Variation Of Thermal Resistance Research Articles

Numerical study on natural convection heat transfer of supercritical CO2 in an infinite space

A numerical study of natural convection heat transfer from horizontal heating wire to supercritical carbon dioxide (sCO2) in an infinite space was performed. The qw ∼ ∆T curves of sCO2 natural convection heat transfer were calculated in a wide range of parameters, where initial bulk temperature (Tib) and operating pressure (P) varied in the range: 275 K < Tib < 345 K and 7.5 MPa < P < 11 MPa, respectively. Then, we reported from the viewpoint of pseudo-phase transition for supercritical fluids, the characteristics and mechanism of sCO2 natural convection heat transfer were revealed through comparing with qw ∼ ∆T curve of subcritical pool boiling. The similarity between film-like heat transfer at Tw > Tpc for supercritical pressure and film boiling at Tw > Tsat for subcritical pressure was demonstrated. The influence of operating parameters such as Tw, Tib, and P on sCO2 heat transfer was also discussed. For supercritical film-like heat transfer, thermal resistance of film-like which reflects the effect of gas-like film thickness and its thermal conductivity on heat transfer was proposed. Expectedly, the variation of thermal resistance explained successfully the difference of sCO2 heat transfer under different operating parameters. The investigation is of paramount importance in comprehension of sCO2 heat transfer mechanism for designing heat exchanger component in the advanced sCO2 application.

Thermal Resistance Modeling for the Optimal Design of EE and E/PLT Core-Based Planar Magnetics

With the integration of power electronic converters and components, an accurate thermal design becomes essential. Hence, precise thermal models for components are needed for their optimal design. This paper focuses on the development of an analytical model for the design of thermal resistance of planar magnetic cores (PMC). Based on computational fluid dynamic (CFD) simulations, the PMC design thermal resistance variation is studied, according to ambient temperature and level of losses. Then, a polynomial equation is developed to model those variations, and coefficients are deduced for all the sizes of PMC. This analytical model, useful for designers, is finally validated with thermal measurements on a planar transformer prototype.

Solar thermoelectric generator and thermoelectric cooler performance: analysis and comparison using a different shape geometry

Abstract Thermoelectric devices are one of the technologies used either to generate electricity by applying a temperature difference using thermal energy or as a heating/cooling system by applying an electrical voltage. The number of materials required to produce a product is an important factor in determining its price. Production costs associated with these materials, as well as their availability and quality, play a crucial role in price determination by manufacturers. In this context, a method that employs a uniform volume distribution was implemented. This approach enabled the analysis to focus on other variables, thereby promoting a more precise and relevant evaluation of overall performance. Based on the finite element method, this study investigated the influence of geometric shape, including Rect-leg, Y-leg, Pin-leg and X-leg designs, on the performance of solar thermoelectric generators and thermoelectric coolers. The study was conducted considering the same hot alumina junction surface that receives solar radiation; however, the effective surface, which corresponded to the heat flow area and had a similar area near the exposed surface, varied depending on the chosen leg geometry, thus impacting the heat flux due to the variation in thermal resistance. In the case of a solar thermoelectric generator, the Rect-leg model, having the same effective surface area, presented the lowest heat loss value resulting from convection and radiation in the heat spreader and the hot alumina plate. Under the same conditions, the Y-leg showed the highest value. The Rect-leg design generated, by using thermal and optical concentration, the highest output power of 0.028 and 0.054 W, and efficiency of 3.47% and 4.7%, respectively, whereas the Y-leg generated lower values of 0.006523 and 0.018744 W for power, and 2.83% and 2.71% for efficiency, respectively. In the case of the thermoelectric coolers, the Y-leg generated the highest temperature difference between the hot and cold sides of 67.28 K at an electric current value of 1.8 A, whereas the Rect-leg, Pin-leg and X-leg generated ~66.25, ~67.02 and ~67.19 K at 6.1, 2.7 and 2.6 A.

Experimental study on the thermal performance of a liquid-cooled heat sink integrating heat pipes for dual CPU servers

The increasing power consumption of server CPUs poses a challenge for data center cooling. In this work, a liquid-cooled heat pipe heat sink (LHPHS) with compactly arranged heat pipes is designed and fabricated for CPU cooling in dual CPU servers. Water is selected as the working fluid in the heat pipes. Experiments are conducted to explore the heat dissipation potential of LHPHS under high inlet coolant temperature and high heat load conditions, and to investigate the operational stability of LHPHS under the conditions of uneven heat generation between two CPUs. Furthermore, a one-dimensional thermal resistance network and numerical model of the LHPHS are established to quantitatively analyze the thermal resistance distribution of LHPHS and the variation of thermal resistance with coolant flow rate. The results show that, firstly, the LHPHS operates stably under high inlet coolant temperature and high heat load conditions, meeting the heat dissipation requirements of up to 37.5 W/cm2 with a coolant flow rate of 1 L/min and inlet coolant temperature of 35℃. Secondly, the LHPHS exhibits excellent performance in controlling temperature difference between CPUs and even when there is discrepant heat generation between them. Thirdly, the LHPHS performs a low thermal resistance, measuring 0.044 ℃/W at a coolant flow rate of 0.5 L/min, and this resistance further decreases as the flow rate increases. Additionally, it is observed that the thermal resistance of the liquid-cooled plate decreases with the increasing flow rate, and heat transfer within the heat pipes becomes a critical aspect under high coolant flow rates. Therefore, reducing the thermal resistance of the heat pipe is crucial for the thermal performance improvement of the LHPHS.

Performance of Orbicella faveolata larval cohorts does not align with previously observed thermal tolerance of adult source populations.

Orbicella faveolata, commonly known as the mountainous star coral, is a dominant reef-building species in the Caribbean, but populations have suffered sharp declines since the 1980s due to repeated bleaching and disease-driven mortality. Prior research has shown that inshore adult O. faveolata populations in the Florida Keys are able to maintain high coral cover and recover from bleaching faster than their offshore counterparts. However, whether this origin-specific variation in thermal resistance is heritable remains unclear. To address this knowledge gap, we produced purebred and hybrid larval crosses from O. faveolata gametes collected at two distinct reefs in the Upper Florida Keys, a nearshore site (Cheeca Rocks, CR) and an offshore site (Horseshoe Reef, HR), in two different years (2019, 2021). We then subjected these aposymbiotic larvae to severe (36°C) and moderate (32°C) heat challenges to quantify their thermal tolerance. Contrary to our expectation based on patterns of adult thermal tolerance, HR purebred larvae survived better and exhibited gene expression profiles that were less driven by stress response under elevated temperature compared to purebred CR and hybrid larvae. One potential explanation could be the compromised reproductive output of CR adult colonies due to repeated summer bleaching events in 2018 and 2019, as gametes originating from CR in 2019 contained less storage lipids than those from HR. These findings provide an important counter-example to the current selective breeding paradigm, that more tolerant parents will yield more tolerant offspring, and highlight the importance of adopting a holistic approach when evaluating larval quality for conservation and restoration purposes.

THERMOELECTRIC MONITORING OF THERMAL RESISTANCE IN ELECTRONIC SYSTEMS

The article proposes to apply the method of thermoelectric testing to determine the thermophysical parameters of the thermal interface. A thermal interface is located between metal surfaces, between which, thermoelectromotive force occurs during heating at any stage of the device operation. The obtained graphs of the temperature difference dependence on the heating time, measured by thermocouples, and measured using thermoelectromotive force confirm the accuracy of the thermoelectric method of testing. Graphs visualize the heat transfer process with thermal resistance variation, temperature fluctuations and the resulting thermoelectromotive force. The proposed method makes it possible to monitor thermal resistance with an error of less than 8 %.

Investigating the disparities between experimental and computational analyses of thermal performance in prefabricated wall panels

Prefabricated buildings are increasingly popular worldwide, driven by technological advancements and the demand for sustainable and energy-efficient construction. However, the thermal performance of typical multi-layered and compacted prefabricated wall panels remains uncertain, leading to overdesigned HVAC and building envelope systems. This results in higher utility bills, unnecessary expenses, and increased material, labor, and transportation costs. While experimental analysis and computational simulations have proven effective for measuring the thermal performance of on-site wall panels (e.g., precast concrete, masonry), their effectiveness for exterior insulation finishing system (EIFS) prefab wall panels is unclear. This study aims to test the hypothesis that both experimental and computational analyses accurately measure the thermal performance of EIFS prefab wall panels under various climate conditions and yield to similar results. To achieve this, (1) experimental test analysis was conducted in an environmental chamber based on ASTM C1363 and ISO 8990 standards, and (2) computational simulation was conducted by employing THERM® at hot (65 °C) and cold (−16 °C) climates. The results showed that the experimental thermal resistance exceeded the computational thermal resistance by about 17% across hot and cold climates. These thermal resistances were then incorporated into a typical hospital building prototype in the U.S. modeled by U.S. Department of Energy in EnergyPlus™, resulting in substantial cost savings of approximately $396,000 over a 20-year lifespan due to the thermal resistance variations between experimental and computational analyses. The savings were achieved through a reduction in construction costs, accurate HVAC system selection, and lower utility bills. This study contributes to the improvement of prefabricated building design and energy performance analysis, leading to potential cost reductions in HVAC and building envelope system overdesign.

Functional principles of the envelope characterized by the variable thermal resistance of the wall as a result of parietodynamic exhaust effect (PDE)

The paper presents the opaque envelope system in the endowment of inhabited buildings characterized by the use of the heat of the air discharged from the inhabited spaces, circulated through the space adjacent to the living environment / outdoor environment in order to reduce the need for heat / sensitive cold of the inhabited spaces (PDE). It is presented mathematical modeling of the processes of heat transfer and flow of indoor / outdoor air circulated through parallel vertical channels. The dimensions of vertical channel opening and air flows are based on the solutions of the property transfer equations associated with Model 1 (forced laminar flow) and Model 2 (globally representative model of both laminar flow and turbulent flow). Model 2 includes both laminar flow (space heating in cold season and cooling spaces in summer season hours with sun) and turbulent flow (cooling living spaces in summer season, hours of night – free cooling). A case study and the interpretation of the results obtained regarding the energy performance of the living space in relation to similar results associated with the conventional envelope is presented.

Performance evaluation of a pipe-embedded phase change material (PE-PCM) roof integrated with solar collector

Utilizing renewable energy in buildings is important for the development of zero-energy buildings to achieve the goals of carbon peak and carbon neutrality. A novel roof with a pipe-embedded phase change material (PE-PCM) integrated with a solar collector is proposed to fully utilize solar energy and improve building efficiency. The PE-PCM layer is attached to the inner side of the roof for effective utilization of solar energy and enables all-day indoor heating via thermal storage. In this study, a simplified model of a PE-PCM structure was developed based on its physical configuration, which included a variable thermal capacitance and thermal resistance model (VarRC model). The simplified PE-PCM roof model integrated with the VarRC model is validated using a numerical model. Additionally, a platform for a room with the PE-PCM roof and solar collector is developed to investigate the system performance. The results showed that the heating demand can be met, except under extreme weather conditions. When the indoor temperature is controlled above 17 °C by auxiliary heating, the total energy required for heating the room with the PE-PCM roof can be reduced by 54% compared to that of a conventional roof room.

Determining the Thermal Resistance of Enclosed Reflective Airspace

Enclosed airspaces of various effective emittances exist in the building envelope in walls, roofs, and double/triple glazing windows, curtain walls and skylight devices. Assessing the energy performance of a building component with enclosed reflective airspaces requires evaluation of all modes of heat transport inside the airspace. The thermal resistance (R-value) of the enclosed airspaces depends on the dimensions and orientation of the airspace, and the emittance and temperature of all surfaces that bound the airspace. To the best of our knowledge, the existing methods used around the world to evaluate the airspace thermal performance (e.g., ISO 6946, AUS/NZ 4859, and methods based on the U.S. NBS data) are one-dimensional and assume isothermal conditions on the hot and cold surfaces. In actual applications, however, the temperatures of both the hot and cold surfaces vary (i.e., they are non-isothermal), and the heat transfer takes place via conduction, convection and surface-to-surface radiation. In some cases, convection is absent or negligible, as is the case for downward heat flow or situations with a small temperature difference between the hot and cold surfaces. Radiation transport is significantly reduced by the presence of a low-emittance surface in the heat flow path. One of the goals of this study is to answer the question “is it a good assumption to use the isothermal conditions on both the hot and cold surfaces for determining/reporting the R-value of enclosed airspaces for different building applications?” A complete evaluation of the thermal performance of enclosed airspaces that includes the impact of bounding materials can now be undertaken in multiple dimensions with convective transport described by computational fluid dynamics. In addition to providing a relatively complete evaluation of the thermal performance of the enclosed airspaces, the adequacy of popular simplifying assumptions can be evaluated. This paper describes the computational process used, along with examples of the variation in thermal resistance with the airspace aspect ratio, realistic thermal boundary conditions and radiative heat transport on all surfaces that bound the airspace. The analysis completed in this research shows that the assumption of isothermal hot and cold surfaces affects calculated R-values for enclosed reflected airspaces by less than 3%. This was demonstrated for the five conventionally considered heat-flow directions and effective emittances from 0 to 0.82.

Comparative analysis of CLPHP working with binary mixtures in different concentrations

The advanced technologies in design and manufacturing of electronic components have resulted considerable heat flux through component miniaturization. For highly performing microelectronic devices production, thermal management has the priority. Other applications also like material science, fabrication technology, solar applications etc. which directly impact cost, reliability and performance. When it comes to overcome these problems efficient cooling approaches development and use will come into play. The best solution for this is Pulsating heat Pipe (PHP). A simple PHP is a two-phase heat transfer device consists of a tube with a suitable working fluid, along with an evaporator (heating)section, adiabatic section and a condenser (cooling)section. In the present work multi turn Closed Loop Pulsating Heat Pipe (CLPHP) is chosen with capillary dimensions. The experimentation is carried out with different working fluids viz. water, water-based mixtures (75%-25%) at different volumetric fill ratios 50%,60% and 75%. The heat inputs of20W, 40 W, 60 W, 80 W, 100 W applied. At various heat inputs for all the working fluids the variation of thermal resistance is plotted and observed that upon increasing heat input, thermal resistance of fluid decreases. Comparative study is also presented for same experimental setup when the binary mixtures are considered in 50%-50% as mixture concentration. This gives the better understanding of Closed Loop Pulsating Heat Pipe performance when operated at different fill ratios and concentrations for different working fluids considered.

A combined solution of thermoelectric coolers and microchannels for multi-chip heat dissipation with precise temperature uniformity control

• Conbined TEC and microchannel cooling proposed for multi-chip heat dissipation. • Based on the change of applied current, the equivalent thermal resistance of TEC is changed to realize the active temperature control of chip. • Heat dissipation with temperature uniformity control of multi-chip combined cooling is studied. Effective thermal management with precise temperature uniformity is necessary to improve the performance and stability of multi-chip devices such as active phased array antennas, semiconductor laser radar systems, and light emitting diode (LED) arrays. This study proposes a combined solution for multi-chip devices heat dissipation that integrates thermoelectric coolers (TECs) and microchannel heat sink to tune the temperature of each chip dynamically by controlling multiple TEC currents independently. The equivalent variable thermal resistance of the TEC can be dynamically adjusted under different TEC currents, thereby realizing the equivalent thermal resistance value of TECs change on the heat dissipation path of different chips and the precise temperature control and continuity at desired temperature range. A simplified thermal resistance network of multi-chip is established to illustrate the dynamic control mechanism of multi-chip temperature uniformity based on the variable thermal resistance. Not only the heat dissipation and temperature control of single chip under different operating conditions (TEC current, flow rate, and heat flux), but also the temperature uniformity control of multi-chip is studied. The combined cooling scheme is compared and analyzed with the microchannel cooling. The results show that this combined cooling scheme can achieve precise temperature control of multiple chips with maximum temperature difference less than 0.3 °C and temperature standard deviation less than 0.07 °C, which is far less than the maximum temperature difference of 7.89 °C and temperature standard deviation of 3.55 °C when the heat flux is 50 W/cm 2 . This represents a feasible solution for the realization of precise temperature uniformity and dynamic temperature control in multi-chip devices.

Experimental study on heat transfer performance of sCO2 near pseudo‐critical point in airfoil‐fin PCHE from viewpoint of average thermal‐resistance ratio

The printed circuit heat exchanger (PCHE) is one of the leading heat exchanger candidates in the supercritical carbon dioxide Brayton cycle, which has high efficiency and compactness. In the present paper, a novel type of PCHE with airfoil fins was proposed and tested. The effect of the thermal resistance variation on the heat transfer performance near the pseudo-critical point was analyzed. Meanwhile, a non-dimensional factor defined as the ratio of bulk temperature to the pseudo-critical temperature was applied to evaluate the heat transfer performance. The results indicate that the variation of the heat transfer performance can be attributed to the different average thermal-resistance ratios, where a larger the average thermal-resistance ratio corresponds to a better heat transfer performance. When the non-dimensional factor is around 1.03, the heat exchanger exhibits the highest heat transfer performance. In this regard, a working condition analysis method based on the non-dimensional factor and the average thermal-resistance ratio is suggested. Finally, a modified heat transfer correlation considering the non-dimensional working parameters near the pseudo-critical point is obtained.

Impact of ambient temperature and thermal resistance on device performance of junctionless silicon-nanotube FET

In this article, a comprehensive analysis of the impact of electrothermal characteristics in the junctionless silicon-nanotube (Si-NT) field-effect-transistors is carried out using the Sentaurus TCAD. The combined study of the variation in thermal contact resistance (1 × 10−9 to 1 × 10−8 m2 W K−1), ambient temperature (300–400 K), and spacer length (5–20 nm) are performed. Significant improvements are observed in carrier temperature by 14%, lattice temperature by 13.7%, and gate leakage current from 0.787 nA to 0.218 fA due to the change in the spacer length. Further, a change in the drain current of 25.6% for thermal resistance (R th) and of 11.62% due to ambient temperature is observed. We also show that the junctionless device suffers significantly less from self-heating effects because of the electric field intensity, which is much lower in the channel region.

Nanoscale Thin-Film Boiling Processes on Heterogeneous Surfaces

Acquiring rapid and efficient boiling processes has been the focus of industry as they have the potential to improve the energy efficiency and reduce the carbon emissions of production processes. Here, we report nanoscale thin-film boiling on different heterogeneous surfaces. Through nonequilibrium molecular dynamics simulation, we captured the triple-phase interface details, visualized the bubble nucleation, and recorded the internal fluid flow and thermal characteristics. It is found that nanoscale thin-film boiling without the occurrence of bubble nucleation shows excellent heat and mass transfer performance, which differs from macroscale boiling. In general, rough structures advance the onset time of stable boiling and improve the efficiency. The heat transfer coefficient and heat flux on a rough hydrophilic surface respectively reach to 7.43 × 104 kW/(m2·K) and 1.3 × 106 kW/m2 at a surface temperature of 500 K, which are 100-fold higher than those of micrometer-scale thin-film boiling. However, due to the resultant vapor film trapped between the liquid and the surface, the rough hydrophobic surface leads to heat transfer deterioration instead. It is revealed that the underlying mechanism of regulatory effects resulting from surface physicochemical properties is originated from the variation of interfacial thermal resistance. It is available to reduce the overall interfacial resistance and further improve the heat and mass transfer efficiency through increasing surface roughness, enhancing surface wettability, and increasing the area proportion of the hydrophilic region. This work provides guidelines to achieve rapid and efficient thin-liquid-film boiling and serves as a reference for the optimized design of surfaces utilized for high-heat flux removal through vaporization processes.

Experimental Investigations on Performance Analysis of a Wickless Thermosiphon Heat Pipe With Two Heat Sources and Multiple Branches

Abstract In an electronic circuit of laptops, supercomputers with multiple central processing units, spacecraft etc., it is required to arrange the cooling system for multiple heat loads in the smallest possible space in view of power-saving opportunities. In the present study, the experimental investigations are carried out on a wickless multi-branch heat pipe in gravity-assisted mode with two evaporators and one condenser on each of the individual branches. The start-up and dynamic characteristics were studied with different filling ratios (range, 40–70%), with equal heat loads (range, 0–200 W) and unequal heat loads (range, 0–100 W) on evaporators. The results are analyzed in terms of temperature variation in axial direction, thermal resistance, and heat transfer coefficient for a multi-branch thermosiphon heat pipe (MBTHP). It was found that the optimal filling ratio depends on the applied heat load under tested conditions. In a thermosiphon mode, the heat pipe was capable of transporting a maximum heat load of 210 W and maximum heat flux of 20.31 W/cm2 with the maximum evaporator temperature lower than 100 °C. The minimum wickless thermal resistance of heat pipe was found to be 0.21 °C/W at 50% filling ratio and 160 W and maximum total heat transfer coefficient was found as 6.33 k W/m2 °C.

Failure investigation of a Ag sinter-joining SiC power device under harsh cycling conditions

Silicon-Carbide (SiC) power devices have become the prime candidates for future high-performance energy conversion on account of their superior performance over traditional silicon power devices. The superior electro-thermal properties of SiC power devices permit higher temperature of operation and enable higher power density. Nevertheless, the high operation temperature has introduced new changes in die attachment packaging of the SiC power device, which is one of the most vulnerable places due to a direct connection to the SiC chip. Traditional Pb-free solder-joining is hardly to withstand high temperatures because of the low melting point of solder materials. Besides, severe temperature swings of the SiC chip can introduce severe deterioration in the solder layer due to brittle alloy compounds. Currently, Ag sinter-joining is an ideal connection technique for high reliable power electronics packaging. Due to excellent thermal stability and chemical resistance, the Ag sinter-joining is applicable to various harsh conditions, such as extreme temperatures, high humidity, high heavy current, etc., showing promising application scenarios in die attachment packaging. In this work, we applied Ag sinter-joining to die attachment of a molded SiC device and focused reliability of Ag sinter joining under harsh thermal cycling and power cycling conditions. The SiC device was assembled by a SiC diode and direct bonding copper (DBC) substrate via the Ag sinter-joining. Ribbon bonding was used for the topside connection of the SiC device. An imide resin was applied to mold the SiC device. The thermal cycling test was conducted at a temperature range from -50 °C to 200 °C and the power cycling test was conducted at an initial junction of 200 °C. The structures inspection, thermal resistance variations, and voltage-current curve were investigated to understand failures in the SiC device. The thermal cycling can introduce crazing cracks in the sintered layer due to drastic thermal stress caused by a huge temperature swing. However, the thermal resistance, of the SiC device measured by T3ster, was not significantly affected by the crazing pattern. The Ag sinter-joining also shows an excellent resistance during the power cycling test, while the ribbon lift-off will introduce a final failure of the SiC device. Figure 1 shows a cross section of the structure of the SiC power device with and without a power cycling test. Obvious cracks can be differentiated at the ribbon bonding interface after cycling, while there is no deterioration happened in the sintered Ag layer. Base on the present results, it can be concluded that the Ag sinter-joining can withstand a harsh cycling condition either in huge temperature swings or heavy currents. Figure 1

Thermal Analysis of a MEMS-Based Self-Adaptive Microfluidic Cooling Device.

This study presents a thermal analysis of a temperature-driven microfluidic cell through a nonlinear self-adaptive micro valve that provides the mechanisms for the system to maintain a given critical temperature in an efficient way. For the description of the dynamics of the microfluidic cell, a system of two ordinary differential equations subjected to a nonlinear boundary condition, which describes the behavior of the valve, is proposed. The solution of the model, for determined conditions, shows the strong nonlinearity between the overall thermal resistance of the device and the heat flux dissipated due to the action of the thermostatic valve, obtaining a variable thermal resistance from 1.6 × 10−5 to 2.0 × 10−4 Km2/W. In addition, a stability analysis of the temperature-driven microfluidic cell is presented. The stability of the device is essential for its proper functioning and thus, to prevent its oscillating behavior. Therefore, this work focuses on assessing the range of design parameters of the self-adaptive micro valve to produce a stable behavior for the entire system. The stability analysis was performed by studying the linear perturbation around the stationary solution, with the model solved for various heat flows, flow rates, and critical temperatures. Finally, a map of the design parameters space, which specifies the region with asymptotic stability, was found. In this map, the critical temperature (temperature at which the valve initiates the buckling) plays and important role.

Hybrid microchannel/multi-jet two-phase heat sink: A benchmark and geometry optimization study of commercial product

In this paper, an experimental study of two-phase cooling heat sink using the dielectric working fluid Novec/HFE-7000 is performed. The liquid flow is directed into microchannel fins through multiple impinging jets. A copper microchannel heat sink with a fin height of 3mm, fin thickness of 0.1 mm and channel width of 0.1 mm was used for removing heat flux from the heated surface. A copper block was used to mimic computer chip with a surface area of 1” x 1” (6.45 cm2). The experiment was performed at heat flux from 5 W/cm2 to 80 W/cm2, subcooling of 5 °C, 15 °C, 25 °C, and flow rates 1, 1.25 and 1.75 lpm. The results in the two-phase regime for high heat fluxes are compared with analytical correlations. Reduced subcooling can results in enhancement of heat transfer coefficient by 25 % but will increase the pressure drop through the heat sink. Heat transfer coefficient increases with flow rate in the single-phase region and in the convective boiling region. In nucleate boiling region, heat transfer coefficient increases with heat flux rather than flow rate. Geometry optimization of heat sink was carried out considering effects of fin height on thermal and hydraulic performance. It is found that thermal resistance variation with fin height shows a parabolic trend and an improvement of 46 % in thermal performance can be obtained. A new correlation was also developed to predict results of experimental data with mean absolute error of 7 %. The inclusion of Reynolds number and hydraulic diameter was found to considerably reduce error when compared to values obtained with the previously available correlation, in which only the subcooling and heat flux were the parameters for modelling the heat transfer.

Experimental investigation of the thermal performance of a heat sink with U-shaped heat pipes

To meet the urgent requirement of high heat flux cooling of the chips in current electronic equipment, a unique and reliable U-shaped heat pipe bonded heat sink was developed and experimentally investigated for CPU cooling in computers. This novel heat sink was bonded with air-cooled fin stacks and two U-shaped heat pipes through the base plate. In this work, the effects of relevant parameters, including orientation of the heat sink, working conditions (air velocity and ambient temperature) of the computer on the variation of CPU temperature and overall thermal resistance of the heat sink are considered. The tests indicate that the overall thermal resistance of the heat sink at orientation α = 90° is lower than that in another five orientation cases, and reaches the minimum value of 0.263 °C/W. If the operating temperature of the CPU is limited to below 70 °C, the maximum heat dissipation power of the heat sink reaches 130 W. It can be seen that the orientation and working conditions of the heat sink have significant effects on the cooling capacity which results in variation in the overall thermal resistance of the heat sink. In addition, the comparison of overall thermal resistance between the novel heat sink and a traditional CPU fin-bonded heat sink indicates that the novel heat sink provides greater reliability.