Showing papers on "Electronics cooling published in 2002"

Current and future miniature refrigeration cooling technologies for high power microelectronics

Abstract: Utilizing refrigeration may provide the only means by which future high-performance electronic chips can be maintained below predicted maximum temperature limits. Widespread application of refrigeration in electronic packaging will remain limited, until the refrigerators can be made sufficiently small so that they can be easily incorporated within the packaging. A review of existing microscale and mesoscale refrigeration systems revealed that only thermoelectric coolers (TECs) are now commercially available in small sizes. However, existing TECs are limited by their maximum cooling power and low efficiencies. A simple model was constructed to analyze the performance of both existing and predicted future TECs, in an electronic packaging environment. Comparison with the cooling provided by an existing high-performance fan shows that they are most effective for heat loads less than approximately 100 W, but that for higher heat loads, fan air cooling actually yields a lower junction temperature. Thermal resistance between the refrigerator and the chip is not as critical as the thermal resistance between the refrigerator and the ambient air.

Two-phase flow patterns, pressure drop, and heat transfer during boiling in minichannel flow passages of compact evaporators

Abstract: The small hydraulic diameters employed during flow boiling in compact evaporator passages are becoming more important in diverse applications including electronics cooling and fuel cell evaporators. The high pressure drop characteristics of these passages are particularly important as they alter the flow and heat transfer, especially in parallel multichannel configurations. The pressure drop oscillations often introduce dryout in some passages while their neighboring passages operate under single-phase mode. This article presents a comprehensive review of literature on evaporation in small-diameter passages along with some results obtained by the author for water evaporating in 1-mm-hydraulic-diameter multichannel passages. Critical heat flux is not covered in this article due to space constraints.

Design and performance evaluation of a compact thermosyphon

Abstract: Thermosyphons are a promising option for cooling of high heat dissipating electronics. In this paper, the first known implementation of a compact two-phase thermosyphon for cooling of a microprocessor in a commercial desktop computer is presented. The implemented thermosyphon involves four components in a loop: an evaporator with a boiling enhancement structure, a rising tube, a condenser and a falling tube. The performance of the thermosyphon with water and PF5060 as working fluids, and the effect of inclination are studied experimentally under laboratory conditions. Experimental observations are also made at actual operating conditions to monitor the thermal behavior with changes in power output of the microprocessor. The inside cabinet of the desktop computer is also numerically simulated to understand the airside performance of the condenser.

A Natural Circulation Model of the Closed Loop, Two-Phase Thermosyphon for Electronics Cooling

Abstract: We present a model for the two-phase flow and heat transfer in the closed loop, two-phase thermosyphon (CLTPT) involving co-current natural circulation. The focus is on CLTPTs for electronics cooling that exhibit complex two-phase flow patterns due to the closed loop geometry and small tube size. The present model is based on mass, momentum, and energy balances in the evaporator, rising tube, condenser, and the falling tube. The homogeneous two-phase flow model is used to evaluate the friction pressure drop of the two-phase flow imposed by the available gravitational head through the loop. The saturation temperature dictates both the heat source (chip) temperature and the condenser heat rejection capacity. Thermodynamic constraints are applied to model the saturation temperature, which also depends upon the local heat transfer coefficient and the two-phase flow patterns inside the condenser. The boiling characteristics of the enhanced structure are used to predict the chip temperature. The model is compared with experimental data for dielectric working fluid PF-5060

Method and arrangement for a controlling strategy for electronic components in a hybrid elecrtic vehicle

Abstract: A method for controlling a cooling arrangement for electronic components in a hybrid electric vehicle is disclosed. The method includes arranging an electronics cooling loop aboard a hybrid electric vehicle in which a DC to DC converter, DC to AC inverter, and an electronics radiator are placed in fluid communication, one with the others, for cooling the DC to DC converter and the DC to AC inverter; sensing a temperature condition in the electronics cooling loop; and discontinuing operation of the DC to DC converter when a predetermined upper threshold temperature condition is sensed in the electronics cooling loop. A voltage condition of a 12 volt battery may also be sensed, and operation of the DC to DC converter may be resumed when the voltage condition sensed falls below a predetermined lower voltage limit.

Characterization of laminar jet impingement cooling in portable computer applications

Abstract: A thermal characterization study of laminar air jet impingement cooling of electronic components within a geometry representative of the CPU compartment of a typical portable computer is reported. A finite control volume technique was used to solve for the velocity and temperature fields. Convection, conduction and radiation effects were included in the simulations. The range of jet Reynolds numbers considered was 63 to 1500; the applied compartment heat load ranged from 5-15 W. Radiation effects were significant over the range of Reynolds numbers and heat loads considered, while the effect of natural convection was only noticeable for configurations when the ratio Gr/Re/sup 2/ exceeded 5. The predicted importance of Re rather than jet size was confirmed with test data. Proof of concept was demonstrated with a numerical model representative of a full laptop computer. Both simulations and lab tests showed that low flow rate JI cooling schemes can provide cooling comparable to a high volume flow rate configuration, while using only a fraction of the air flow. Further, under the conservative assumption of steady state, fully powered components, a hybrid cooling scheme utilizing a heat pipe and laminar JI was capable of cooling the processor chip within 11 C of the vendor specified maximum temperature for a system with a total power dissipation of over 21 W.

Loop thermosyphons for cooling of electronics

Abstract: Recent innovations to loop thermosyphon (LTS) design are anticipated to have a significant impact on electronics thermal solutions. Future electronics systems including high-density desktop computers, multi-processor rack mounted servers, and telecommunications cabinets are reaching volumetric thermal densities beyond the limits of direct air-cooling. Therefore, alternative cooling solutions need to be developed, for example, using loop thermosyphons. This paper presents experimental data obtained on two innovative loop thermosyphons with capillary structures that are prime candidates for electronics cooling solutions to replace typical air-cooled systems, where commercially available heat pipes cannot be used due to the high power and transport length limitations. The performance characteristics of the two loop thermosyphons with capillary structures are discussed: (a) with a horizontal square U-tube evaporator and (b) with horizontal transport lines and a flat evaporator heated from both sides.

Air cooled heat sinks integrated with synthetic jets

Abstract: This paper discusses the development of active air-cooled heat sinks using synthetic jet ejector arrays for high power dissipation electronics. The heat sinks typically consist of a plate fin heat sink augmented with a synthetic jet module such that each fin is straddled by a pair of synthetic jets thereby creating a jet ejector that entrains cool ambient air upstream of the heat sink and discharges it into the channels between the fins. The present work characterizes three configurations of active integrated heat sinks designed for around 100 W power dissipation levels with respect to power dissipation, thermal effectiveness and package weight and volume. The flexibility of adapting synthetic jets for different heat sink designs is demonstrated by changing the relative orientation of the entrained and discharged air flow for the different cooling module designs, indicating the potential for controlling and utilizing limited air flow in spatially constrained environments. The performance of the heat sinks is assessed using an Intel thermal die instrumented with thermocouples. Using air temperature and velocity measurements the thermal effectiveness of heat sinks is compared with an off-the-shelf heat-sink-fan combination as well as with a steady flow past the heat sink. The jets generate an airflow in the range of 3-5 CFM, resulting in /spl sim/25 Watts/CFM for each device with a thermal effectiveness as high as 60-70%.

Advanced thermal architecture for cooling of high power electronics

Abstract: This paper presents a thermal architecture concept for analysis of thermal problems and solutions existing in electronics systems. The thermal problems are categorized into a total of seven levels from chip to system. Advanced thermal technologies for addressing the thermal problems at all seven levels are discussed. Integrating the thermal architecture with the electronic architecture can significantly improve the effectiveness of the thermal management.

Smart, low-cost, pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces

Abstract: Two-phase cooling of a square simulated electronic device surface of 21.3 mm side was successfully carried out without the need for a pump. This smart, passive, low-cost cooling system incorporates a self-enhancing and self-sustaining mechanism, wherein the system inherently enhances its cooling capacity by increasing the velocity of the two-phase mixture along the boiling surface when an increase in heat flux is sensed. Other practical attributes of this pumpless loop are small liquid inventory requirements and absence of the incipient boiling temperature drop. It is shown small surface tension and contact angle render dielectric coolants such as FC-72 ideally suited for flow in narrow gaps. These unique properties are responsible for very small bubble size, precluding any appreciable blockage of the replenishment liquid flow even in narrow gaps. Critical heat flux (CHF) was found to generally increase with decreasing boiler gap. CHF for flat, micro-channel (with 0.2 mm rectangular fins) and mini-channel (with 1.98 mm rectangular fins) surfaces was 4.5, 5.9, and 5.7 times greater than for pool boiling from a flat surface for corresponding gaps. A pressure drop model was formulated to predict coolant mass flow rate, boiling surface inlet and exit velocities, and pressure drop components throughout the loop. The model predictions illustrate the pumpless loop's self-sustaining and self-enhancing attributes, and relate CHF trends to those of the two-phase mixture acceleration along the boiling surface.

Experimental Investigation of an Air Microjet Array Impingement Cooling Device

Abstract: A microjet impingement cooling device for high power electronics was constructed from silicon wafers using microelectromechnical systems fabrication techniques. The array of 221, 0.277-mm-diam jets was tested using air as the coolant for jet diameter Reynolds numbers from 4.65 × 10 2 to 1.405 x 10 3 . Heat transfer and pressure drop data were obtained for a range of mass rates extending up to the point of choked flow and also for variable heat fluxes. The results were compared to an existing Nusselt correlation for jet impingement arrays that was found to significantly under-predict the heat transfer. A new correlation is provided that also accounts for variable air properties

Spray cooling at low system pressure

Abstract: Spray cooling has been shown to be the best cooling technique for applications that require efficient high heat flux removal. This research presents a study of spray cooling at low system pressure that allows control of the surface temperature by controlling the boiling point of the fluid. Such cooling provides an attractive, low-temperature thermal control to industries such as electronics, avionics, lasers, and electro-optics. Experiments were conducted with a 1 cm/sup 2/-heated surface cooled by a spray nozzle at reduced system pressures. System pressures range from 0.015 to 1 bar. The heat transfer coefficient was determined by measuring the surface temperatures at increasing heat fluxes. The present study has shown that spray cooling with water at reduced system pressure can significantly decrease the surface temperature as compared to ambient pressure spray cooling. This study shows that at a pressure of 0.015 bar, the surface temperature can be cooled to 54/spl deg/C with 11/spl deg/C liquid at a heat flux of 380 W/cm/sup 2/, while the surface temperature at 380 W/cm/sup 2/ is about 120/spl deg/C with same liquid temperature at ambient pressure. The heat transfer coefficients measured here were on the order of 100,000 W/m/sup 2//spl middot/K. This study is limited by the heating source, which cannot exceed 400 W/cm/sup 2/. The maximum heat flux that can be removed by spray cooling at low pressure has yet to be determined.

High performance liquid-cooled heat sink with twisted tape inserts for electronics cooling

Abstract: A liquid-cooled heat sink for use in combination with a heat exchanger to cool electronic or electrical devices attached to said heat sink comprises a metal block having a plurality of circular passageways therethrough defined by cylindrical walls. An inlet is in fluid communication with one end of the passageways for receiving a cooling liquid, and an outlet is in fluid communication with an opposite end of the passageways for outputting the cooling liquid. At least one tape insert is mounted within at least one of the circular passageways, wherein the tape insert longitudinally bisects the circular passageway and further wherein edges of the tape insert abut cylindrical wall.

Reliability of Heat Sink Optimization Using Entropy Generation Minimization

Abstract: The design and optimization of electronics cooling systems is not easily accomplished through conventional analysis tools such as analytical and numerical methods The enhancement in thermal performance causes a rise in pressure drop, which increases the load of pumping power Consequently, the overall assessment of cooling system requires a trade-off between thermal performance and pressure drop Entropy generation minimization (EGM) method is based on the theory that a thermodynamically optimized system is the least irreversible, or minimum entropy generation in the system EGM method has been used as an optimization tool in thermodynamic systems for a broad range of engineering fields, but its reliability and accuracy has not been investigated In this study, the fin pitch of a plate fin heat sink in free convection environment was optimized by EGM method Results were compared with both analytical and numerical optimization results The EGM method predicted slightly higher fin pitch than the analytical and CFD methods but with the same trend The discrepancy between the analytical and EGM method was within 7-12%, while that between the CFD and EGM method was 5-9% With this small discrepancy, it was concluded that the EGM method was very reliable to be used in the fin pitch optimization of the plate fin heat sink subject to free convection

Fouling of high density heat sinks - theoretical origins and numerical analysis

Abstract: Heat sinks and other components must operate in a thermal envelope that calls for continuous operation over long periods of time. Increasing thermal loads are resulting in widespread acceptance of densely packed heat sinks with closely spaced fins. However, the environment surrounding heat sinks and other electronic components is not ideal. Particulate contaminants are present in the ambient supply air in nearly every office, home or other zone of operation. These contaminants are ingested by the cooling systems and subsequently introduced to these heat sinks. Repeated exposure to particulate contamination can lead to the phenomena known as thermal fouling, where contaminant particles adhere themselves to the surface of the heat sink, acting as an insulating layer and thus reducing thermal performance. This analysis seeks to quantify the reduction in performance experienced by a dense pin fin array exposed to a contaminated operating environment. Results of numerical simulations illustrate the potentially drastic effects on fin effectiveness and overall airflow rate in a uniformly coated, fouled array. The fundamental theories of particulate adhesion are discussed and an experimental course of action for verification of the reduction in performance owing to fouling is suggested.

Numerical simulation of phase change heat transfer in PCM-encapsulated heat sinks

Abstract: The enthalpy-based computational model is developed for analysing PCM-encapsulated heat sinks for electronics chips. Solution is obtained by developing a control volume-based finite difference code and results are validated by comparing results with that given by analytical solution available for a limiting case problem. Preliminary results based on a parametric study indicate the two-dimensional code developed for this study can be used in evaluating PCM, and selecting geometrical dimensions of the PCM encapsulated heat sink.

Getting the most out of your CFD program [for electronics cooling]

Abstract: Modern CFD codes dealing with air-cooling of electronics components have evolved to the stage where, properly handled, they can produce useful and fairly reliable estimates of coolant flow rates and component temperatures. For the most part, the outputs of these codes are "maps" of flow and temperature distribution. Useful as they are for identifying potential problems, these maps provide little or no guidance as to how to solve the problems they reveal. This paper proposes a set of diagnostic parameters that should be calculated for each critical component. From these parameters, a heat transfer person can tell not only whether an overheating problem exists but what is the most likely cause: low coolant flow, thermal stratification in the coolant, excessive heat conduction or radiation from hot neighbors, or low heat transfer coefficient. The recommended parameters are: (1) operating temperature, (2) adiabatic temperature, (3) mixed-mean temperature of the coolant, (4) net radiation and conduction input, and (5) the heat transfer coefficient, hadiabatic, and (6) the pressure drop. These can all be obtained without undue effort either by integration of values already available over appropriate areas or by "freezing" the flow calculation and sequentially changing the power distribution (relying on the linearity of the energy equation and the fact that hadiabatic does not change when the temperature distribution changes). The principle computational requirement for implementing this method is to develop a method for "freezing" the flow field and sequentially calculating the operating temperatures for different thermal boundary conditions. Techniques for doing this have been discussed by two previous authors. It remains only to complete the task and use that approach to calculate the diagnostic parameters.

Gap-reduced thermal paste package design for cooling single flip-chip electronic modules

Abstract: A method of reducing the thermal paste chip-to-cap interface gap is presented to achieve enhanced cooling of single flip-chip electronic modules. The structure and assembly process steps of the gap reduction design are described. The thermal reliability of the design is evaluated by measuring the thermal resistance for several permutations of the structural design variables, allowing identification of an optimum design configuration.

CAD-based methods for thermal modeling of coolant loops and heat pipes

Abstract: As air cooling of electronics reaches the limits of its applicability, the next generation of cooling technology is likely to involve heat pipes and single- or two-phase coolant loops. These technologies are not suitable for analysis using 2D/3D computational fluid dynamics (CFD) software, and yet the geometric complexities of the thermal/structural models make network-style schematic modeling methods cumbersome. This paper describes CAD line-drawing methods to quickly generate 1D fluid models of heat pipes and coolant loops within a 3D thermal model. These arcs and lines can be attached intimately or via lineal contact or saddle resistances to plates and other surfaces, whether those surfaces are modeled using thermal finite difference methods (FDM) or finite element methods (FEM) or combinations of both. The fluid lines can also be manifolded and customized as needed to represent complex heat exchangers and plumbing arrangements. To demonstrate these concepts, two distinct examples are developed: a copper-water heat pipe, and an aluminum-ammonia loop heat pipe (LHP) with a serpentined condenser. A summary of the numerical requirements for system-level modeling of these devices is also provided.

Microscale technology electronics cooling overview

Abstract: NASA requirements and subsequent technology solutions for high heat flux electronics are generally different that those for the terrestrial applications. Unlike terrestrial operations. NASA spacecraft have limited opportunities for air cooling, for example, and must rely on less efficient thermal radiation to reject heat to space. The terrestrial commercial electronics industry, as well as other Government agencies, is investing in advanced technologies for electronics cooling at the microscale. This paper gives a brief summary of metrics used in high heat flux electronics cooling, the difference between solutions developed for terrestrial requirements and those for space, and a short description of challenges as well as possible solutions for space-based high heat flux electronics cooling. The argument is made that high heat flux electronics cooling is indeed a core technology required by NASA, since the thermal and other environmental requirements are unique to NASA space missions and are not addressed ...

Use of shell conduction plates for compact models of extruded heat sinks in forced convection environments

Abstract: Shell conduction plates (thin conducting plates) are computational fluid dynamics (CFD) objects that may be used to model three-dimensional conduction in thin, plate-like objects used in electronic cooling applications. The use of zero thickness conducting plates for fins of extruded heat sinks can result in significant reduction in mesh elements when compared to an equivalent case using thick fins. Shell conduction plates model three dimensional conduction in a solid as well as viscous stresses at a fin surface accurately. However, they do not account for the flow impedance caused by the thickness of the actual plates. Therefore, volumetric resistances with appropriate flow loss coefficients derived from existing channel flow correlations were used to account for the flow blockage caused by the fins. The validity of using such a hybrid approach to model extruded heat sinks was studied using Icepak, a CFD software for electronics cooling applications. Results suggest that the present approach can be used to reduce the cost of computational analysis while maintaining accuracy. The above approach resulted in savings in CPU requirements by a factor of approximately 4 to 5. For the cases considered, the grid sizes were reduced by approximately 60%.

Numerical study of mixed-convective cooling of a rectangular cavity with differentially heated side walls

Abstract: A numerical study is conducted to investigate mixed-convective cooling of a two-dimensional rectangular cavity with differentially heated vertical sidewalls. The horizontal walls are assumed to be adiabatic. Cold air enters the cavity through the left vertical wall and exits from the right vertical walls. A finite volume based computational procedure with collocated placement of the variables is used in the calculations. The algebraic equations are solved using the strongly implicit procedure. Six different configurations for various inlet and outlet placements have been tested for a range of Reynolds number and Richardson number. The aim of the study is to optimize the relative location of the air inlet and outlet for maximum heat transfer and minimum average temperature inside the cavity. The results have been presented in the form of thermal and dynamic fields. Influence of the Reynolds number, Richardson number, and location of inlet and outlet openings on the flow field and thermal field has been discussed. Maximum cooling efficiency, minimum average temperature, and maximum Nusselt number at the hot surface is obtained when the inlet is at the bottom of the cold wall and outlet is at the top of hot wall.

Thermal Management Research for Power Generation. Delivery Order 0002 - Volume 1: Plain Fin Array Cooler for Electronics Cooling

Abstract: : A fin array cooler was developed to cool a substrate of high-heat flux electronics. Plain copper fin strips were soldered onto the substrate to minimize the contact thermal resistance between the electronics and heat sink. Two new types of fin arrays based on offset fin strips and aligned fin strips were employed in order to mitigate the thermal stress problem found in the integral finned substrate concept. The cooler with different fin strip layouts was tested using polyalphaolefin as the coolant for flow Reynolds number variation from 53 to 482. The fin strip gaps of 0.13, 0.38, and 1.0 mm were experimented. Heat transfer data for different fin strip layouts were obtained under various operating conditions and compared. It was shown that, in general, the heat transfer coefficient was 29 to 36 percent higher for the offset fin strip layout than for the aligned fin strip layout. New heat transfer correlations for offset fin strip layout and aligned fin strip layout are presented.