment Systems

The practice of ducting racks to a dropped ceiling or containing entire cold or hot aisles in data centers is being implemented with more frequency in an attempt to improve reliability and efficiency. While CFD and other numerical modeling tools are widely used to optimize data center cooling, they are not particularly effective at modeling containment systems; the performance of such systems is dominated by small and complex leakage paths (e.g., through, around, and under racks), which are difficult or impossible to include in a practical full-scale model. We propose a compact model which uses a flow network to determine airflow rates inside containment systems while the traditional “parent” numerical model continues to handle predictions in the rest of the facility. The two models are coupled at flow boundaries such as where ducting meets a dropped ceiling and leakage paths cross rack surfaces. The compact-model approach has the opportunity to be much faster and more robust than fully-explicit CFD models since leakage path resistances can be established through experimental measurements. We discuss the characterization of rack leakage paths and demonstrate the use of the compact model in a full data center simulation in which the role of parent numerical model is played by a potential flow model.

A Capacitance-Based Technique for Characterization of Dielectric Interfaces Using a Grid of Electrode Junctions

A sensor for detecting imperfections in the distribution of a dielectric thermal interface is proposed. The sensor can detect imperfections such as voids, cracks, and interface gap changes on the millimeter scale. A rake of long, parallel electrodes is imbedded flush into each opposing substrate face of a narrow gap interface, and exposed to the gap formed between the two surfaces. Electrodes are oriented such that their lengthwise dimension in one substrate runs perpendicular to the other. Capacitance measurements taken at each crossing point (junction) allow for characterization of the region, and subsequently, detection of voids present or changes in gap size. The electric field associated with each electrode junction is numerically simulated and analyzed. Design criteria for the electrode junctions that localize the electric fields are presented. The electrode configuration employed gives rise to a non-trivial network of interacting capacitances. Due to these interactions, the actual capacitance at any given junction cannot be measured directly; instead, the measurement represents an equivalent capacitance resulting from this network. A generalized solution for analyzing the circuit network is presented. An experimental test unit is described, and experimental data are presented for measurements from a typical electrode junction. The results agree with predictions from the network model for cases that meet the design criteria for electric field localization; when the localization criteria are not met, the measurements deviate from the model predictions as expected.

Thermal Transport Phenomenon in Circular Pipe Flow Using Different Nanofluids

The aim of the present study is to investigate the thermal fluid flow transport phenomenon of nanofluids in the heated horizontal circular tube. Consideration is given to the effects of volume fraction of the nanoparticle on the laminar heat transfer and thermal properties. Alumina (Al2O3) and oxide copper (CuO) are employed here as nanoparticles. It is found from the study that (1) the viscosity ratio of nanofluids increases in accordance with an increase of the volume fraction of the nanoparticles, (2) the nanofluids have substantially higher value of Nusselt number than the same liquids without nanoparticles and the Nusselt number of nanofluids increase with an increase of the Reynolds number, and (3) the dispersibility of particle in the nanofluid becomes worse slightly with an increase of the volume fraction of the nanoparticles.