Heat Dissipation Potential Research Articles

Evaluation of Free Cooling Technique at the Telecom Cell Site in Ghana

Air Conditioning (AC) is the primary source of cooling in a typical Cell Site (CS) although it consumes a large quantity of electricity. Excessive energy consumption due to air conditioning increases the operational expenses of telecommunication companies. Therefore, network operators have attempted various techniques to reduce the exorbitant cost of a cell site’s high-power consumption. A Free Cooling (FC) System is one such approach. This paper uses computer modeling to investigate how the FC System will function at a CS in Ghana for 12 hours (from 6 pm to 6 am). A shelter with dimensions of 3 m × 2.47 m × 2.45 m (L × B × H) is used in the study, with three window positioning scenarios (different window heights), namely: (a) an inlet window 0.5 m high, and the outlet window 1.5 m high, (b) an inlet window and outlet window both 1.0 m high and (c) an inlet window 1.5 m high, and the outlet window 0.5 m high. The analysis revealed that the shelter with inlet and outlet windows at the same height has the most efficient heat dissipation potential. Furthermore, at a set temperature threshold of 25 °C, the annual percentage share for the two operating modes is approximately 67% for AC and 33% for FC in 2020, 76% for AC and 24% for FC in 2021, and 61% for AC and 39% for FC in 2022. However, when the set temperature threshold is increased from 25 °C to 30 °C, the annual percentage share is approximately 0.01% for AC and 99.9% for FC in 2020, 0% for AC and 100% for FC in 2021, and 1% for AC and 99% for FC in 2022. Thus, the proportion of free cooling increases as the set temperature threshold is raised, indicating potential energy savings. It could be concluded that in Ghana, installing a cell site with an FC system and setting the temperature of its control system to 30 °C or higher will result in significant energy savings.

The optimization of roll-bond cold plate based on Taguchi-grey theory for server chips thermal management

The roll-bond cold plate, an economical and straightforward electronics liquid cooling approach, is highly promising to address the increasing electronics chip thermal management challenges. However, the roll-bond cold plate with staggered cellular flow channels, while having a sizable solid-liquid contact area and high heat dissipation potential, suffers from inadequate temperature uniformity and fluidity. This study addresses flow and temperature uniformity issues in current roll-bond cold plates by proposing a design—Direct Liquid Cooling Roll-Bond Plates (DLCRBP) tailored for high-performance server architectures. To investigate how regional combination, cellular diameter, and longitudinal spacing affect both thermal and flow performance, orthogonal experiments with diverse flow channel structures were conducted using the Taguchi-grey theory. COMSOL-based numerical simulation were used to analyze these experiment conditions. Multi-objective optimization, considering pressure drop, heat source temperature uniformity, and coolant temperature difference, was then performed using the grey correlation method. The optimized DLCRBP (A3B5C1) was fabricated, and its performance was compared with the staggered cellular DLCRBP (SCS), which is the control group without optimization, under various inlet flow velocities, heat source powers, and coolant inlet temperatures. Experimental comparisons illustrate that the optimized structure ensures outstanding temperature uniformity even at low inlet flow velocities. Specifically, at an inlet flow velocity of 0.2 m/s, the average and maximum temperature differences on the heat source's upper surface are 36.5 % and 75.3 % lower, respectively, compared to pre-optimization levels. Furthermore, A3B5C1's temperature uniformity improves with higher coolant inlet flow velocity and lower coolant inlet temperatures. At a coolant inlet flow velocity of 0.5 m/s or higher, A3B5C1 sustains the surface temperature of a 600 W chip below the thermal design's safe temperature of 77.8 °C, highlighting its enhanced performance.

Integrated coplanar waveguide coil on diamond for enhanced homogeneous broadband NV magnetometry.

Nitrogen-vacancy (NV) centers in diamond have emerged as promising quantum sensors due to their highly coherent and optically addressable spin states with potential applications in high-sensitivity magnetometry. Homogeneously addressing large ensembles of NV centers offers clear benefit in terms of sensing precision as well as in fundamental studies of collective effects. Such experiments require a spatially uniform, intense, and broadband microwave field that can be difficult to generate. Previous approaches, such as copper wires, loop coils, and planar structures, have shown limitations in field homogeneity, bandwidth, and integration in compact devices. In this paper, we present a coplanar waveguide (CPW) gold coil patterned on a 3 × 3 mm 2 diamond substrate, offering full integration, enhanced stability, and broad bandwidth suitable for various NV sensing applications. Coil fabricated on diamond offers several advantages for magnetometry with NV centers ensemble, including enhanced heat dissipation, seamless integration, scalability, and miniaturization potential. We optimize critical geometrical parameters to achieve a homogeneous magnetic field with a coefficient of variation of less than 6% over an area of 0.5 mm 2 and present experimental results confirming the performance of the proposed CPW coil.

Pool boiling heat transfer performance of micro-embossing molds for the fabrication of polymer wicks

As flexible heat dissipation devices are urgently needed for flexible electronics, polymers with the advantages of good flexibility, insulation, and low density have attracted wide attention. It would be interesting to fabricate thermal functional structures on polymers to overcome their low thermal conductivity. This study aims to design and fabricate a micro-embossing mold to process polymer three-dimensional microgroove wicks (3D-MWs), which are applied in flexible heat pipes and achieve high thermal conductivity. Three kinds of tungsten steel molds were processed by using two wire electrical discharge machining processes (WEDM), two electrical discharge shaping machining processes (EDSM), and a combination of WEDM and EDSM. The 3D-MWs of the tungsten steel mold were reprinted onto the polymer films by a two-step embossing process. To investigate the effects of microstructures on boiling heat transfer performances, tungsten steel molds fabricated by different methods were tested to reflect the heat dissipation potential of the polymer 3D-MWs. The results showed that the 3D-MWs fabricated by the combination of WEDM and EDSM exhibit good boiling heat transfer performance and high processing efficiency, with a critical heat flux of 1040.1 kW/m2 and a maximum heat transfer coefficient of 38.4 kW/m2·°C. These values are 46% and 611% higher than those of the smooth tungsten steel surface, respectively, making it a superior choice for micro-embossing molds.

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.

Assessment of cooling performance of mini/micro-channel stacked double layer heat sink

A study is accomplished here to investigate the performance of a novel double-layer mini/micro-channel stacked heat sink (DL-MMCHSHS). The hydraulic diameter and aspect ratio of mini-channel heat sink are 0.12 cm and 3, respectively. The width of the fins between two channels is 0.04 cm. The total width of two runners is the same, and its width is 1.44 cm. The heat dissipation potential of this new type of heat sink is investigated. It was observed that the pressure loss is reduced by using the DL-MMCHSHS instead of a single-layer micro-channel heat sink (SL-MCHHS). The reduction in the pressure loss reduces the pumping power, which leads to decrease in operating costs of the system. For the total flow rate of 345.69 cm3/min, the pressure loss reduces about 35.41% by using the DL-MMCHSHS instead of a SL-MCHHS. The uniformity index in the range of 0.077 to 0.192 is achieved in this study. For the total flow rate of 346.64 cm3/min, the uniformity index enhances about 60% with boosting the flow rate ratio from 0.5 to 3.5. For the total flow rate of 1039.26 cm3/min, the thermal resistance increases about 76.22% as the flow rate ratio increases from 0.5 to 3.5.

Concepts of heat dissipation of a disposal canister and its computational analysis

The stability of engineered barriers in high-level radioactive waste disposal systems can be influenced by the decay heat generated by the waste. This study focuses on the thermal analysis of various canister designs to effectively lower the maximum temperature of the engineered barrier. A numerical model was developed and employed to investigate the heat dissipation potential of copper rings placed across the buffer. Various canister designs incorporating copper rings were presented, and numerical analysis was performed to identify the design with the most significant temperature reduction effect. The results confirmed that the temperature of the buffer material was effectively lowered with an increase in the number of copper rings penetrating the buffer. Parametric studies were also conducted to analyze the impact of technical gaps, copper thickness, and collar height on the temperature reduction. The numerical model revealed that the presence of gaps between the components of the engineered barrier significantly increased the buffer temperature. Furthermore, the reduction in buffer temperature varied depending on the location of the gap and collar. The methods proposed in this study for reducing the buffer temperature hold promise for contributing to cost reduction in radioactive waste disposal.

Lactating SKH-1 furless mice prioritize own comfort over growth of their pups.

Lactation is the most energetically demanding physiological process that occurs in mammalian females, and as a consequence of this energy expenditure, lactating females produce an enormous amount of excess heat. This heat is thought to limit the amount of milk a mother produces, and by improving heat dissipation, females may improve their milk production and offspring quality. Here we used SKH-1 hairless mice as a natural model of improved heat dissipation. Lactating mothers were given access to a secondary cage to rest away from their pups, and this secondary cage was kept either at room temperature (22°C) in the control rounds or cooled to 8°C in the experimental groups. We hypothesized that the cold exposure would maximize the heat dissipation potential, leading to increased milk production and healthier pups even in the hairless mouse model. However, we found the opposite, where cold exposure allowed mothers to eat more food, but they produced smaller weight pups at the end of lactation. Our results suggest that mothers prioritize their own fitness, even if it lowers the fitness of their offspring in this particular mouse strain. This maternal-offspring trade-off is interesting and requires future studies to understand the full interaction of maternal effects and offspring fitness in the light of the heat dissipation limitation.

Numerical investigation of heat transfer and pressure drop characteristics of flow boiling in manifold microchannels with a simple multiphase model

The Manifold Microchannel (MMC) heat sink is an emerging micro-scale electronic cooling technology with high heat dissipation potential and application prospects. This paper numerically studied the subcooled flow boiling in MMC heat sinks. The feasibility of using the Mixture multiphase flow model to simulate the subcooled flow boiling in the MMC heat sink is validated. As the vapor-liquid interfaces are not reconstructed in this model, less computational resources are needed compared with other multiphase models. The results show that at constant channel width, the maximum and average temperatures of the MMC decrease with the increase of the microchannel aspect ratio. For cases with an inlet velocity of 0.35 m/s, the average wall temperature decreases from 388.53 K to 374.26 K when the aspect ratio increases from 6.67 to 16.67. The difference between the maximum and the minimum temperature on the heated surface increases with the increase of channel aspect ratio, resulting in an uneven temperature distribution. The thickness of temperature boundary layer near the divider is thicker than that near the heated base at low aspect ratio. Low pressure drops are obtained for cases with high aspect ratios due to the increase in cross-sectional area. The pressure drop of MMC with aspect ratio of 6.67 is nearly twice of the case with aspect ratio of 16.67. Although the Mixture model could not characterize the bubble shapes, the numerical results are similar to those obtained by the VOF model and the experimental visual results. A complex case of subcooled flow boiling in Z-type MMC with a hotspot heat flux of 800 W/cm2 is also conducted using the whole computational domain. Mal-distributions of mass flux, wall temperature and vapor void fraction are observed for this complicated problem. The difference between the maximum and the minimum temperatures is about 12 K. The Mixture model is more suitable to obtain reasonable numerical results of flow boiling in complex MMC with much less computational resource.

Enhanced thermal properties of epoxy composites by constructing thermal conduction networks with low content of three-dimensional graphene

Micro/nano electronic devices heat dissipation depends heavily on the thermal interface materials (TIMs). Despite notable progress, it is hard to efficaciously enhance the thermal properties of the hybrid TIMs with high-load additives due to an absence of effective heat transfer routes. Herein, the low content of three-dimensional (3D) graphene with interconnected networks is adopted as the additive to improve the thermal properties of epoxy composite TIMs. The thermal diffusivity and thermal conductivity of the as-prepared hybrids were dramatically improved by constructing thermal conduction networks after adding 3D graphene as fillers. The 3D graphene/epoxy hybrid’s optimal thermal characteristics were observed at 1.5 wt% of 3D graphene content, corresponding to a maximum enhancement of 683%. Besides, heat transfer experiments were further performed to determine the superb heat dissipation potential of the 3D graphene/epoxy hybrids. Moreover, the 3D graphene/epoxy composite TIM was also applied to high-power LED to improve heat dissipation. It effectively reduced the maximum temperature from 79.8 °C to 74.3 °C. These results are beneficial for the better cooling performance of electronic devices and provide useful guidelines for advancing the next-generation TIMs.

Self-hygroscopic and smart color-changing hydrogels as coolers for improving energy conversion efficiency of electronics

Phase change materials as a potential passive cooling solution is widely used to cool electronic devices, since the high temperature accompanied by the continuous operation of these electronic devices significantly affects their efficiencies and causes irreversible damage. However, there is a lack of the feedback on cooling potential and recessive failures such as ineffective cooling due to overuse. Here, we report a self-hygroscopic and smart color-changing Co@Li-PAM hydrogel, composed of polyacrylamide (PAM)-based polymer chains, H2O molecules, and functional ions including Li+, Co2+, [Co(H2O)]2+, Cl-, and Br-. It has been found that the hydrophilic porous PAM network acts as the main framework with excellent biocompatibility and reliable chemical stability, LiBr acts as the adsorbent in the PAM network to absorb water molecules in the air during electronic device downtime, and Co ions are introduced into hydrogels through coordination bonds formed with amide groups of polymer chains. The reversible transformation between [Co(H2O)6]2+ and Co2+ driven by the water content in Co@Li-PAM triggers color changes, which indicates the current heat dissipation potential of hydrogels. Applying the as-designed Co@Li-PAM hydrogel to cooling commercial polycrystalline silicon solar cells can increase its energy conversion efficiency by 1.26 % under the illumination of 1 kW m-2. This strategy is expected to provide exotic solutions for the development of electronic devices, carbon neutrality, and global sustainable development goals.

Demonstration of high-stable self-mode-locking pulses based on self-focusing in fiber lasers

Passive ultrafast photoelectric devices based on the self-focusing effect have been deeply studied in ultrashort pulse formation, and great progress has been made in solid-state oscillators known as Kerr-lens mode-locking (KLM). The intensity-dependent change of refractive index and the 1–2 fs electronic response time of materials are the basics of KLM. Recently, the advantages of fiber lasers such as compact structure, outstanding environmental stability, excellent heat dissipation potential, and low cost have stimulated great interest among researchers to explore stable and reliable fiber lasers. Based on the feasibility of KML in fiber lasers that have been reported, we demonstrate a high-stable self-mode-locking Er-doped fiber laser in this work. Diverse stable self-mode-locking pulses phenomena can be obtained through adjusting the polarization state of the cavity, including the output pulses with a fundamental frequency of 3.93 MHz, dark-bright soliton pulses, the 3rd harmonic operation, the 81st harmonic operation with the repetition rate of 318.4 MHz and the signal-to-noise ratio of 80 dB, and dual-wavelength output.

Comparative Analysis of Heat Transfer and Fluid Flow in Circular and Rhombus Pin Fin Heat Sink Using Nanofluid

Abstract Numerical investigation has been carried out to compare the heat transfer performance and fluid flow behavior of microchannel heat sinks with circular and rhombus pin fins which are arranged in an in-line manner. Diameter and sides are 1 mm for circular and rhombus fins. Three-dimensional (3D) computational domain has been simulated using two types of cooling medium, i.e., water and Al2O3–H2O nanofluid. A comprehensive comparative analysis has been presented considering the coolants and pin fin profiles as variable parameters. Two operating variables, i.e., heat flux (q) and Reynolds number (Re), are varied in the range of q = 100–400 kW/m2 and Re = 100–400. A total of 64 cases have been simulated to identify the promising features of both the pin fins attributed to improved heat transfer and overall thermal performance. Comparison has also been made between the coolant medium to find out their heat dissipation potential and flow characteristics in the heat sink. Results obtained in terms of average bottom wall temperature, heat transfer coefficient, Nusselt number (Nu), and pressure drop demonstrate that heat sink with rhombus pin fins dissipates more heat compared to its counterpart. It is attributed to the shape and geometry of rhombus fins that facilitate distinct fluid flow behavior; nevertheless, the pressure drop is less in the circular fin heat sink. Moreover, for constant value of Re, nanofluid extracts more heat compared to water in both configurations of the heat sink.

Boiling Heat Transfer Performance of Parallel Porous Microchannels

Flow boiling in microporous layers has attracted a great deal of attention in the enhanced heat transfer field due to its high heat dissipation potential. In this study, flow boiling experiments were performed on both porous microchannels and a copper-based microchannel, using water as the coolant. As the heat flux was less than 80 W/cm2, the porous microchannels presented significantly higher boiling heat transfer coefficients than the copper-based microchannel. This was closely associated with the promotion of the nucleation site density of the porous coating. With the further increase in heat flux, the heat transfer coefficients of the porous microchannels were close to those of the copper-based sample. The boiling process in the porous microchannel was found to be dominated by the nucleate boiling mechanism from low to moderate heat flux (<80 W/cm2).This switched to the convection boiling mode at high heat flux. The porous samples were able to mitigate flow instability greatly. A visual observation revealed that porous microchannels could suppress the flow fluctuation due to the establishment of a stable nucleate boiling process. Porous microchannels showed no advantage over the copper-based sample in the critical heat flux. The optimal thickness-to-particle-size ratio (δ/d) for the porous microchannel was confirmed to be between 2–5. In this range, the maximum enhanced effect on boiling heat transfer could be achieved.

On the role of confinement plate wettability on pool boiling heat transfer

Remarkably high heat transfer coefficient observed during boiling makes it one of the most sought-after thermal management solutions. However, the heat dissipation potential deteriorates in practical scenarios of confined spaces typically encountered in electronic packages and compact heat exchangers, among others. Contrary to unconfined pool boiling, confinement hinders buoyancy aided vapor removal from the heater surface. Accordingly, vapor crowding inhibits the continuous supply of fresh liquid to the heater surface, eventually resulting in premature dryout/critical heat flux (CHF) in comparison to the baseline case of unconfined boiling on the same surface. While studies in literature report the effect of various parameters such as gap dimensions, heater surface to confinement plate area ratio, and surface inclination on confined pool boiling, the role of confinement plate wettability on bubble behavior and heat transfer has yet not been reported. Here we perform controlled experiments to show that superhydrophobic (SHPo) surfaces attract vapor/air bubbles and can be used to enhance heat transfer during confined boiling. In comparison to the typical case of boiling with hydrophilic (HPi) confinement plate, nucleating bubbles underneath a SHPo confinement plate detach easily from the heater surface to provide a continuous pathway for liquid re-supply and heater surface rewetting. Using this strategy, we shown a significant increase of up to 82% in heat dissipation capacity using SHPo confinement plate in comparison to the baseline case of the HPi confinement plate. Detailed parametric investigation of the effect of confinement gap and heater surface to confinement plate area ratio on heat transfer performance is also reported. We believe that the ability to improve confined boiling heat transfer performance by merely tuning the wettability of the confinement plate will find wide applications in two-phase heat sinks for high heat flux applications.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

Thermal Management System Modeling and Simulation of a Full-Powered Fuel Cell Vehicle

Abstract Thermal management is an important factor in securing the safe and effective operation of a fuel cell vehicle (FCV). A parameterized stack model of a 100 kW proton exchange membrane fuel cell (PEMFC) is constructed by matlab/Simulink to design and asses the thermal management characteristics of a 100 kW full-powered FCV. The cooling components model, with parameters obtained by theoretical calculation based on the cooling requirement, is developed in the commercial solver GT-COOL. A thermal management simulation platform is constructed by coupling the stack model and cooling components. The accuracy of the modeling method for the stack is validated by comparing with the experimental data. The relationship between the operating temperature and output performance of the fuel cell stack is revealed based on the simulation model. The simulation results show that the operating temperature has a considerable influence on stack performance under high-current operation, and the inlet and outlet temperatures of the stack change nearly linearly with the increasing environmental temperature. The heat dissipation potential of the thermal management system under the high-load condition is also verified. The temperatures and coolant flow of core components, including the stack, DC/DC, air compressor, and driving motor, can meet the cooling requirements.

Numerical simulation of subcooled flow boiling in a manifold microchannel heat sink

Recently, various miniaturized electronic systems and the rapid increase in power density of microelectronic devices have created additional requirements for micro-scale heat dissipation technology. To dissipate heat in a micro-scale region efficiently, several creative cooling methods for high heat flux dissipation have been designed on the order of 1000 W/cm2. Due to its high surface-to-volume ratio, the microchannel heat sink has become the most popular one among emerging technologies and an effective tool for high heat flux thermal management. Nevertheless, in common electronic microchannel cooling systems, the traditional parallel microchannel causes a dramatic pressure loss along channel length. In this regard, the manifold microchannel heat sink cooling technology becomes promising with its high heat dissipation potential and moderate pressure drop penalty. The shortened effective flow length compared with the traditional microchannel provides a smaller pressure drop and delays the onset of the critical-heat-flux point. However, while a number of numerical and experimental studies have been conducted for the single-phase flow in the manifold microchannel, two-phase flow boiling in this kind of channel is not widely employed because of its issues with flow pattern prediction. This work develops a new solver based on open source software OpenFOAM to perform the transient process of subcooled flow boiling in the manifold microchannel heat sink. Additionally, heat transfer in the silicon substrate is considered to improve the accuracy of simulation. Similar to single-phase flow simulation for the manifold microchannel, a unit cell model extracted from the whole cooling plate is employed for the two-phase boiling flow to avoid huge computational costs. Through several reasonable assumptions, the symmetry boundary condition is applied on four lateral surfaces in the unit cell, while uniform heat flux is adopted on the bottom wall. A uniform inlet velocity boundary estimated from the total volume flow rate condition is coupled with a fixed temperature value of 7°C lower than saturation temperature. After the mesh independence test, the self-programming solver and numerical methods are validated by comparing numerical results with experimental data. It is determined that the boiling curve obtained by the empirical rate phase-change model with parameters r l= r v=100 agrees well with experimental results. Furthermore, heat transfer and pressure drop performance differences are caused by microchannel width w c and fin width w f. In comparison to sample A ( w c= w f=15 μm), decreasing w c and w f simultaneously leads to lower average wall temperature of the heated surface; the channel pressure drop between inlet and outlet surfaces rises dramatically when the widths decrease. When the total number of microchannels in the manifold microchannel cooling plate remains unchanged, increasing w c results in a decreased w f. Based on experimental results, heat sink sample C shows that the pressure loss can be considerably reduced under the premise of sacrificing a small degree of heat transfer performance, which can be attributed to the increased microchannel hydraulic diameter. The manifold microchannel heat sink with w c> w f is recommended for practical application due to lower pressure loss and average temperature variation.

Entropy generation minimization of thermoelectric systems applied for electronic cooling: Parametric investigations and operation optimization

Two categories of thermoelectric systems, i.e. thermoelectric active cooling (TAC) and thermoelectric self-cooling (TSC), were broadly applied for electronic cooling. Energy efficiency maximization and entropy generation minimization will be simultaneously achieved for both thermoelectric systems. Thermodynamic models of TAC and TSC systems are established to clarify the effects of main influencing parameters (i.e., applied current, thermal conductance in hot and cold ends) on the energy and entropy performances. Subsequently, under the situation of electronic cooling with constant material properties, a comprehensive performance comparison between TAC and TSC has been conducted, including decision targets, irreversible characteristics and environmental impacts. Furthermore, in order to improve the heat dissipation potential, the effects of material properties on the system performance are also sensitively identified for both TAC and TSC systems. For both systems, internal irreversibility is more pronounced than external one. Numerical results demonstrated that system efficiency of TAC could be higher than that of TSC; the operating mode for TAC could be completely switched to that for TSC when the input power was turned into zero. Present investigation could give suitable guidelines for thermoelectric systems applied for electronic cooling.

A Novel Thermal Network Model Used for Temperature Calculation and Analysis on Brushless Doubly-Fed Generator With Winding Encapsulating Structure

In recent years, magnetic-barrier rotor has been put forward for brushless doubly-fed generator (BDFG) application owing to its desirable performance, such as high power density and strong magnetic coupling ability. However, it also causes significant losses induced by harmonics and high temperature problem. Hence, it is a major challenge to evaluate Bref_DFG temperature rise fast and accurately. So far, aiming at magnetic-barrier rotor, there has been no accurate thermal network model established achieving this goal. In addition, wingding end encapsulating technology has been used for a few years and its heat dissipation potential is determined by the materials. Therefore, the application of novel material in winding encapsulating structure is desirable to improve the heat dissipation potential. In this paper, an equivalent thermal network model, thermal resistance models of magnetic-barrier rotor, and a winding encapsulating structure with novel materials are further established for thermal analysis. Subsequently, temperature distributions for BDFG components are also calculated by finite element method, while the results are compared with those by analytical method. The BDFG prototype is manufactured with experimental tests performed. The correctness of the equivalent thermal network models proposed in this paper is verified by the test results.