查看更多>>摘要：To meet the well-known energy transition challenge, a rapid shift from fossil fuels to the broader exploitation of renewable energy sources is needed; solar energy represents the most abundant and readily available resource amongst the renewable energy sources. This vision paper aims at shedding light on the current knowledge and emerging pathways for solar energy utilisation. Specifically, after a general introduction and a brief overview of the current knowledge, open issues are discussed regarding photovoltaic/thermal (PV/T) collectors, building integrated photovoltaic/thermal (BIPV/T) systems, concentrating solar power plants, solar thermochemistry, solar-driven water distillation, and solar thermal energy storage technologies. Subsequentially, this vision article defines key fundamental challenges that need to be addressed for these technologies to play a significant role in future sustainable energy systems. The identified challenges include developing new materials, enhanced performance, accelerated system installation and improved manufacturing processes, combining solar energy with other clean energy production and storage systems, and integrating solar energy utilisation with local energy utilisation patterns.
查看更多>>摘要：Superheated steam is produced in a super-heater using the energy of hot flue gases generated in the premixed combustion and it requires a larger surface area. Recently, unmixed combustion (UMC), an environmentally better process to premixed combustion, a variant of chemical looping combustion (CLC), has been investigated successfully for transferring heat to air, with a relatively compact system. Hence to address the limitations of the size of a super-heater and environmental aspects, it is decided to investigate the application of UMC for generating superheated steam. With this objective, the existing design of UMC based heat transfer system was modified for heating saturated steam to a desired temperature on a continuous basis. The modified system with Cu-based Oxygen Storage and Release Material (OSRM) was investigated using modeling and simulation study. A 2-D computational fluid dynamics (CFD) model was developed and simulated using COMSOL (TM). Oxidation and reduction cycles of UMC were studied separately and the study was restricted to laminar steam flow conditions. Results of the representative cases showed that the superheated steam of desired bulk temperature of 650 - 720 K at atmospheric pressure and heat flux of 13,000 - 15000 W/m(2) at the interface can be obtained with sustained combustion (the bed temperatures between 723 and 873 K) in both the cycles. The effect of four important operating parameters viz. inlet steam velocity, inlet reactive gas velocity and concentration, metal loading, was also investigated and optimum ranges were suggested to achieve the maximum possible heat transfer without affecting the bed operation. The simulation study was then extended to generate low and medium pressure superheated steams and obtained results have indicated that it is still possible to generate superheated steam at desired temperatures using UMC.
查看更多>>摘要：Micro-channel heat exchangers are widely used due to their superior performance compared to the existing fin and tube heat exchangers, but are limited to condensers due to refrigerant non-uniformity problems in evaporator operating conditions. Although many studies have been made to solve the refrigerant non-uniformity of the microchannel heat exchanger having a vertical header, a solution has not yet been proposed. In this study, a solution to the non-uniformity of the refrigerant was presented by applying a vertical header in the form of a double tube that can be applied to mass production. The effect of a double-tube form of the header on the refrigerant distribution and performance of a microchannel heat exchanger was explored by experimental means, using R-410A as the refrigerant. To investigate the effect of the shape of the header on the distribution of refrigerant in the final path of the heat exchanger and its performance, three types of multi-baffle and two types of multi-room header were considered. The tests were performed with an evaporation pressure in the micro channel heat exchanger of 800 kpa and a refrigerant flow rate of 50 to 80 kg/hr. The header with 5 independent refrigerant paths from the header inlet showed the most uniform distribution. The results showed a linear relationship between refrigerant distribution and heat exchanger performance. This study showed the effect of improving heat exchanger performance by up to 12% with the improved refrigerant distribution by applying a vertical header in the form of a double tube.
查看更多>>摘要：As the core component of the linear compressor which is a research hotspot of Joule-Thomson (J-T) throttling refrigerators for space applications, the discharge valve has a significant impact on their reliability and effi-ciency. Therefore, studying the dynamic characteristics of the discharge valve is indispensable which contributes to improving the compressor efficiency. Based on the oil-free linear compressor independently developed by the laboratory, a visual and real-time test system was set up to investigate the dynamics of the discharge valve and the compressor performance. While varying the parameters including piston stroke, valve stiffness, and operating frequency, the movement characteristics of the discharge valve and the thermodynamic efficiency of the compressor were measured and studied. The detailed principle of the valve movement characteristics on exhaust loss is discussed in the analysis of the results. The better discharge valve thickness for the linear compressor was selected. The experimental results indicated that the thermodynamic efficiency with the valve thickness of 0.152 mm is higher than 0.102 mm and 0.203 mm. The research in this paper will provide a valuable reference to design a discharge valve with high-efficiency and positive behavior for the linear compressor.
查看更多>>摘要：Film cooling using gaseous hydrocarbon fuel is an effective way to meet the thermal protection and friction reduction demands of hydrocarbon-fueled scramjet engines. The cooling and friction reduction characteristics of the hydrocarbon film can be influenced by the wall thermal state through the variation of endothermic and exothermic chemical reactions in the film. Numerical investigations based on the RANS method have been performed to evaluate the effects of the wall thermal state on the hydrocarbon slot film cooling performance. Four wall thermal states which represent different regenerative cooling levels are simulated. Results show that an increase of the wall cooling heat flux is beneficial to the film cooling performance while has unfavorable effects on the film friction reduction performance. Further analysis reveals that the net energy effects of the chemistry in the hydrocarbon film can be equivalent to an additional constant wall cooling heat flux of -0.36 MW/m(2) within a certain range. Besides, the proportion of this equivalent wall cooling heat flux in the total wall cooling heat flux decreases from 100% to 26% for different wall thermal states. Therefore, the cooling enhancement brought by the endothermic chemistry is comparable to the regenerative cooling. However, the wall shear stress is less sensitive to the wall thermal state. The maximum deviation of the total wall friction for different wall thermal states is less than 8% compared to the reference value.
查看更多>>摘要：A three-dimensional numerical model is developed and validated against experimental data for a cylindrical lithium-ion battery module with a compact hybrid cooling system of phase change material (PCM) and heat pipes. This cooling system is particularly well-suited for small or medium-sized battery module or pack. The PCM process and its effect on battery thermal behavior are numerically investigated for cases based on different C-rate discharge processes (0.5C, 1C and 2C), different PCM properties (different mass fractions of expandable graphite) and different charge-discharge cycles of the battery module. The results indicate that the PCM liquid fraction distribution is not uniform during the discharge process, the PCM starts to melt from its outermost neighboring the batteries and its upper part gets more liquid due to natural advection of liquid PCM. The different melting rates within the PCM lead to slightly enlarged temperature difference between batteries. Increasing discharge C -rate further accentuates this temperature difference. A tested composite PCM of 12 wt% EG (expanded graphite) shows better heat dissipation performance than that of pure PCM, and consequently it is better suited for battery thermal management systems. The simulations for different charge-discharge cycles indicate that lowering C -rate and increasing recovery time can improve thermal performance of the battery module and thermal control is thus more effective.
查看更多>>摘要：Numerical modelling and study of metal hydride bed with heat and mass transfer is a key to identify and predict the state of the storage device during absorption and desorption process. It is a great challenge to numerically study this problem considering cracks and volumetric expansion for conventional approaches due their limitations in treating discontinuities and large deformations. To effectively predict the hydrogen charging performance with the influences of cracks and volumetric expansion, a novel peridynamic (PD) model considering hydrogen absorption was proposed, which has not been used in most existing studies. The developed PD model was applied to simulate metal hydride reaction including depleted uranium (DU) and zirconium cobalt (ZrCo) in a thin-layer bed. The simulation results agreed well with both existing numerical solutions and experimental observations. The cracks were introduced to identify their influences on the performance of hydrogen absorption in a DU bed as a prototype. It is demonstrated that crack length and location had a significant influence on the absorption rate and average temperature evolution. With the increase in the crack length, the hydrogen absorption efficiency of the metal hydride bed (MHB) decreases dramatically due to the high bed temperature. The crack approaching the cooling boundary would greatly affect the temperature evolution, which causes the average H/M value to reach saturation later. Finally, the effects of the volumetric expansion were taken into consideration in the present PD model. Unlike ZrCo, the hydrogen kinetics performance would be overrated due to the neglection of the volumetric expansion in a DU bed.
查看更多>>摘要：Liquid-cooled miniscale channel heat sink has been widely used in electronic chip cooling due to its characteristics of energy saving, high heat dissipation, and easy integration. Although the cooling performance of the miniscale channel heat sinks can be further improved by the optimization of channel structures, the study on the characteristics of the heat transfer process not only help to improve the cooling performance, but also provide the guiding significance for the optimization of heat sinks. This paper conducts an experimental study on the optimization of the heat transfer performance for the proposed liquid-cooled miniscale channel heat sink with Y-shaped unit channels (MCHSYC) based on the constructal law and entransy theory. The models of entransy transfer are developed for the MCHSYC to analyze the effects of the flowrate and heat flux on the entransy dissipation rate, entransy dissipation-based thermal resistance, and efficiency of entransy. And then the models to determine the optimal flowrate of the MCHSYC is proposed by considering both heat transfer ability and average surface temperature. The results show that the optimal heat dissipation performance of the MCHSYC is obtained under the flowrate of 0.055 m(3)/h by considering both the amount of heat transfer and the heat transfer ability. Furthermore, a higher heat transfer ability and a lower average surface temperature of the MCHSYC can be achieved simultaneously from the estimated optimal range of flowrate using the proposed optimization models.
查看更多>>摘要：Heat pipe cooled reactor (HPR) is an ideal technology and a potential future niche where reliability and simplicity are key requirements. The heat transfer capacity of heat pipes determines the application range of HPR. Heat transfer limits (HTLs) of heat pipes are investigated, and the models of HTLs are evaluated in this study. Various HTLs of heat pipes with different filling ratios are tested. For the capillary limit, there is a sudden dryout in the evaporator section, in which the model of Chi is applicable for the horizontal conditions with a deviation of 19.0% but not for the inclined conditions. There is a possibility of sonic limit for a temperature rise in the adiabatic and condenser sections. The sonic limit predicted by Levy model is always higher than the experimental results. The entrainment limit is accompanied by a sudden rise in temperature of the evaporator section and temperature fluctuation in the condenser section, which can be estimated by using the wave-induced model with a relative error of 45.3% and is affected by the wick and inclination angle. The Knudsen number is used to determine the vapor state and the recommended critical Knudsen number of 0.01 is less than experimental data, which neglects the effect of inclination angle. The experimental results provide references for the heat pipe design and the accuracy of the present models are estimated.
查看更多>>摘要：This paper presents a fast parameterized modeling method for fractal channel and constructs a unified parametric model for Y-shaped, V-shaped, T-shaped fractal channels. This parametric model establishes the relationship between multiple parameters (levels ratio, bifurcation level, pipe diameter ratio, bifurcation angle, aspect ratio) and performance of fractal channel. Parameterized modeling method can reduce repetitive workload and time, improve efficiency. Meanwhile, parametric model allows for a more comprehensive study, and it is important for reduce the occasionality of results. Using the parameterized modeling method, the aspect ratio and fractal channel type are numerically studied. The numerical study of the aspect ratio reveals that with the increase of aspect ratio, Nusselt number decreases, the peak temperature rises and then falls, and the pressure drop first decreases and then increases. The numerical study of fractal channel types shows that the V-shaped channel has the best cooling performance and Y-shaped channel has the best flow performance. Compared to the T-shaped channel, Nusselt number of V-shaped channel increases by 11.29%-19.74%, the thermal resistance of V-shaped channel decreases by 8.18%-19.73%, and the pressure drop of Y-shaped channel decreases by 51.04%-61.75%. The fractal channel parametric model can be applied not only for parameter studies, but also for parameter optimization.
NSTL
Elsevier