Showing papers on "Electronics cooling published in 2012"

Thermal Management of Electronics Using PCM-Based Heat Sink Subjected to Cyclic Heat Load

Abstract: Designing a heat sink based on a phase change material (PCM) under cyclic loading is a critical issue. For cyclic operation, it is required that the fraction of the PCM melting during the heating cycle should completely resolidify during the cooling period, so that that thermal storage unit can be operated for an unlimited number of cycles. Accordingly, studies are carried out to find the parameters influencing the behavior of a PCM under cyclic loading. A number of parameters are identified in the process, the most important ones being the duty cycle and heat transfer coefficient (h) for cooling. The required h or the required cooling period for complete resolidification for infinite cyclic operation of a conventional PCM-based heat sink is found to be very high and unrealistic with air cooling from the surface. To overcome this problem, the conventional design is modified where h and the area exposed to heat transfer can be independently controlled. With this arrangement, the enhanced area provided for cooling keeps h within realistic limits. Analytical investigation is carried out to evaluate the thermal performance of this modified PCM-based heat sink in comparison to those with conventional designs. Experiments are also performed on both the conventional and the modified PCM-based heat sinks to validate the new findings.

Design of Complex Structured Monolithic Heat Sinks for Enhanced Air Cooling

Abstract: The design and characterization of monolithic heat sinks, which can take the form of complex structures, is reported. The designs were conceived to augment heat transport for enhanced air cooling by exploiting clearly identified physical mechanisms, i.e., by streaming the flow through a 2-D array of polygonal ducts, by introducing flow-obstacle-induced local mixing, and by exploiting hydrodynamic instabilities to sustain flow unsteadiness. Fabrication of these unconventional designs was achieved by 3-D printing plastic patterns and converting them into monolithic copper structures by investment casting. A direct simulation approach aided by analytical solutions and experimental validation was undertaken to quantify fluid flow and heat transfer parameters. This paper concludes by quantifying the performance enhancement of the proposed heat sink geometries relative to a conventional longitudinally finned heat sink. On an equal pumping power basis, finned foam and slotted hexagonal heat sinks outperform conventional parallel plate finned heat sinks. On the other hand, the parallel plate heat sinks are better for pressure drop less than 20 Pa and slotted honeycombs are better for higher pressure drops (>;20 Pa).

Numerical Simulation of Micro-Channel Heat Sink with Dimpled Surfaces

Abstract: Problem statement: The power density of electronic devices has been increasing along with the rapid technology development. Cooling off electronic systems is essential in controlling the component temperature and avoiding any hot spot. The micro-devices are now extensively used in electronic application especially for cooling the computer chip. Approach: Micro-channel heat sinks are adopted in electronics cooling together with different technologies to enhance the heat transfer process. To improve the cooling process in heat sink, dimples are used because they are simple and cheap technologies. Therefore, this study aims to investigate the micro-channel heat sink with dimples. Results: The heat transfer enhancement in micro-channel heat sink using dimples is useful when the Reynolds number is greater than 125. Conclusion: The results of this study can help designing micro-channel heat sinks for electronics cooling by employing the concept of dimples.

Investigation and application of an advanced dual piezoelectric cooling jet to a typical electronics cooling configuration

Abstract: In recent years, electronics have significantly reduced in size at maintained or increased functionality. This trend has led to an increased demand for more capable thermal management solutions at smaller scales. However, miniaturization of conventional fan and heat sink cooling systems introduces significant size, weight and efficiency challenges. In this study the flow performance of a novel thin form-factor cooling solution, the advanced dual piezoelectric cooling jet(DCJ), is evaluated. A DCJ is a micro-fluidic device that disturbs the boundary layer over a hot component and hence increases heat transfer. The design of an equivalent fan-curve experiment is described in detail. A first ever fan curve for a bimorph DCJ device is presented. This is coupled to a thermal performance analysis using an experiment simulating thin profile consumer electronics.

Zinc Oxide Nanowire Forest for Pool Boiling Heat Transfer

Abstract: Zinc oxide (ZnO) nanowire forest is applied for the electronics cooling by means of pool boiling heat transfer. The forest was composed of lengthwise grown backbone and branched nanowires, which were highly dense and tree-like hierarchical structures. The platinum heater and resistance temperature detector were fabricated by microfabrication on one side of silicon chips and nanowire forest was synthesized on the other side. The superheat and heat flux were evaluated at steady state while the voltage in the heater was increased gradually. The heat flux gradually increased linearly with superheat at the initial stage of heating due to convective heat transfer and abruptly increased once the coolant started to boil. It seemed that the nanowire forest played a role of increased bubble nucleation sites with superhydrophilic nature leading to enhancement of boiling heat transfer. This sheds light on application of nanostructured surface as an effective electronics cooling by boiling heat transfer.

Design of the KMTNet large format CCD camera

Abstract: We present the design for the 340 Mpixel KMTNet CCD camera comprising four newly developed e2v CCD290-99 imaging sensors mounted to a common focal plane assembly. The high performance CCDs have 9k x 9k format, 10 micron pixels, and multiple outputs for rapid readout time. The camera Dewar is cooled using closed cycle coolers and vacuum is maintained with a cryosorption pump. The CCD controller electronics, the electronics cooling system, and the camera control software are also described.

Modeling of Thermal Phenomena in Liquid Cooling System for Aircraft Electric Unit

Abstract: The aim of this paper is to show the complex thermal analyses used for examining cooling properties of a liquid cooling system integrated into an aircraft fuel metering pump. The cooling is provided by flow of aviation fuel-kerosene. The main advantage of this solution is easy implementation in the place of engine bay. Two methods of thermal analyses are used according to the level of cooling system development. Thermal network analysis is intended for first estimation of thermal conditions and cooling system properties. Numerical thermal analysis based on the finite-element method is used for examination of limit operating conditions which cannot be realized during laboratory testing. The experimental measurement by an infrared camera is used for result verification and identification of thermal properties in laboratory conditions. The goal is to maintain the temperature conditions of the control electronics within the temperature limits.

Active cooling of downhole instrumentation for drilling in deep geothermal reservoirs

Abstract: To ensure efficient geothermal energy exploitation, hot and deep reservoirs have to be drilled. During the drilling process all components of a downhole drilling and evaluation system, which can be a measurement while drilling (MWD) or a wireline tool, are exposed to harsh environmental conditions like strong vibrations and temperatures up to 250°C. These high temperatures can impact the electronics or sensors of the bottom hole assembly. Therefore, for deep and hot geothermal wellbores, active cooling provides an option to extend the life time of the components and improve the reliability of the entire system. Techniques that lower the temperature inside the cooling system below the downhole temperature that exists in the wellbore are considered as active cooling methods. Thermal insulation is critical for the efficiency of an active cooling system. Phase change from solid to liquid as well as from liquid to gas provide a good compromise between cooling power, cooling time and independence from electrical power. Several active cooling methods were investigated. An example of a phase change from liquid to gas in combination with a sorption process is explained in more detail and in combination with experimental results.

Microfabrication of short pin fins on heat sink surfaces to augment heat transfer performance

Abstract: Plate-fin heat sinks have been a successful technology in electronics cooling. Thermal performance of such heat sinks, however, has been driven to improve due to increasing heat generation in modern electronics devices. This paper proposes to introduce short pin fins on surfaces of plate-fin heat sinks to address such challenges. A microfabrication approach based on photolithography and electroplating technologies is devised to fabricate short copper pin fins on copper plates. The photolithography implements desired patterns of pin fins, and the electroplating enables pin fins to directly grow out of the base plate. A series of pin-fin coupons were fabricated using the devised method. A heat transfer test was designed to evaluate heat transfer augmentation by the pin fins. Fabricated coupons were tested in a rectangular channel and their thermal conductance and channel pressure drop were measured. A Design of Experiments (DoE) procedure via the Taguchi method was employed to find the influence of four factors: pin-fin height, diameter, spacing, and cross sectional shape, on the combination of thermal conductance and channel pressure drop for the coupons of different pin-fin parameters. Compared with similar plain coupons, pin-fin coupons of the best design parameters increase the thermal conductance by 78.3 % with only 7.8% increase of channel pressure drop. The devised micro-pin-fin fabrication has been proved as an effective approach to augmenting heat transfer of air-cooled plate-fin heat sinks.

Development on thermal contact resistance

Abstract: Thermal contact resistance(TCR)is a hot topic in electronics cooling,cryogenic superconducting thin films,etc.In this paper,the fundamental research method,metrology macro-and nanotechnology,and diminishing method for TCR are reviewed.In order to investigate the heat transfer mechanism of TCR,the scattering and radiation of phonon and electron should be considered besides the quantitative analysis in macro.For the experimental measurement,the accuracy is to be further improved.For reducing the contact resistance,in addition to the commonly used methods,producing new materials with high thermal conductivity(such as carbon nano-tubes)on the contact surface is a useful way.Based on the reported studies,the future researches are discussed.

Optimized and microfabricated ionic wind pump array as a next generation solution for electronics cooling systems

Abstract: This work presents a microfabricated ionic wind pump array that has the potential to meet industry requirements as a next generation solution for thermal management of electronic systems. The optimized single device provides an improved COP of 26.5. The main purpose of the work presented here is to demonstrate that the optimized microfabricated ionic wind pump employed in an array provides superior cooling performance as compared to conventional CPU fans.

Pool Boiling of FC-87 Over Microchannel Surfaces at Atmospheric Pressure

Abstract: Advances in technology and trends towards higher processing speeds have generated a greater need for thermal management. Two-phase cooling (boiling) has the ability to dissipate large amounts of heat and is attractive because of lower mass flow requirement and uniform substrate temperature. Further improvements can be obtained through passive surface enhancements. The objective of this work is to investigate the effect of microchannel surfaces on pool boiling performance at atmospheric pressure with FC-87. Being a dielectric fluid with a low normal boiling point, FC-87 has desirable characteristics for an electronics cooling fluid. A maximum heat flux of 550 kW/m 2 at a wall superheat of 37°C was obtained with the microchannel surface. Surface area increase is noted as the primary reason for the enhanced performance for FC-87 on microchannel surfaces.

Comparison of synthetic and steady air jets for impingement heat transfer over vertical surfaces

Abstract: Natural convection air cooling is the method of choice for many low-power electronics applications due to cost, availability, and reliability considerations. This method is not only limited to low-power applications, but is also constrained by the buoyancy dependence of the flow. Therefore, further enhancement of natural convection is needed. Enhanced natural convection allows higher heat dissipation while maintaining the simplicity of passive cooling. Synthetic jet devices operating on the microfluidics principle provide unique cooling advantages for local cooling with high coefficients of performance. Synthetic jets used in the current study are piezoelectrically driven, small-scale, pulsating devices capable of producing highly turbulent jets formed by periodic entrainment and expulsion of the fluid through an orifice. The compactness of the jet actuator coupled with the high exit air velocities can significantly reduce the size of thermal management systems. In this paper, we present experimental results for impingement heat transfer for both steady and unsteady jets over a Reynolds number range of 100 to 3,000. A range of nozzle-to-plate surface distances is discussed. To mimic a comparable electronics component, we used a 25.4-mm square heated surface.

Exergy Analysis of an Absorption Refrigeration System Using an Iconic Liquid as a Working Fluid in the Chemical Compressor

Abstract: The first and second law of thermodynamics are applied to an ionic-liquid (IL) based absorption refrigeration system for high power electronics cooling. The IL is a salt in a liquid state usually with an organic cation and inorganic anion. It provides an alternative to the normally toxic working fluids used in the chemical compression loop, such as ammonia in conventional absorption systems. The use of ILs also eliminates crystallization and metal-compatibility problems of the water/LiBr system. In this study, mixtures of refrigerants and imidazolium-based ILs are theoretically explored as the working fluid pairs in a miniature absorption refrigeration system, so as to utilize waste-heat to power a refrigeration/heat pump system for electronics cooling. A mathematical model based on exergy analysis is employed to characterize the performance for specific refrigerant/IL pairs. Both the coefficient of performance (COP) and the exergetic coefficient of performance (ECOP) of the absorption system and components are evaluated. The thermodynamic properties of ILs are evaluated using the correlations based on group contribution methods. A non-random two-liquid (NRTL) model is built and used to predict the solubility of the mixtures. The properties of the refrigerants are determined using REFPROP 6.0 software. Saturation temperatures at the evaporator and condenser are set at 25 o C and 50 o C, respectively. The power dissipation at the evaporator is fixed at 100 W with the operating temperature set at 85 o C, which are the benchmark conditions for high performance microprocessor chip cooling. The desorber and absorber outlet temperatures are adjusted to evaluate the system performance variation with respect to the operating condition change. The effect of the refrigerant/IL compatibility, alkyl chain length of the IL cation, and thermodynamic properties of the refrigerants, such as latent heat of evaporation, on the ECOP is investigated. Also the exergy destruction of each component of the cycle is evaluated and discussed as a means to identify the critical component(s) of the system that would require optimization.

Development Of Microclimate Cooling Systems For Increased Thermal Comfort Of Individuals

Abstract: Miniature cooling systems have lately gained increased attention due to ever increasing needs to locally cool hot spots. Miniaturized cooling is needed in a variety of different applications, for example to cool powerful yet highly compact electronics or to increase the thermal comfort of individuals through man-mounted systems. This paper focuses on the development of components suitable to be used in miniaturized vapor compression systems. Of particular interest is the achievable cooling output to system mass ratio. Miniaturized aluminum microchannel heat exchangers, positive displacement compressors, and passive expansion devices have been designed, developed, and investigated both experimentally and numerically. Relevant performance data are presented and improvement potentials are revealed and assessed. A measured cooling capacity of 57 W at 35 C and a system mass of 2.2 kg (including power source) yields, with 26 W kg one of the highest cooling output to system mass ratios ever reported in the open literature available for miniature cooling technology. It is clear that vapor compression technology can outperform many other approaches, including cooling systems based on phase change materials with respect to cooling output per unit mass. Human subject system evaluations confirm the laboratory measurements. The tested system impressively demonstrates much slower increases of core body temperature and heart rate over time in humans experiencing high levels of physical activity in hot ambient conditions in comparison to the same test person exercising at identical activity levels, but without having a man-mounted cooling system.

Comparison of Deionized Water and FC-72 in Pool and Jet Impingement Boiling Thermal Management

Abstract: High heat fluxes and stringent constraints on surface temperature and its uniformity during thermal management of electrical and electronic components often necessitate use of boiling heat transfer. This paper compares the pool and jet impingement boiling heat transfer characteristics of deionized water and FC-72 at an equivalent fluid saturation temperature of 57°C and for identical experimental conditions. To lower the saturation temperature of water down to 57°C, experiments with water are performed at a reduced absolute system pressure of 0.176 bar. Despite the reduction in pressure, pool boiling critical heat flux with deionized water is found to be 3.6 times larger than with FC-72. Furthermore, jet impingement is seen to enhance boiling heat transport more significantly for water than for FC-72. Consequently, heat transfer coefficients during jet impingement boiling are as much as 3.9 times larger for water compared to FC-72 at identical Reynolds numbers and surface temperatures. The heat transfer advantage of using water is mainly associated with the superior thermophysical properties of this fluid. However, in addition to the large fluid saturation temperature at atmospheric conditions, direct cooling of electronics is frequently not possible using water due to the incompatibility of the fluid with electrical components. To assess the practical utility of subatmospheric deionized water through indirect cooling of electronics, a 1-D heat sink analysis is performed on a multichip module geometry. The overall thermal resistance of the heat sink using water is determined to be around two times lower than that of direct cooling of FC-72 on a silicon substrate. Under saturation condition of the working fluids, dissipation of heat fluxes in excess of ~45 W/cm2 with the multichip module using water is constrained by the chip surface temperature limit.

A sensitivity analysis of a miniature-scale linear compressor for electronics cooling using a comprehensive model.

Abstract: A comprehensive model of a linear compressor for electronics cooling was previously presented by Brads haw et al. (2011). The current study expands upon this work by first developing methods for predicting the resona nt frequency of a linear compressor and for controlling its pist on stroke. Key parameters governing compressor performance ‐ leakage gap, eccentricity, and piston geometry ‐ ar e explored using a sensitivity analysis. It is demo nstrated that for optimum performance, the leakage gap and eccentricity should be minimized. In addition, the ratio of p iston stroke to diameter should not exceed a value of one to min imize friction and leakage losses, but should be la rge enough to preclude the need for an oversized motor.