查看更多>>摘要：Phase change material (PCM) is a hot topic in the field of battery thermal management (BTM) for their high latent heat and excellent temperature homogeneity. However, it is limited by the problems such as easy leakage, low thermal conductivity, high rigidity. Herein, we prepared a novel stable and flexible composite phase change materials (FCPCM) successfully. Paraffin (PA)was employed as PCM, Ethylene-ethylene-butadiene-styrene block copolymers (SEBS) as support material, polyethylene octene co-elastomers (POE) as flexibility enhancer, and Expanded graphite (EG) was selected to boost the thermal conductivity. Experimental results showed that the FCPCM leakage rate remained below 1%. Furthermore, the thermal resistance test showed that the contact thermal resistance decreases from 0.735 degrees C/W to 0.119 degrees C/W as the temperature rises from 37.35 degrees C to 53.41 degrees C. In addition, the battery cycle test showed that during 3C rates, FCPCM could control the temperature of the battery module at 41.56 degrees C, which was lower 11.56. than NOPCM module. What's more, Regardless of 1C or 3C cycle, the temperature difference of FCPCM module can be controlled within 1.35 degrees C.
查看更多>>摘要：Several investigations related to increasing engine thermal efficiency focusing on the engine hydraulic circuits (oil and coolant) were performed over the last years. According to literature, more than 20% of the fuel energy is rejected to the coolant in steady state conditions. Thus, better use of that energy, especially during warm up of the engine, would lead to an improvement of engine performance and fuel consumption. In the present work, a complete engine and its subsystems were modeled and validated in a 0D/1D in-house software. After, several case studies based on modifying the volume of the engine hydraulic circuits were simulated and analyzed. The simulations were performed for steady state and transient conditions. On one hand, the impact of the present thermal management strategy was practically negligible in steady state conditions. On the other hand, for transient conditions and ambient boundary conditions, the results showed that by reducing the coolant volume by 45% the reduction in warm up time and fuel consumption compared with the base case were 7% and 0.4% respectively. Additionally, for cold conditions, the impact was even higher, obtaining a reduction of warm up time and fuel consumption of 13% and 0.5% respectively.
查看更多>>摘要：To meet the power supply development needs of electronic devices in spacecraft, the optimized design and fabrication of geometric configuration of thermoelectric (TE) legs to increase the power output of radioisotope thermoelectric generators are proposed. The principle of performance enhancement is the increase of heat dissipation on the increased side area, which is analyzed according to Fourier's law. Helix-shaped and spoke-shaped TE legs with special geometrical shapes are proposed and fabricated by 3D printing technology. The geometric design makes the TE legs produce a larger temperature difference and inevitably brings about an increase in resistance. Variation of TE legs' output performance with the geometrical parameters is analyzed based on the finite element method. Results show that the helix-shaped TE legs with the appropriate geometrical parameters can produce maximum output power of 2.78 mW, which are 2.55 times of the traditional cylinder-shaped TE legs with the same mass. And the spoke-shaped TE legs can produce maximum output power of 2.28 mW, which are 2.09 times of the traditional cylinder-shaped TE legs. Nine TE legs with special shapes and sixteen with traditional shapes are integrated into the same space (16.5 mm x 16.5 mm). The results show that the combination of TE legs with special shapes produce a higher power density, more than 66%-98% compared with traditional shapes due to lighter mass. TE modules with different shapes are fabricated by direct-writing 3D printing technology, and their output performances are tested. TE modules with special shapes exhibit greater open-circuit voltage and output power than the traditional shapes, which are consistent with the simulation results. The simulation and experimental results indicate that the geometric design of TE legs with larger side areas can improve the output performance of TE devices. This design concept of optimizing the geometry to increase the temperature difference can be applied to thermoelectric generators under the condition of natural heat dissipation to improve power output and heat dissipation, and reduce weight.
Zanoni, Marco A. B.Wang, JiahaoTorero, Jose L.Gerhard, Jason I....
16页
查看更多>>摘要：Moisture transport as well as evaporation and condensation are key mechanisms in the energy management of many processes occurring in different porous media. The role of moisture varies significantly among several applications such as food drying, concrete heating, deep geological barriers, and smouldering combustion of wet fuels. Currently, there is a lack of numerical models able to accurately predict these processes within a complex porous matrix. A one-dimensional numerical model was developed to predict the transient and spatial movement of evaporative and condensation fronts in an air heated inert porous bed partially saturated with water. The results showed that by introducing simple calibration constants in the effective thermal properties, the model was able to accurately predict experimental results. The numerical results revealed that temperatures rapidly increase to boiling conditions because of vapour condensation. Moreover, the model identified that the temperature plateau occurred because all the available energy was used to evaporate water. Evaporative cooling was also predicted and showed a rapid temperature decrease when air was initiated. When the air supply was initiated along with the heater, evaporative cooling resulted in a temperature plateau lower than air-off conditions. This was caused because vapour was carried forward much faster by the forced air, resulting in lower condensation and consequently lower energy released. Overall, this work provides a valuable representation in space and time of the key moisture transport processes in the gas phase as well as key phase-change and heat transfer processes occurring in porous media.
查看更多>>摘要：Dynamic optimization of the fluid loop is critical of the active thermal control system (ATCS) for future spacecraft. In this paper, the transient heat current model of the parallel heat exchanger system is constructed by the thermoelectric analogy method to analyze the optimal control problem of dynamic flow allocation. The continuous free time optimal flow control problem is transformed into a fixed-time nonlinear programming problem which can be solved by the sequential quadratic programming (SQP) algorithm through the control variable parameterization (CVP) and time-scaling methods. The simulation showed that the optimized flow allocation significantly reduced the process time required for system heat dissipation. Compared with the non-gradient particle swarm optimization (PSO) algorithm, the SQP method constructed in this paper decreased the optimization index by about 4%similar to 6% and the calculation time by about one order of magnitude. To diminish the sensitivity of the SQP algorithm to the initial value, this paper simplifies the model according to the system characteristics. Through the Pontryagin extremum principle, it is found and proved that, under the simplified model, the optimal flow rates are a set of time-independent variables, which significantly reduces the difficulty of the optimization problem, according to which a hybrid algorithm is designed that uses the optimal result of the simplified model as the initial value predictor. Compared with the average of using the random initial value, using the estimated initial value decreased the optimization index by more than 2% and the computation time by about 60%, under the same SQP algorithm.
Di Marcoberardino, G.Morosini, E.Di Bona, D.Chiesa, P....
17页
查看更多>>摘要：Nowadays supercritical CO(2 )cycles are considered as a promising alternative to the traditional steam cycle for the power block in CSP plants with the aim of enhancing the system efficiency and reducing costs. This work deals with the experimental characterisation of a CO2 blend as working fluid in transcritical cycle: the addition of C6F6 as a dopant increases the fluid critical temperature allowing for a condensing cycle in hot environment with ambient temperature higher than 40 ?. The potential benefits on adopting this mixture passes through thermal stability test for identifying its maximum operating temperature and Vapour Liquid Equilibrium measurements for tuning the Equations of State, thus having a good prediction of the thermodynamic properties. The static method with thermal stress test at different operating temperatures shows that the mixture can withstand to about 600 C in an Inconel 625 vessel. Furthermore, the standard Peng-Robinson with the optimised binary interaction parameter is selected for a preliminary thermodynamic assessment of the power cycle. An efficiency of 41.9% is found for an optimum mixture composition with a CO2 molar content of 84% considering a turbine inlet pressure of 250 bar and a maximum and minimum cycle temperature of 550 ? and 51 ? respectively.
查看更多>>摘要：This paper presents a developed isobaric expansion heat engine and preliminary measurements made with the engine operating as a pump, as well as a comparison of the experimental results with a thermodynamic model. Experiments were carried out at heat source temperature in the range 30-90 degrees C and heat sink temperature around 11 degrees C; refrigerant R134a was used as the engine working fluid. The pressure difference generated by the engine-pump varied from 2.5 bar at the heat source temperature of 30 degrees C to 23 bar at the heat source temperature of 86 degrees C. At an engine cycle frequency of about 0.25 Hz, the engine operates with a useful power up to 500 W, and a water pumping flowrate up to 15 L/min. Depending on the temperature of the heat source the obtained efficiency was 3.5-6 %. This efficiency looks very high, considering such a low-temperature difference (20-75 degrees C) and low power (<1 kW). The difference between the experimental and thermodynamic efficiency is <25%, which indicates low mechanical and thermal losses. The results are very promising showing that the engine is a valuable alternative to the current technologies, especially at low temperatures (<100 degrees C) and low power range (<500 kW). The presented technology is easily scalable and reproducible in all industries where there is a temperature difference > 20 degrees C. High pressure pumps for water desalination and heat engines for generating mechanical energy (electricity) using geothermal, biomass and solar energy, waste heat of diesel engines, as well as of SOFCs and LT-PEM fuel cells are examples of promising applications.
查看更多>>摘要：To alleviate the consequences of the freshwater crisis and fully utilize marine resources in tropical coastal areas, this study presents a low-pressure flash evaporation desalination system driven by ocean thermal energy (OTE). Through gravity and atmospheric pressure, a natural vacuum can be formed in the evaporator, which achieves flash evaporation for warm surface seawater. Then, the vapour is condensed into freshwater via an inner coil condenser with flowing cold deep seawater. Using a water pump to fill the evaporator intermittently, the non-condensable gas accumulated in the desalination process can be thoroughly discharged. Based on the process of mass and heat transfer inside the system, a thermodynamic model was developed. Several performance evaluation indices, including water productivity, specific electrical energy consumption (SEEC) and recovery ratio (RR), were experimentally investigated under different parameters. The results indicate that productivity and SEEC are higher with increasing seawater flow rate and decreasing deep seawater temperature. Under a warm seawater temperature of 30 degrees C and a deep cold seawater temperature of 8 degrees C, the system obtained a maximum water productivity of 5.3 kg/h, and the corresponding SEEC and RR were 0.126 kWh/kg and 1.5%, respectively. Finally, compared with solar-driven desalination systems, the proposed OTE desalination system requires less energy consumption, which shows an attractive application prospect.
查看更多>>摘要：Membrane-based liquid desiccant dehumidifiers avoid the desiccant carryover, which is better than the packed bed. In the process of liquid dehumidification, the dehumidification performance is reduced due to heat release, and internally-cooled liquid dehumidification is a method to improve the dehumidification performance. This study develops an internally-cooled hollow fiber membrane-based liquid desiccant dehumidifier (IHFMLDD). IHFMLDD is composed of two tubes. The inner tube is not permeable to moisture but only permeable to heat, while the outer tube is a hydrophobic semi-permeable membrane, which can transmit moisture and heat simultaneously. The advantage of this technology is that the liquid desiccant does not directly contact the air in the process, preventing the problem of droplet carryover in the air stream. In addition, cold water reduces the temperature of the solution and improves the dehumidification performance. This paper studies the heat and mass transfer characteristics of IHFMLDD, which uses a staggered triangle arrangement and takes LiCl solution as a cycling fluid. Heat and mass transfer capacities are adopted as performance indices. The effects of the inlet parameters, including air and solution flow rates, air inlet temperature and humidity ratio, solution inlet temperature and concentration, water flow rates, on the indices are investigated. Besides, a coupled heat and mass transfer model is developed and solved with the finite difference method. The results show that the air and solution flow rate greatly influence the heat and mass transfer coefficient, revealing the coupling mechanism of heat and mass transfer and the mutual influence law in IHFMLDD. Correlation expressions of heat and mass transfer coefficients are proposed, in good agreement with the experimental data. The dehumidification capacity of the internally-cooled membrane liquid dehumidifier is 75-140% higher than that of the traditional hollow fiber membrane liquid dehumidifier.
查看更多>>摘要：The aim of this work is to analyze different tube-side inlet boundary conditions in order to properly model heat transfer mechanism in a crossflow air-to-water fin-and-tube heat exchanger (FTHEX). For accurately describing conjugated heat transfer, the entire flow length on both fluid sides should be considered. For problem simplifying, domain that include a segment inside the heat exchanger in the water flow direction, is used. This paper studies the effect of fully conjugated heat transfer, including air-side and water-side thermal resistances. Three different numerical models are analyzed by varying the tube-side inlet boundary conditions. In one of these models the water-side thermal resistance is neglected and other two models consider the water-side thermal resistance, but with different velocity and temperature boundary conditions at the water inlets: uniform and fully developed. To validate the proposed numerical models, measurements have been performed on a crossflow airto-water plain FTHEX. The results show that the numerical model with the fully developed water inlet boundary condition coincides best with the experimental data. It can be concluded that the water-side thermal resistance cannot be neglected, especially when evaluating the air outlet temperature and the overall heat transfer coefficient of heat exchanger.
NSTL
Elsevier