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REVIEW AND DEVELOPMENT OF ELECTRONIC COOLING TECHNOLOGY AND APPLICATION GUIDE1
Abstract
title>ABSTRACT
The cooling load required to cool power electronics in modern combat vehicles is continuously increasing due to the rise of power consumption and the high thermal load of current theaters of operation. Advances in semiconductor technology have led to increased power dissipation from electronic chips while reducing their size. This has resulted in increased demand to remove the heat from these chips in order to prevent them from overheating leading to failure. In many circumstances, electronics cooling is provided through enhanced ventilation or air-handling units of the vehicle interior. However, the military has unique requirements for achieving thermal management of electronic devices such as Line Replaceable Units (LRUs). Some of these requirements include low weight, high efficiency, and reliability. Therefore, it is important that critical electronics be cooled effectively using conventional and/or advanced cooling technologies.
The primary objective of this work is to develop a Line Replaceable Unit Cooling Technology (LRUCT) summary database and application guide of current and future cooling technologies, and to assess readiness level of each technology, location/situation to be used, and characteristics. The guide will allow rapid development of LRU cooling concepts and to assist in the evaluation of the different cooling methods that best fit a specific application; thus, supporting system and subsystem level selection and LRU designs.</p
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This paper presents results of tests carried out to investigate the potential application of heat pipes and phase change materials for thermoelectric refrigeration. The work involved the design and construction of a thermoelectric refrigeration prototype. The performance of the thermoelectric refrigeration system was investigated for two different configurations. The first configuration employed a conventional heat sink system (bonded fin heat sink) on the cold side of the thermoelectric cells. The other configuration used an encapsulated phase change material in place of the conventional heat sink unit. Both configurations used heat pipe embedded fins as the heat sink on the hot side. Replacement of the conventional heat sink system with an encapsulated phase change material was found to improve the performance of the thermoelectric refrigeration system. In addition, it provided a storage capability that would be particularly useful for handling peak loads and overcoming losses during door openings and power-off periods. Results showed that the heat sink units employing heat pipe embedded fins were well suited to this application. Results also showed the importance of using a heat pipe system between the cold junction of the thermoelectric cells and the cold heat sink in order to prevent reverse heat flow in the event of power failure.
Sorption heat pipe (SHP) combines the enhanced heat and mass transfer in conventional heat pipes with sorption phenomena in the sorbent bed. SHP consists of a sorbent system (adsorber/desorber and evaporator) at one end and a condenser + evaporator at the other end. It can be used as a cooler/heater and be cooled and heated as a heat pipe. SHP is suggested for space and ground application, because it is insensitive to some “g” acceleration. This device can be composed of a loop heat pipe (LHP), or capillary pumped loop (CPL) and a solid sorption cooler. The most essential feature is that LHP and SHP have the same evaporator, but are working alternatively out of phase. SHP can be applied as a cryogenic cooler, or as a fluid storage canister. When it is used for cryogenic thermal control of a spacecraft on the orbit (cold plate for infrared observation of the earth, or space), or efficient electronic components cooling device (lased diode), it is considered as a cooler. When it is applied as a cryogenic storage system, it insures the low pressure of cryogenic fluid inside the sorbent material at room temperature.
This paper presents results of tests carried out to investigate the potential application of phase change materials (PCMs) integrated with thermosyphons in a thermoelectric refrigeration system. The work involved design, fabrication and test of a 150 W thermoelectric refrigeration system. The system was first fabricated and tested using a conventional heat sink system (bonded fin heat sink system) at the cold heat sink. In order to improve the performance and the storage capability, the system was reconstructed and tested using an encapsulated PCM as a cold sink. Results of tests of the latter system showed an improved performance compared with the former system. However to improve the storage capability, in particular during off-power periods, it was found necessary to integrate the PCM with a thermal diode, which would allow heat flow in one direction only. Results of tests carried out on the new system showed considerable improvement in the storage capability of the thermoelectric refrigeration system compared with the previous ones. Overall the system suits operation with renewable energies, e.g., solar energy.
The paper is devoted to the development of miniature loop heat pipes (mLHPs) with a nominal capacity of 25–30 W and a heat-transfer distance up to 250 mm intended for cooling electronics components and CPU of mobile PC. It gives the results of investigating several prototypes of mLHPs incorporated into remote heat exchanger (RHE) systems in different conditions. It has been established that in the nominal range of heat loads orientation does not practically affect the mLHPs operating characteristics. Under air cooling the total thermal resistance of such a system is 1.7–4.0 °C/W and depends strongly on the cooling conditions and the radiator efficiency. In this case the mLHP’s own thermal resistance is in the limits from 0.3 to 1.2 °C/W, and the maximum capacity reaches 80–120 Bt. The obtained results make it possible to regard mLHPs as quite promising devices for RHE systems providing thermal regimes for electronics components and personal computers.
A hybrid heat sink concept which combines passive and active cooling approaches is proposed. The hybrid heat sink is essentially a plate fin heat sink with the tip immersed in a phase change material (PCM). The exposed area of the fins dissipates heat during periods when high convective cooling is available. When the air cooling is reduced, the heat is absorbed by the PCM. The governing conservation equations are solved using a finite-volume method on orthogonal, rectangular grids. An enthalpy method is used for modeling the melting/re-solidification phenomena. Results from the analysis elucidate the thermal performance of these hybrid heat sinks. The improved performance of the hybrid heat sink compared to a finned heat sink (without a PCM) under identical conditions, is quantified. In order to reduce the computational time and aid in preliminary design, a one-dimensional fin equation is formulated which accounts for the simultaneous convective heat transfer from the finned surface and melting of the PCM at the tip. The influence of the location, amount, and type of PCM, as well as the fin thickness on the thermal performance of the hybrid heat sink is investigated. Simple guidelines are developed for preliminary design of these heat sinks.
An analytical approach has been employed to study liquid flows in an isotropic wick structure of a flat plate heat pipe with multiple heat sources so as to determine the optimum heat pipe performance. In this study, the heat sources have been modeled as point sources using the Dirac Delta function to describe the heat distribution. The two dimensional pressure and velocity distribution are shown and discussed. The study has been extended to locate the positions of the multiple heat sources for optimum heat pipe performance. The optimum performance of the heat pipe is accomplished when the minimum pressure drop is attained across the wick structure.
Electronic device thermal management improvements are critical to continued increases in computing performance. Thermoelectric coolers (TECs) show promise in meeting this need. This paper compares the performance of Bismuth Telluride (BiTe) in three different forms: thin film, bulk, and a hypothetical nanostructured bulk material. This hypothetical materials is based on recent experimental demonstrations in Lead Telluride. Performances of the TEC based on the three different BiTe forms are compared with respect to heat pumping capacity and optimum thickness. The simulations are based on 1-D models that include the effects of thermal and electrical contact resistance. Simulated results show a significant enhancement in maximum heat pumping capacity of ‘Nanostrutured-Bulk TEC’. Modified definitions of heat pumping capacity and coefficient of performance (COP) are proposed for evaluating TECs used to cool objects with temperatures above ambient temperature. It is observed that the appropriate heat sink selection is key factor for achieving the improved TEC performance while maintaining minimum thickness.
The sorption heat pipe (SHP) is a new heat transfer device, which can be used as a sorption cooler or as a heat pipe. The SHP has a sorbent bed (adsorber/desorber and evaporator) at one end and a condenser+evaporator at the other end. This device is insensitive to some “g” acceleration and could be suggested for space and ground application. The most crucial feature of this device is that in different cases it can be used, for example, as a loop heat pipe, because they have the same evaporator and condenser, or as a SHP. The SHP can be used also as a cryogenic cooler. The SHP is convenient for cryogenic fluid storage, when the system does not work at low pressure and room temperature, and for use in the active cryogenic thermal control systems of spacecraft in orbit (cold plates for infrared observation of the Earth or space), or as an efficient electronic component cooling device.
Experiments were performed to study the fully developed turbulent flow heat transfer and friction factor characteristics of shrouded, rectangular cross-sectioned longitudinal fin arrays with uninterrupted fins subjected to a uniform heat flux boundary condition at the fin base. The inter-fin spacing was varied from 9.3 to 53 mm, the fin height from 0 to 40 mm, and the fin tip-to-shroud clearance from 0 to 20 mm. The overall height and width of the duct (flow passage) were held constant at 40 mm and 228.6 mm, respectively. The working fluid was air, and tests were conducted for Reynolds numbers ranging from 2000 to 50,000. The equivalent diameter, defined as four times the flow area divided by the wetted perimeter, was taken as the characteristic dimension. In all, seven different fin configurations and the rectangular duct configuration with no fins were investigated. The heat transfer tests showed that the Petukhov-Popov equation is applicable for the rectangular duct configuration with no fins and that the Dittus-Boelter equation is applicable for the configurations with no clearance. The Dittus-Boelter equation is also applicable for fin configurations with clearance if the clearance-to-spacing ratio is small or the spacing-to-fin height ratio is large. Lower values of the Nusselt number were obtained for the fin configuration that did not satisfy the above conditions. As expected, the pressure drop tests showed that the Prandtl equation for evaluating friction factor is applicable for the rectangular duct configuration with no fins as well as for the fin configurations with no clearance. The configurations with clearance yielded friction factor values different from the Prandtl equation. These values have been correlated.
Blood cells, vaccines, and other drugs and biological products should be kept at a temperature within certain range. In this paper, a medical thermoelectric cooling kit is introduced. The kit keeps medical products at temperatures between 6 and 10 °C while being stored or transported. This range is within the standards recommended by WHO to keep biological, medical and food products. Tests were carried out under various environmental temperatures in laboratory conditions as well as outside the laboratory and on a transportation vehicle. Under all these conditions, the kit provided cooling at a satisfactory level. This paper presents the technical specifications of the kit, theory of its operation, and test results. The kit is supported with a microprocessor unit. The block diagram of the microprocessor program is also presented in the paper.
This work reports the laboratory test results of a Li-ion battery designed for electric scooter applications. Four different modes of heat dissipation were investigated in this experimental study: (1) natural convection cooling; (2) presence of aluminum foam heat transfer matrix; (3) use of phase change material (PCM); and (4) combination of aluminum foam and PCM. The objective of using the PCM is to lower the temperature rise of the Li-ion cells and create a uniform temperature distribution in the battery module. This is clearly justified looking at the experimental results presented in this work. The use of high thermal conductivity aluminum foam in the voids between the cells reduces the temperature rise of the Li-ion cells but is insufficient when operated in high ambient temperature such as those usually occur in summer. The use of aluminum foam with PCM causes a significant temperature drop of about 50% compared to the first case of no thermal management. It also provides uniform temperature distribution within the battery module, which is important for the efficient performance of the cells used. The laboratory results were modeled using a 2-D thermal model accounting for the four different modes of heat dissipation and good agreement was obtained between the simulation and experimental results.
In this paper, an experimental investigation was performed to study the heat transfer performance of metal foam heat sinks of different pore densities subjected to oscillating flow under various oscillatory frequencies. The variations of pressure drop and flow velocity along the kinetic Reynolds number of oscillating flow through aluminum foams were compared. The measured pressure drops, velocities and surface temperatures of oscillating flow through aluminum 10, 20 and 40 PPI foams were presented in detail. The calculated cycle-averaged local temperature and Nusselt number for different kinetic Reynolds numbers were analyzed and compared with finned heat sinks. The results of length-averaged Nusselt number for both oscillating and steady flows indicate that higher heat transfer rates can be obtained in metal foams subjected to oscillating flow. For the purpose of designing a novel heat sink using metal foam, the characteristics of the pumping power of the cooling system for aluminum foam with different pore densities were also analyzed.
The goal of this analysis has been the search for the optimal configuration for a finned plate (with rectangular and vertical fins) to be cooled in natural convection. Utilizing a simplified relation of the fins heat exchange some simple expressions for the determination of the optimum value of the fins spacing have been developed as a function of the parameters which feature in the configuration: dimensions, thermal conductivity, fins absorption coefficient and fluid thermo-physical properties.
A cooling device using stacked centrifugal fans and circular heat sinks was designed for cooling a semiconductor chip with a heat flux near 125 W/cm(2). In this device, heat is conducted from the chip to a copper heat distribution block and then distributed to multiple heat sinks via four heat pipes. The copper block with embedded heat pipes or evaporator block was optimized using finite element analysis, and several cases were validated with experimental data. The experiments showed great benefits by having a second fan/heat sink in the device. The copper block by itself was found to contribute more than half of the overall thermal resistance of the cooling device. The thermal spreading resistance of the block can be reduced by about 70% if a piece of high-conductivity material, such as a diamond-copper composite, is inserted into its base. The thermal spreading resistance is generally lower when the thickness of the high-conductivity base piece increases. However, the analysis shows that the benefit of using a high-conductivity base tapers off as the thickness of the base piece nears the diameter of the heat pipes (delta(*)=1) and weakly worsens after (delta(*)>1). The base will also not have a benefit when the size of the chip approaches that of the copper block (A(*)=1).
Computational modeling, temperature measurements, and flow visualizations were conducted to investigate the enhancement of combined natural convection, conduction, and radiation heat transfer from a 25.4mm×25.4 mm discrete heat source in a 127mm×127mm and 41.3 mm high enclosure, using pin–fin heat sinks. The pin–fin array was attached to the heated component, which was flush mounted on the enclosure base. Effects of flow confinement by enclosure walls on heat transfer from the pin–fin heat sink were found prominent. The existing natural convection heat transfer correlations for pin–fin arrays in free space showed large deviations from present measurements. Radiation was also found to contribute significantly to the overall heat transfer. Experimental results show that enhancement of heat transfer using pin–fin heat sinks was significantly different for horizontally and vertically heated orientations of the enclosure.
An experimental investigation was conducted to determine the thermal behavior of arrays Of Micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels 45 mum wide and 80 um deep and the other that used an anisotropic etching process to produce triangular channels 120 mum wide and 80 mum deep. Once fabricated, a clear pyrex co ver plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtainedfor a plain silicon wafer and indicated that incorporating an array of micro heat pipes as an integral part of semiconductor devices could significantly increase the effective thermal conductiuity; decrease the temperature gradients occurring across the wafer, decrease the maximum wafer temperatures; and reduce the number and intensity of localized hot spots. A t an input power of 4 W, reductions in the maximum chip temperature of 14.1-degrees-C and 24.9-degrees-C and increases in the effective thermal conductivity of 31 and 81 percent were measuredfor the machined rectangular and etched triangular heat pipe arrays, respectively. In addition to reducing the maximum wafer temperature and increasing the effective thermal conductivity, the incorporation of the micro heat pipe arrays was found to improve the transient thermal response of the silicon test wafers significantly.
A computer model was developed to aid in the design of a flexible bellows heat pipe for cooling small discrete heat sources or arrays of small heat sources. This model was used to evaluate the operational characteristics and performance limitations of these heat pipes and to formulate and optimize a series of conceptual designs. Three flexible bellows heat pipes approximately 40 mm in length and 6 mm in diameter were constructed and tested using three different wick configurations. The test pipes were found to be boiling limited over most of the operating temperature range tested. Heat fluxes in excess of 200 W/cm{sup 2} were obtained and thermal resistance values of less than 0.7C/W were measured. Although the computer model slightly underestimated the experimentally determined transport limit for one of the wicking configurations, the remaining transport predictions were consistently within 8% of the experimental values.
Boiling jet impingement cooling is currently being explored to cool power electronics components. In hybrid vehicles, inverters are used for DC–AC conversion. These inverters involve a number of insulated-gate bipolar transistors (IGBTs), which are used as on/ off switches. The heat dissipated in these transistors can result in heat fluxes of up to 200 W/cm 2 , which makes the thermal management problem quite important. In this paper, turbulent jet impingement involving nucleate boiling is explored numerically. The framework for these computations is the CFD code FLUENT. For nucleate boiling, the Eulerian multiphase model is used. The numerical results for boiling water and R113 jets (submerged) are validated against existing experimental data in the literature. Some representative IGBT package simulations that use R134a as the cooling fluid are also presented.
In this paper, we present a new development of loop heat pipe (LHP) technology in its applications to cooling systems for high-power IGBT elements. An advanced method of LHP evaporator wick manufacturing has been proposed. Following this approach, a 16 mm outer diameter and 280 mm-length LHP evaporator was designed and manufactured. Nickel and titanium particles were used as raw material in LHP evaporator wick fabrication. LHP with a nominal capacity as high as 900 W for steady-state condition and more than 900 W for a periodic mode of operation at a temperature level below 100 °C and a heat transfer distance of 1.5 m was designed through the cooling of a high-power electronic module. An experimental program was developed to execute LHP performance tests and monitor its operability over a span of time. An investigation of the effects of LHP performance of parameters such as evaporator and condenser temperatures and LHP orientation in a gravity field was brought about. As regards the results of this initial series of tests, it was found that LHP spatial orientation within the nominal range of heat loads has no drastic effect on overall LHP functioning, whereas condenser temperature does play an important role, especially in the range of heat load close to critical. A 2D nodal model of the evaporator was developed and provides us with confirmation of the suggestion that when high-power dissipation levels are available, low wick conductivity is well adapted for LHP applications.
FC-72 and HFE-7100 dielectric liquids are favored for immersion cooling applications of computer chips. This paper reports the results of series of experiments that systematically investigated nucleate boiling of both FC-72 and HFE-710 on a plain porous graphite surface measuring 10 × 10 mm. The obtained values of the nucleate boiling heat transfer coefficient and critical heat flux (CHF), at different surface orientations, 0° (upward facing) to 180° and liquid subcooling up to 30 K are compared. In addition to the absence of temperature excursions prior to boiling incipience, the CHF increases linearly with increased liquid subcooling but at a rate that is ∼10% higher for HFE-7100 than that for FC-72. The CHF for HFE-7100 is typically 10% to 15% higher than that for FC-72, however, the maximum nucleate boiling heat transfer coefficient, occurring at or near the end of the fully developed nucleate boiling region, is ∼57% higher for FC-72 than that for HFE-7100 and increases also linearly with increased liquid subcooling. The CHF correlation developed accounts for the effects of surface characteristics (smooth, porous, or micro-porous), liquid properties and subcooling, and surface inclination and fits present and reported values by other investigators to within ±10%.
The search and selection for a suitable thermoelectric cooler (TEC) to optimize a cooling system design can be a tedious task as there are many product ranges from several TEC manufacturers. Although the manufacturers do provide proprietary manuals or electronic search facilities for their products, the process is still cumbersome as these facilities are incompatible. The electronic facilities often have different user interfaces and functionalities, while the manual facilities have different presentations of the performance characteristics. This paper presents a methodology to assist the designer to size and select the TECs from different manufacturers. The approach will allow designers to find quickly and to evaluate the devices from different TEC manufacturers. Based on the approach, the article introduces a new operational framework for an Internet based thermoelectric cooling system design process that would promote the interaction and collaboration between the designers and TEC manufacturers. It is hoped that this work would be useful for the advancement of future tools to assist designers to develop, analyze and optimize thermoelectric cooling system design in minimal time using the latest TECs available on the market.
Experiments are conducted which investigated enhancements in nucleate boiling of FC-72 dielectric liquid on porous graphite and compared results with those on a smooth copper surface of the same dimensions (10 × 10 mm). Also investigated is surface temperature excursion at boiling incipience and the obtained values of CHF are compared with those of other investigators on copper, silicon, and micro-finned silicon surfaces and micro-porous coatings. Results showed no temperature excursion at boiling incipience on porous graphite but as much as 14.0 K in saturation and 9.0–13.3 K in subcooled boiling on copper. The nucleate boiling heat transfer coefficients on porous graphite are significantly higher than on copper and the values of CHF (27.3, 39.6, 49.0, and 57.1 W/cm2 in saturation and at ΔTsub = 10, 20, and 30 K, respectively) are 63–94% higher than on copper (16.9, 21.9, 26.9, and 29.5 W/cm2, respectively). The surface superheats at CHF on porous graphite (11.0 K in saturation and 17.2–19.5 K in subcooled boiling) are lower than on copper (21.3 K and 22.9–24.9 K, respectively). The nucleate boiling heat transfer coefficient increase with subcooling and CHF increased linearly with ΔTsub. The subcooling coefficient of CHF on porous graphite (0.041) is slightly smaller than those on micro-porous coatings (0.044 and 0.049) but much higher than those on micro-finned silicon surfaces (0.022, 0.036), and on Cu, Si, and enhanced SiO2 (0.018) surfaces.
Experiments are performed, which investigated the effect of inclination angle, θ, on saturation pool boiling of HFE-7100 dielectric liquid from a smooth, 10×10 mm copper surface, simulating a microelectronic chip. For θ⩽90° and surface superheats, ΔTsat>20 K, nucleate boiling heat flux decreases with increased θ, but increases with θ for ΔTsat<20 K. Similarly, at higher inclinations and ΔTsat>13 K, nucleate boiling heat flux decreases with increased inclination, but at lower surface superheats the trend is inconclusive. The developed nucleate boiling correlation is within ±10% of the data and the developed correlations for critical heat flux (CHF) and the surface superheat at CHF are within ±3% and ±8% of the data, respectively. Results show that CHF decreases slowly from 24.45 W/cm2 at 0° to 21 W/cm2 at 90°, then decreases fast with increased θ to 4.30 W/cm2 at 180°. The surface superheat at CHF also decreases with θ, from 31.7 K at 0° to 19.9 K at 180°. Still photographs are recorded of pool boiling at different heat fluxes and θ=0°, 30°, 60°, 90, 120°, 150° and 180°. The measured average departure bubble diameter from the photographs taken at the lowest nucleate boiling heat flux of ∼0.5 W/cm2 and θ=0° is 0.55±0.07 mm and the calculated departure frequency is ∼100 Hz.
A unique process for the fabrication of high-thermal-conductivity carbon foam was developed at Oak Ridge National Laboratory (ORNL). This process does not require the traditional blowing and stabilization steps and therefore is less costly. The resulting foam can have density values of between 0.2 and 0.6 g/cc and can develop a bulk thermal conductivity of between 40 and 180 W/m K. Because of its low density, its high thermal conductivity, its relatively high surface area, and its open-celled structure, the ORNL carbon foam is an ideal material for thermal management applications. Initial studies have shown the overall heat transfer coefficients of carbon foam-based heat sinks to be up to two orders of magnitude greater than those of conventional heat sinks.
Piezoelectric fans are thin elastic beams whose vibratory motion is actuated by means of a piezoelectric material bonded to the beam. These fans have found use as a means to enhance convective heat transfer while requiring only small amounts of power. The objective of the present work is to quantify the influence of each operational parameter and its relative impact on thermal performance. Of particular interest are the vibration frequency and amplitude as well as the geometry of the vibrating cantilever beam. The experimental setup consists of a piezoelectric fan mounted normal to a constant heat flux surface. Temperature contours on this surface captured via an infrared camera are used to extract the forced convection coefficient due to the fluid motion generated from the fan. Different fans, with fundamental resonance frequencies ranging from 60 to 250 Hz, are considered. Results show that the performance of the fans is maximized at a particular value of the gap between the fan tip and the heated surface. It is found that when a fan operates at this optimum gap, the heat transfer rate is dependent only on the frequency and amplitude of oscillation. Correlations based on appropriately defined dimensionless parameters are developed and found to successfully predict the thermal performance across the entire range of fan dimensions, vibration frequency and amplitude. An understanding of the dependence of thermal performance on the governing variables allows for improved design of piezoelectric fans as a method of enhancing heat transfer.
In this study, micro-heat pipe arrays etched into silicon wafers have been investigated for electronic cooling purposes. Micro-heat pipes of triangular cross-section and with liquid arteries were fabricated by wet anisotropic etching with a KOH solution. The microchannels (230 μm wide) are closed by molecular bonding of a plain wafer with the grooved one. A test bench was developed for the micro-heat pipe filling and the thermal characterisation. The temperature profile on the silicon surface is deduced from experimental measurements. The results show that with the artery micro-heat pipe array, filled with methanol, the effective thermal conductivity of the silicon wafer is significantly improved compared to massive silicon.
An analytical solution for both the liquid and vapour flows inside a flat micro heat pipe (MHP) coupled to an analytical solution for the temperature inside the MHP wall is presented. The maximum heat transfer capability of a flat MHP, on which several heat sources and heat sinks are located, is calculated. The capillary structure inside the MHP is modeled by considering a porous medium, which allows to take into account capillary structures such as meshes or sintered powder wicks. The thermal model is able to calculate the part of heat flux transferred only by heat conduction in the MHP wall from the heat transferred by change of phase.
An experimental study was conducted on the cooling of portable hand-held electronic devices using n-eicosane as the phase change material (PCM) placed inside heat sinks with and without internal fins. The effects of the PCM, number of fins, orientation of the device, and the power level (ranging from 3 to 5 W), on the transient thermal performances were investigated under frequent, heavy and light usage conditions. The results indicated that PCM-based heat sinks with internal fins are viable options for cooling mobile devices but the effectiveness of the approach may require optimization with respect to the amount of PCM used, the number of fins, the power level of the heat source, and the usage mode of the device.
Pin fins are widely used as elements that provide increased cooling for electronic devices. Increasing demands regarding the performance of such devices can be observed due to the increasing heat production density of electronic components. For this reason, extensive work is being carried out to select and optimize pin fin elements for increased heat transfer. In the present paper, a procedure is described to select heat exchanger surfaces with pin fins in accordance with their location in a performance diagram. Such a diagram provides performance comparisons of pin fins with respect to two operating parameters: the heat transfer rate per unit base surface area and the power input for the same area. It is shown that elliptical cross-sections offer the best performance compared with all other investigated cross-sections for pin fins. The present work demonstrates that the heat exchanger performance plot allows also the selection of the best elliptical cross-section design within the initial design set (design set obtained numerically) in analogy with the Pareto-optimality approach. However, the real optimal geometry of the elliptical cross-section is deduced from commercial optimization software, modeFRONTIER. It is shown that by subsequent use of the virtual solutions from the response surface modelling (RSM) of that software and their validation with Star-CD, a complete “Pareto-frontier solution” can be obtained.
This paper presents an analytical and numerical study on the heat transfer characteristics of forced convection across a microchannel heat sink. Two analytical approaches are used: the porous medium model and the fin approach. In the porous medium approach, the modified Darcy equation for the fluid and the two-equation model for heat transfer between the solid and fluid phases are employed. Firstly, the effects of channel aspect ratio (αs) and effective thermal conductivity ratio (k̃) on the overall Nusselt number of the heat sink are studied in detail. The predictions from the two approaches both show that the overall Nusselt number (Nu) increases as αs is increased and decreases with increasing k̃. However, the results also reveal that there exists significant difference between the two approaches for both the temperature distributions and overall Nusselt numbers, and the discrepancy becomes larger as either αs or k̃ is increased. It is suggested that this discrepancy can be attributed to the indispensable assumption of uniform fluid temperature in the direction normal to the coolant flow invoked in the fin approach. The effect of porosity (ε) on the thermal performance of the microchannel is subsequently examined. It is found that whereas the porous medium model predicts the existence of an optimal porosity for the microchannel heat sink, the fin approach predicts that the heat transfer capability of the heat sink increases monotonically with the porosity. The effect of turbulent heat transfer within the microchannel is next studied, and it is found that turbulent heat transfer results in a decreased optimal porosity in comparison with that for the laminar flow. A new concept of microchannel cooling in combination with microheat pipes is proposed, and the enhancement in heat transfer due to the heat pipes is estimated. Finally, two-dimensional numerical calculations are conducted for both constant heat flux and constant wall temperature conditions to check the accuracy of analytical solutions and to examine the effect of different boundary conditions on the overall heat transfer.
Liquid water jet impingement cooling was investigated experimentally for both free-surface jet arrays and confined submerged jet arrays. The jet arrays consisted of straight holes of 1.0 mm diameter arranged in rectangular arrays with spacings of 3, 5 and 7 jet diameters between adjacent jets. For the impingement surface area of 780 mm2, these jet array configurations can be considered well populated, with a total of 21, 45 and 121 jets impinging on the surface. Average heat transfer and pressure drop measurements are presented for volumetric flow rates in the range of and dimensionless jet-to-target spacings between 2 ⩽ H/dn ⩽ 30. For the submerged jet arrays a strong dependence on both jet-to-target and jet-to-jet spacing is observed and correlations are presented that adequately predict the experimental measurements. The free-surface jets show a non-monotonic change with jet-to-target spacing with a local minimum in the heat transfer coefficient at approximately H/dn = 10. Here a transition from a submerged to a free jet flow configuration occurs. Once again, a correlating equation is presented that adequately predicts the free-surface jet array heat transfer data. The pumping power required to form the submerged and free jet flows show a different relationship to the heat transfer coefficient. Generally, submerged jets have a higher heat transfer coefficient for a given pumping power requirement.
In this work the influence of the cooling fin shape on the pressure drop caused by flow resistance of a heat sink is studied.Anewmannertooptimiseheatsinksis developed,bringingtheemphasisnotonlyonmaximumheat transfer flux, but also on minimum flow resistance. The study is done numerically using the computational fluid dynamic softwareFLUENT.Theresultsshowtheadvantagesofusing aerodynamicshapedfinsiftheReynoldsnumber,based on the spacing between the cooling fins, is greater or equal than about 800. Some preliminary profile shapes are suggested.Theauthorsconsiderthisresearchasaquite uniqueapproachascomparedtootherresearchwherethe flow resistance has not been taken into account. 2002 Elsevier Science Ltd. All rights reserved.
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