查看更多>>摘要：Jet impingement as a highly efficient cooling method has been broadly used in the field of heat dissipation for high heat flow electronics. Well-designed structures including jet hole and pin-fin arrangement can significantly alter flow fields to provide a favorable trade between increased heat transfer performance and total pressure loss. In order to solve the imbalance problem of temperature uniformity and power consumption caused by nonuniform heat flux distribution in multi-core integrated circuits, a novel jet impingement cooling concept, for hotspot-targeted, energy efficient cooling of non-uniform heat generation electronics, was proposed in this paper. A three-dimensional numerical simulation was carried out to investigate the effects of jet hole diameter and distribution, heat transfer plate type and inlet area on heat transfer and resistance performance. The heat transfer coefficient, total pressure drop, maximum temperature rise and temperature non-uniformity were selected as the performance parameters. Compared with the traditional jet impingement heat sink, the well-designed heat sink in this work achieves an improvement of 58% in temperature non-uniformity while the total pressure drop is reduced by 68.7% at the same mass flow rate. In addition, the pumping power consumption is only 0.093% and 0.061% of the total heating power for 150 W/cm(2) and 300 W/cm(2) hotspot heat flux respectively while ensuring the acceptable temperature non-uniformity.
查看更多>>摘要：The efficiency of the photovoltaic (PV) panel would drop significantly as the absorbed solar radiation converts into heat. To improve the performance of PV, a novel cooling system with phase change material (PCM) and air forced convection cooling is developed, the flow and heat transfer of molten PCM (mPCM) under flow boundary is focused and investigated through numerical method. Three Rayleigh numbers for heat flux boundary (10(4), 10(5), and 10(6)), five Reynolds numbers for forced convection boundary (1 x 10(4), 1.5 x 10(4), 2 x 10(4), 3 x 10(4) and 4 x 10(4)), three aspect ratios (5, 8, and 10) and tilt angles (30 degrees, 45 degrees, and 60 degrees) for design parameters are considered. A group of dimensionless numbers is performed to describe heat transfer characteristics. It is concluded that different boundary conditions would cause different flow and heat transfer characteristics in the enclosure. Especially for the forced convection boundary, increasing Reynolds number can improve the heat transfer efficiency and reduce the temperature at the same time. Further, the influence of each factor on heat transfer is quantified, as the Rayleigh number increases by two orders of magnitude, Reynolds number increases by 33.33%, aspect ratio enlarged by double variation and tile angle varied by 50%, the heat transfer efficiency changes by 36.53%, 10.76%, 28.45% and 20.34%, respectively.
查看更多>>摘要：Insulating engine combustion chamber surfaces with thermal barrier coatings (TBCs) provides engine thermal efficiency improvement by reducing the heat transfer to the coolant when done appropriately. However, reducing heat losses using traditional ceramic insulation increases combustion chamber wall temperatures during the whole engine cycle, including intake stroke resulting in a decreased engine volumetric efficiency due to excessive intake air heating. This paper presents the concept of partial insulation coverage on a selective area of combustion chamber wall by simulations to improve the engine's thermal efficiency. A combined combus-tion-conjugate heat transfer analysis methodology was developed to determine the partial-coverage insulation area by analyzing the heat flux and surface temperature distribution across the combustion chamber wall. Engine cycle simulations were run to compare the engine performance with various partial insulation designs, and an optimal insulation area was proposed. To validate the simulation results, the full-coverage coated piston tech-nology with 0.5 mm thick Yttria Stabilized Zrconia (YSZ) coating has been experimentally evaluated on a pro-totype engine and compared to the non-insulated baseline aluminum piston. It was found that the partial coverage insulation showed a thermal efficiency improvement of 1.98% relative to a full-coverage insulated piston efficiency of 2.06%.
查看更多>>摘要：To lower the energy consumption of Li-ion power battery packs thermal management, this paper investigates an improved heat pipe/phase change material coupled thermal management in a 55-Ah Li-ion battery pack, in which fans can be utilized to strengthen the air condensing effect of the heat pipe. The purpose of this paper is to optimize heat pipe/phase change material coupled thermal management and assess its energy-saving potential for long-time running. Based on the numerical calculation with a 2-dimension coupled resistance-capacity model, the role of phase change material is defined comprehensively, and a multi-cycle process is investigated for long-time temperature control stability. Further multi-objective optimization is conducted to minimize both thickness of phase change material and power of fans under the worst working condition. Finally, the energy-saving effects of optimized coupled thermal management under different discharge-charge conditions are assessed. Results demonstrate that phase change material is used to "shift load" during discharging and "fill valley" during charging, and the intensity of "load shifting" has been discussed. For long-time running, battery surface tem-perature fluctuates upward and then steadily when heat accumulated during discharging equals to heat-releasing during charging. Furthermore, it also shows that the maximum energy-saving rate reach up to 81.80% by active-passive thermal management. In conclusion, heat pipe/phase change material coupled thermal man-agement possesses considerable energy-saving potential after optimization and is promising to utilize in battery temperature control field with lower carbon emissions.
查看更多>>摘要：Tangential leakage is an important factor affecting the aerothermodynamic performance of the scroll compressor, and existing methods to reduce tangential leakage usually aim at narrowing leakage path. In this paper, continuous seal-grooves arranged on the sidewall of static scroll was proposed, in order to achieve the passive flow control of the tangential leakage. Numerical methods were conducted to investigate the flow characteristics of tangential leakage in the radial clearance with seal-grooves. The impacts of diverse geometric parameters and arrangement schemes on the performance of the scroll compressor were also discussed. Results showed that throttling effect, flow separation of tangential leakage, and secondary axial turbulence was the main influence mechanism of the seal-grooves. Appropriate groove depth and spacing contributes to the developments of flow separation and secondary axial turbulence in seal-grooves, while increasing seal-groove number brought better flow control effect. Based on the above results, the seal-grooves arranged on the sidewall of compression chamber increased separately the volumetric efficiency and isentropic efficiency by 1.63% and 1.32%, and the volumetric efficiency also improved by 0.99% when the seal-grooves were arranged in the suction chamber. This work may provide a new flow control method of tangential leakage in the scroll compressor.
查看更多>>摘要：Vertical-fin microchannel heat exchanger has better frosting and defrosting performance owing to its better water drainage along vertical fins. Effect of hydrophilic and superhydrophobic coatings on frosting and defrosting performance is experimentally compared and drainage performance is investigated by dynamic dip testing and simulation. Results show that since water has smaller adhesive force on superhydrophobic fins, defrosting water carries some frost while is separated from the evaporator in form of frost-water mixture. Hence, the superhydrophobic vertical-fin microchannel evaporator has shorter defrosting duration by 9.8%. Both hydrophilic and superhydrophobic samples have satisfactory cyclic repeatability without obvious capacity attenuation. During defrosting, most water can flow down the leading edge extension. The residual water of the superhydrophobic sample is 61 g/m(2), which is 19.6% less than that of hydrophilic one.
查看更多>>摘要：Due to their high conductivity, fractures play a vital role in geothermal exploitation. Previously, multiple random fractures, considered as the primary channel in enhanced geothermal systems (EGS), were studied extensively. However, natural fractures may not be developed in hot dry granite and fluid flow may be controlled by faults or hydraulic fractures. Based on fractal theory, this paper considers that hydraulic fractures have fractal-like tree characteristics and focuses on the influence of their geometric and heterogeneous characteristics on heat extraction. The results show that a higher fracture number, fracture length and a staggered fracture layout are the key factors affecting heat extraction, which obviously enhance the heat extraction efficiency. Additionally, a higher fracture length ratio and aperture ratio and a greater distance between the primary fracture and the lateral well are suggested in EGS, which are important factors influencing heat extraction. However, the effect is weaker than the above 3 factors. The fracture stage, the spacing between the primary fractures and the fracture angle are secondary factors, which should not be excessively focused on in EGS. The results can provide some reasonable for reservoir
查看更多>>摘要：In this study, a three-dimensional numerical simulation is carried out to implement a comparative analysis between the glazed and unglazed photovoltaic/thermal (PV/T) integrated structures incorporated without or with phase change materials (PCMs) at distinct locations using water as a coolant. Accordingly, six glazed and unglazed PV/T or PV/T-PCM configurations are designed through burying the water pipes within the PCM and positioning an additional PCM layer with different melting temperature from the upper one. The performance of the proposed layouts is evaluated and compared from the energy and exergy points of view. The effect of coolant flow rate is also evaluated in both the laminar and turbulent regions along with assessing the environmental impact of the proposed modules to analyze the daily CO2 reduction. The results show that burying the water pipes inside the PCM beneath the PV/T module yields a significant improvement of the overall performance. Also, the electrical energy generated from unglazed modules is somewhat higher than that generated from the glazed modules, however the daily thermal energy output for the glazed modules is marginally higher than that generated from unglazed layouts having the same layer arrangement. Among the proposed structures, the glazed and unglazed PV/T modules integrated with dual PCMs achieve the highest daily average energetic and exergetic efficiencies, respectively.
查看更多>>摘要：In this study, a compound parabolic concentrating photovoltaic/thermal phase change system with different heatsinks (S-type and H-type) are constructed in an open-air environment to experimentally analyze the influ-ence on photothermal, photovoltaic and heat-electricity cogeneration performance. The non-uniform irradiance distribution at different moments is simulated with the Monte Carlo rays tracing method, and it is demonstrated to be the main cause of non-uniform temperature distribution on photovoltaic module surface. The H-type heatsink system integrating phase change material (PCM) owns better temperature regulation performance, as its photovoltaic modules temperature and non-uniformity factor are about 0.8 celcius and 0.05 lower than those of S-type respectively when PCM begins to melt. The instantaneous electrical and thermal efficiency with H-type heatsink are 0.6% higher and 10.0% lower than those with S-type, respectively. Considering that the pump power consumption of S-type heatsink is over 3 times of H-type, the instantaneous priamry energy-saving effi-ciency of S-type system is 8.0% higher. Simultaneously, the maximum primary energy-saving efficiency of concentrating photovoltaic/thermal phase change system with H-type/S-type heatsink is about 7.9%/14.6% and 10.7%/17.4% higher than that of concentrating photovoltaic and solar collector system respectively, showing great potential of photovoltaic/thermal phase change system in heat-electricity cogeneration performance especially utilizing S-type heatsink. Results present advantages of the system utilizing S-type heatsink and help promote the application of heatsinks in photovoltaic/thermal phase change system.
查看更多>>摘要：A numerical model is developed in the present work for analysing cooling performance of combined impingement-film cooling under realistic engine operating conditions. Effect of jet impingement along with the film cooling is analysed on fluid flow, heat transfer, cooling performance and pressure drop for forward and reverse orientations of the cooling hole. Further, effect of hole orientation on the cooling performance is studied under wide range of blowing ratios and jet-to-plate spacings. It is observed that the effect of jet impingement is significant on the cooling performance. Surface temperature of the metallic plate is found to be 50-90 degrees C lower for the combined jet impingement and film cooling case as compared to the only film cooling case. The improvement in the heat transfer for combined impingement-film cooling is at the cost of increased pressure drop. Film cooling effectiveness for the case of combined impingement-film cooling is found to be dependent on the hole orientation and blowing ratio. The secondary jet penetration into the mainstream increases with increase in the blowing ratio/velocity ratio. Kidney-vortices are formed at a velocity ratio of 1.07 from forward injection which resulted in decreased film cooling effectiveness. These kidney-vortices were not formed for the case of reverse injection and coolant spreads uniformly in the transverse direction. At lower velocity ratios when formation of kidney vortices is not prominent, forward injection results in better film cooling effectiveness compared to the reverse injection. Gas radiation is also considered in the modelling and its effect is found significant on the temperature prediction of the components.
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Elsevier