Heat Dissipation Ability Research Articles

Enhanced Cooling of LED Filament Bulbs Using an Embedded Tri-Needle/Ring Ionic Wind Device

Improving the heat dissipation ability for light-emitting diode (LED) filament bulb is very difficult. A tri-needle/ring ionic wind generator was developed to improve the heat dissipation condition of bulbs. The operation characteristics of the ionic wind generator, such as the electrode gap, inception voltage, and discharge current with regard to the operation voltage, were studied by experiments. The ionic wind velocity within the bulb was investigated under different electrode gaps and applied voltages. The temperature drop achieved by the ionic was were tested with the consideration of many operation parameters for analysis. The experiments showed that ionic wind can provide efficient and stable cooling effect for LED filament bulbs. The temperature drop of LED junction can reach 30 °C at best. Good stability and adjustability of the ionic wind generator were demonstrated by tracing the temperature history of LED filaments in long-term tests. Finally, the light efficiency of LED filament bulbs with ionic wind cooling was studied. The experimental results showed that light efficiency was improved by 7.3% under the best cooling case. The experimental results indicate that the embedded ionic wind generator can provide an effective solution for cooling LED filament bulbs.

Tailoring of Si–C–N–O ceramic-coated reduced graphene oxide by oil/water-solution process for high thermal conductive epoxy composite with electrical insulation

Polymer composites with high thermal conductivity are required for addressing the heat management issues associated with the rapid growth of electronic device technology. As a strategy to improve the heat dissipation ability of composites, graphene has been attracting attention as a filler due to its superior high thermal conductivity. However, its high electrical conductivity poses a challenge for use in electronic applications. In this study, we report the synthesis of silicon oxycarbonitride-coated reduced graphene oxide (SiCNO-rGO) by an oil/water biphasic solution process to improve the thermal conductivity of epoxy composites. The precursors are prepared by controlling the hydrolysis/condensation reactions and are subsequently transformed to polymer-derived ceramics by pyrolysis. The manufactured Al2O3/epoxy composite, with assembled SiCNO-rGO, exhibits excellent interfacial adhesion due to the structural affinity between the filler and matrix, homogeneous dispersion, and a synergistic effect with the hybrid fillers. The thermal conductivity of the SiCNO-rGO/Al2O3/epoxy composite can reach up to 2.29 W m−1 K−1, corresponding to a thermal conductivity enhancement of about 1662%. Moreover, the insulating property of the epoxy composite is still retained up to a SiCNO-rGO loading of 3 wt %. This approach to the surface modification of graphene provides a potential tool for heat management in a wide range of thermal conductive applications.

A novel radiator structure for enhanced heat transfer used in PEM fuel cell vehicle

Fuel cell vehicle (FCV) is regarded as a worldwide promising technology due to its high efficiency and zero emission. Heat management is very important during the operation of FCV, especially under high power output and environment temperature. As a consequence, development of a high heat dissipation ability radiator is necessary for the state-of-the-art FCV. In this paper, heat exchange characteristics of four different radiators are investigated in detail. A special air flow deflection phenomenon in the Windward bend structure is found to be beneficial for heat dissipation. A novel structure of Windward-Louvered fin (WLF) is proposed to be used in the radiator. The result shows that its Nusselt number is 1.2-1.3 times higher than that in Windward bend structure. In addition, field synergy theory is used for the evaluation of the heat transfer ability among the four structures. The synergy angles between velocity vector and temperature gradient are 82.1°, 82.5°, 87.0°, 81.8° at the air flow rate of 12 m•s−1, respectively. The WLF structure has the smallest synergy angle, thus the heat exchange amount is over 1.25 times higher than that of Windward bend structure.

Facile method to fabricate highly thermally conductive UHMWPE/BN composites with the segregated structure for thermal management

ABSTRACTOwing to the thermal management of polymer-based thermal conductive composites provide great performance in various applications, widespread attention has been focused on the design and fabrication of thermally conductive materials. In the work, ultrahigh molecular weight polyethylene (UHMWPE) / boron nitride (BN) composites with segregated structures were fabricated by a facile heat-treatment powder mixing method. SEM and optical images revealed that BN flakes can be tightly overlapped on the surface of UHMWPE particles by heat-treatment in the mixing process and the thermally conductive network was more continuous and homogeneous, leading to the greatly improved thermal conductivity. Laser flash thermal analyzer (LFA) demonstrated that an optimised in-plane thermal conductivity of UHMWPE/BN composites fabricated by the heat-treatment powder mixing method can reach to 9.99 W m−1 K−1 by adjusting the BN loading to 21.6 vol %. In summary, the UHMWPE/BN composite with high thermal conductivity provides excellent heat dissipation ability in the in-plane direction, which is very important for thermally conductive materials.

Enhanced heat dissipation in graphite-silver-polyimide structure for electronic cooling

Thermal dissipation has become a vital issue in electronic devices with highly miniaturization and integration. As a high thermal conduction material, graphite has been used widely in these devices. However, heat dissipation of graphite is limited by the poor thermal radiation in serving as a heat sink. In this work, we proposed a novel composite foil with a sandwich structure of graphite-silver-polyimide, exhibiting high thermal conductivity and excellent selective emission property, which is attractive for application in outdoor space. The heat dissipation performance of the foil was numerically and experimentally investigated. Our simulations confirmed that employing a selective thermal emitter in the sandwich structure prevented object surface from overheating under direct sunlight, augmenting the dissipated heat generated by heat source, reducing the local high temperature of surface. By the solar simulator and infrared camera, we experimentally demonstrated the numerical prediction that the heat dissipation ability of sandwich structure is much better than the graphite.

Design and performance of an internal-cooling turning tool with micro-channel structures

The application of coolant is generally in the metal cutting process. However, conventional flood cooling shows inadequate cooling effect during the machining of superalloys. Internal-cooling approach is a capable alternative to enhance the heat transfer ability of coolant and lower the machining temperature. In this paper, an internal-cooling turning tool with micro-channel structures is proposed for turning Inconel 718. Initially, computational fluid dynamics (CFD) simulation was conducted to evaluate the micro-channel structures inside the tool. The micro-channel diameter shows a great influence on the coolant outlet velocity which appreciably affected the heat dissipation ability of the tool. According to the simulation results, the micro-channel diameter was set to 2 mm. Subsequently, various structures of the tool were machined and assembled. Finally, turning experiments of Inconel 718 were conducted under dry, flood cooling and internal-cooling conditions. Using the same turning parameters, internal-cooling technology is advantageous to offer higher heat transfer efficiency and lead to smaller cutting force, lower cutting temperature and more favorable surface topography, compared with the dry and conventional flood cooling approaches. Under internal-cooling condition, the cutting force and cutting temperature decreases gradually as the coolant inlet pressure increases. Besides, higher inlet pressure also leads to lower surface roughness.

POWER, METALLURGICAL AND CHEMICAL MECHANICAL ENGINEERING CRYOGENIC INSTRUMENT REFRIGERATING BY LIQUID NITROGEN IN UNDERHEATED CONDUCTION

Objectives. Cryogenic surgical instruments have been successfully used to conduct tumor ablation. For a long time, cryoablation procedures in Russia were carried out using cryodestructors cooled with liquid nitrogen, which are able to remove heat from the ablation object with high heat flux density and quickly form a cryoablation zone, while being relatively cheap and easy to operate. However, these instruments turned out to be unsuitable for minimally invasive surgeries; therefore, they are squeezed out of practical medicine and cryosurgical instruments are cooled by throttling argon gas. This led to the purpose of the study - the choice of equipment for the organization of local supercooling of pathological tissue.Method. To solve the problem of optimizing the cryogenic pipeline, a method was chosen for finding the Pareto-optimal solution. To solve this problem, it is sufficient to increase the pressure in the fluid flow directed to the NCS using a liquid micropump. In the role of quality criteria in this task, we selected: hydraulic loss power and heat loss power.Result. The following results were obtained: the minimum pressure in the vessel, which ensures the movement of the fluid in a single-phase state, according to the magi-line of 1 m length is 0.75 MPa; With this pressure, through a line with a diameter of 1 mm, the flow rate is maintained up to 6 kg / h; heat dissipation ability of the instrument reaches 608 watts. The thermal load on the cooling system of the heatdissipating device of the device for CA is unsteady and is formed due to the heat reserve accumulated in the patient's tissues.Conclusion. The use of liquid nitrogen in the undersized cryosurgical equipment makes it possible to overcome the noted drawbacks of liquid cryodestructors.

Mechanically stretchable piezoelectric polyvinylidene fluoride (PVDF)/Boron nitride nanosheets (BNNSs) polymer nanocomposites

Mechanically stretchable piezoelectric materials are required by wearable devices, such as self-powering sensors and energy harvesters that can monitor human movement and vital biosignals. Polyvinylidene fluoride (PVDF) is a common choice for making flexible piezoelectric sensors but has limited stretchability (strain to failure). Herein we present a new method of significantly increasing the strain-to-failure of PVDF nanofibre membranes by incorporating amino-functionalised boron nitride nanosheets (BNNSs). The PVDF-BNNS nanocomposites were prepared by firstly dispersing a small amount of BNNSs, less than 1.5 wt%, into a PVDF solution and electrospinning. Compared to the un-modified PVDF, the addition of BNNSs yielded increases of eleven times, twice, and forty times in strength, stretchability, and in-plane thermal conductivity, while piezoelectric performance remains unaffected. The higher thermal conductivity offers significantly better heat dissipation ability of piezoelectric sensors than the un-modified PVDF material.

Thermal management integrated with three-dimensional heat pipes for air-cooled permanent magnet synchronous motor

Permanent magnet synchronous motors (PMSMs) are widely used in pure or hybrid electric vehicles for transportation applications. The cooling strategy for PMSMs is still a challenge in the field of thermal engineering. This study presents a novel thermal management method/design based on phase-change heat pipes for PMSMs. Two thermal management modules (TMMs) with different heat-pipe layouts are prepared, including the straightly embedded module in an enclosure (SEME) and three-dimensional rounding module (RM). The impacts of different ambient temperature and various working condition on the cooling effectiveness of the two modules are analyzed experimentally and numerically. Experimental results show that by replacing the convectional motor stator with this heat-pipe-based enclosure component, the effective time to control the temperature of a PMSM can be prolonged by 28.6 and 21.4% when the motor operates continuously under high-speed and high-torque conditions, respectively. The peak temperature of the PMSM under rated conditions can be significantly reduced by 22.3%, which helps increase the power density of PMSMs. Besides, both experimental and simulation results show that the use of a SEME promotes higher heat dissipation ability than RM.

Numerical study of heat transfer in underground power cable system

Underground power cable system has been wildly used as an effective transmission line. This paper presents the thermal analysis of the two formations (Flat and Trefoil) of buried system. Temperature-dependent thermal conductivity of soil was considered in the research and the Fluidized Thermal Backfill (FTB) was applied to increase the ability of heat dissipation of power cable system. Further, the influences of heat transfer performance and ampacity on power cable systems were studied when FTB was applied in two formations. Firstly, it is found that the geometric parameters have significantly effects on the heat transfer performance of underground power cable system. Secondly, considering the temperature-dependent thermal conductivity, the maximum temperature of cables will rise rapidly, the limited temperature is easy to reach. The application of FTB can enhance the heat transfer of the whole system, especially for trefoil formation. Finally, the analysis of ampacity shows that the FTB will ensure the higher loading current in a security condition with the temperature-dependent thermal conductivity.

In situ study of field emission vacuum breakdown of individual multi‐wall carbon nanotube

The vacuum breakdown critical condition of individual carbon nanotube (CNT) during field emission was investigated experimentally using the in-situ field emission combined scanning electron microscope system. The results showed that the critical breakdown electric field of CNT was related to its length and diameter. CNT which has a shorter length and larger diameter could bear higher electric field. The underlying mechanism was attributed to the heat dissipation ability of CNT. A smaller length and larger diameter CNT has better heat dissipation. The results met with the theoretical calculations and helped to understand the physics mechanism of vacuum breakdown of CNT during field emission in a visual perspective.

Polyacrylonitrile coupled graphite oxide film with improved heat dissipation ability

Graphene film has potential to act as an excellent heat dissipation material. However, its thermal conductivity suffers from some limitations, including small graphene lateral sizes and low production efficiency from a practical viewpoint. To solve these issues, we propose an industrially viable protocol to synthesize homogeneously oxidized graphite oxide (GtO) with large lateral sizes and easy-purifying features. We employ polyacrylonitrile (PAN) as a coupling agent to repair the defects on GtO sheets and link these sheets together during the annealing treatment. The graphitized GtO/10%PAN film (g-GtO/10%PAN) shows excellent thermal and electrical properties, with an in-plane thermal conductivity of 1629.24 ± 43.88 W m−1 K−1 and an electrical conductivity of 9645.29 ± 199.44 S cm−1. Furthermore, it also exhibits outstanding flexibility; the original thermal conductivity can be maintained after 20,000 times bending. The g-GtO/10%PAN film offers an alternative to replace commercially used graphitized polyimide (PI) film. Our research provides a general method for the fabrication of high quality graphene film.

Experimental study and optimized design on electromagnetic pump for liquid sodium

The electromagnetic pump (EMP) was a critical device and widely applied in Sodium-cooled Fast Reactors (SFR) to deliver liquid sodium. Annular linear induction pump (ALIP) showed relatively higher efficiency than other pumps. Equivalent circuit method was commonly adopted to design the ALIP. Hence, a theoretical model between flow rate and developed pressure of EMP was proposed after detailed deduction based on equivalent circuit theory. To verify the theoretical model, some tests of ALIP were conducted to investigate the characteristic of volume flow rate and developed pressure. Besides, sodium temperature limitation test was also accomplished to test the heat dissipation ability of winding coils through natural air-cooling, results showed that the limit was about 270 °C. Performance analyses was carried out through proposed model which has been verified by experimental results. It turned out that power frequency has positive effect on the pump, whereas sodium temperature showed converse results. Furthermore, the developed pressure increased fast first and then turn to decrease gradually as the annular flow channel width increases. Finally an optimized design was put forward after combining theoretical analyses and experimental research. Compared to the previous ALIP, the volume flow rate, developed pressure and efficiency increased from 9.07 m3·h−1, 386.96 kPa and 28.6% to 10.0 m3·h−1, 574.55 kPa and 48.11% respectively.

Reinforced heat dissipation by simple graphene coating for phosphor-in-glass applied in high-power LED

A well dispersed graphene oxide sol was prepared by the improved Hummer method. The atomic force micrographs showed a monolayer structure and larger sheet area. Aiming at improving the poor heat dissipation ability of Phosphor-in-glass (PiG)for remote packaged LED, the reduced graphene oxide film was prepared by photoreduction that after the sol was coated on PiG. UV–Vis spectra showed that the degree of reduction of graphene oxide was related to the illumination time, while the reduced graphene film had high transmittance without affecting the luminescent properties of the fluorescent material. At the same time, the excellent thermal conductivity of the film significantly enhanced the cooling capacity of the PiG, which has a lower stable operating temperature and a higher luminous intensity under high-power excitation compared with the uncoated ones. Software simulation also showed similar results of heat dissipation.

Energy and Hydraulic Performance of a Vegetated Roof in Sub-Mediterranean Climate

Widespread overbuilding, the prevalence of asphalt surfaces on green areas, and the use of building materials with low heat dissipation abilities are among the main causes of Urban Heat Islands. Within urban areas, evapotranspiration and shade from plants can significantly reduce the UHI phenomenon, help in stormwater management, and reduce building energy consumption. The goal of this work is to analyze the hydraulics and energy performances of an experimental extensive green roof at the University of Calabria (Italy) in Mediterranean area. This study confirmed that green roofs significantly mitigate storm water runoff generation in terms of runoff volume reduction and peak attenuation, and improve the thermal performance of buildings and the internal comfort of indoor spaces.

Macroporous epoxy-carbon fiber structures with a sacrificial 3D printed polymeric mesh suppresses electromagnetic radiation

Metals are known to be highly conducting and can shield electromagnetic waves quite well. In a quest to explore materials that are lightweight, corrosion free, easy to fabricate and integrate/embed, epoxy-carbon fiber composite structures have attracted a great deal of attention for myriad applications. Herein, we have fabricated macroporous structures involving epoxy and bi-directional carbon fiber (CF) for suppressing electromagnetic (EM) radiation, using 3D printed polymer mesh as a sacrificial layer in the laminate. This strategy reduces the weight of the composites by 15% besides retaining the EM blocking capability. In order to further enhance the shield-ability of the composite structures, ferromagnetic nanoparticles were electrodeposited directly on the bidirectional CF and infused with epoxy, using vacuum assisted resin transfer, and the 3D printed mesh. The latter was used as a sacrificial layer and was etched out from the final laminate structure to fabricate macroporous epoxy-CF laminates. The laminates with CF deposited with nickel on one side and cobalt on the other side showed better shielding manifesting in -40 dB (for 1.4 mm) as compared to other laminate structures. Upon etching the 3D printed mesh from the laminates, the resultant macroporous epoxy-modified CF laminates exhibited a shielding of −45 dB (for 1.4 mm thick) along with a thermal stability up to 200 °C, enhanced flame-retardant properties and excellent heat dissipation ability. Taken together, this macroporous epoxy-CF laminates exhibited multifunctional properties and can further be explored for advanced EM suppressing material.

Core-Multishell Heterostructure with Excellent Heat Dissipation for Electromagnetic Interference Shielding.

Herein, we report high electromagnetic interference (EMI) shielding effectiveness of -40 dB in the Ku-band (for a 600 μm thick film) through a unique core-shell heterostructure consisting of a ferritic core (Fe3O4) and a conducting shell (multiwalled carbon nanotubes, MWCNTs) supported onto a dielectric spacer (here SiO2). In recent times, materials with good flexibility, heat dissipation ability, and sustainability together with efficient EMI shielding at minimal thickness are highly desirable, especially if they can be easily processed into thin films. The resulting composites here shielded EM radiation mostly through absorption driven by multiple interfaces provided by the heterostructure. The shielding value obtained here is fairly superior among the different polymer nanocomposite-based EMI shielding materials. In addition to EMI shielding capability, this composite material exhibits outstanding heat dissipation ability (72 °C to room temperature in less than 90 s) as well as high heat sustainability. The composite material retained its EMI shielding property even after repeated heat cycles, thereby opening new avenues in the design of lightweight, flexible, and sustainable EMI shielding materials.

Improved organic-inorganic/graphene hybrid composite as encapsulant for white LEDs: Role of graphene, titanium (IV) isopropoxide and diphenylsilanediol

We report a graphene embedded organic-inorganic hybrid (O-I hybrid) composites that can be utilized as robust encapsulants for white light emitting diodes (WLEDs). The effect of graphene and monomers on the thermal resistance, refractive index (RI), transparency, thermal conductivity, and thermal aging stability of the encapsulant were studied. This study has explicitly unveiled that the embedded graphene played a multifunctional role in producing a high-performance encapsulant. Compared with graphene free O-I hybrid encapsulant, RI, thermal conductivity and heat dissipation ability of the graphene embedded encapsulants were significantly improved. After continuous operation for 10 days, the color rendering index (Ra) of WLED with graphene embedded encapsulants changed from 89.5 to 89.2 (driving current 100 mA), while the correlated color temperature (CCT) altered from 4535 to 4546. Its luminous efficiency changed from 82.9 to 81.2 lm/W. The WLEDs with graphene embedded O-I hybrid encapsulant exhibited long-term stability. This work has paved the way for rational design and assembly of graphene embedded composites as robust materials for various requirements of WLED applications.

Hybrid Excitation Stator PM Vernier Machines With Novel DC-Biased Sinusoidal Armature Current

Stator permanent magnet (PM) machines are gaining more research interest in recent decades. Since the permanent magnets (PMs) are moved from the rotor side to the stator side and the salient rotor structure is adopted, it is claimed that the stator permanent magnet machines exhibit several promising features, such as simple and robust rotor, and good ability of heat dissipation as the main losses are located in the stator. On the other hand, to improve the shortcomings of the inherent limited magnetic field adjustment ability, and the low efficiency and power factor of stator dc current excited machines, hybrid excitation PM machines with additional field windings are proposed. However, the complexity of the machine structure is increased inevitably. At present, it is found that both the stator PM and the stator dc current excited Vernier machines operate with the flux modulation principle, and generally, the armature windings of the stator PM machines are fed with pure sinusoidal current. In this paper, a novel hybrid excitation stator PM Vernier machine is analyzed. In addition to the features of the salient rotor structure, stator located PMs, and concentrated armature windings, the novelty of this machine is dc-biased sinusoidal phase current, which contains an ac component and a dc component. The fundamental theories of stator PM machines fed with dc-biased current are researched, operation principle and slot/pole combination is concluded, and the optimal current configuration is studied theoretically and by finite element analysis. Finally, a prototype was designed and manufactured; the experimental tests were done to validate the proposed analysis.

Multiscale Modeling of Heat Dissipation in 2D Transistors Based on Phosphorene and Silicene

We use multiscale modelings to investigate the heat dissipation in 2D transistors based on phosphorene and silicene. First, molecular dynamics (MD) simulations were used to calculate the thermal interface resistance Rint between the 2D materials (phosphorene and silicene) and dielectrics substrates (SiO2 and TiO2). The calculated Rint of these systems are close to that between graphene and SiO2 and are insensitive to the temperature. The MD values then served as inputs for finite-element simulations at the device scale. It is found that the heat-dissipation ability of the 2D transistors can be improved by increasing the thermal conductivities of the 2D materials as well as of the substrate. However, in contrast to the common belief, it is difficult to largely reduce the hot-spot temperature by tuning the interface thermal resistance. Finally, we show that the cooling performance of silicene/SiO2 system can be significantly improved with a few-layer graphene heat spreader. These results provide important information for the novel design of 2D transistors in terms of thermal management.