毛细管作为一种膨胀装置在小型制冷系统和热泵系统中得到了广泛的应用。尽管毛细管具有简单的几何形状,其中的绝热流动却是极其复杂的。本文对模拟异丁烷(R600a)在毛细管中绝热流动的均相流动模型和分离流模型进行了对比研究。研究了不

The critical temperatures and pressures of the binary mixture of 1,1,1-trifluoroethane (HFC-143a) + trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) were measured by a set of apparatus containing an optical cell that has a variable inner volume. The experimental measurements were carried out based on a static method, and the critical properties were determined by means of the observation of disappearance and reappearance of the meniscus. The experimental uncertainties in pressure, temperature, and mole fraction were estimated to be less than 6.2 kPa, 0.02 K, and 0.003, respectively. The experimental critical properties measured in this work were correlated by the Redlich-Kister method, and the correlated results showed a good agreement with experimental data.

The heat transfer characteristics of supercritical fluids in tubes have been considered indispensable for the design and optimization of the heat exchanger and the energy conversion system. Specifically the cooling heat transfer of supercritical R1234ze(E) in horizontal tubes is a promising heat-power conversion technology; however, there is a scarcity of conducted research in available literature. The present work, the first-ever study in this direction, aims to thoroughly investigate the heat transfer characteristics of supercritical R1234ze(E) which is cooled in horizontal tubes. Experimental work was performed to thoroughly explore and inspect the heat transfer characteristics of supercritical R1234ze(E) passing through the tube of 4.12 mm diameter at 4–5 MPa pressure and 240–400 kg/m² s mass flux. Furthermore, the simulation study, supporting the experimental investigation under the same conditions of pressure and mass flux, extended the range of tube diameter up to 9.44 mm. The effects of pressure, mass flux and tube diameter on the heat transfer coefficient were carefully analyzed in the present research work. Based on the simulation results and experimental results, heat transfer correlations were newly developed by separating the region above and below the pseudo-critical temperature. The average absolute deviation between the calculated Nusselt numbers by the numerical correlation and the simulation results was found 2.87%; the average absolute deviation between the calculated Nusselt numbers by the experimental correlation and the experimental results was found 5.3%.
© 2020 Elsevier Inc.

Investigation on the thermal behavior of the lithium-ion battery which includes the temperature response, heat contribution and generation, is of vital importance for their performance and safety. In this study, an electrochemical-thermal cycling model is presented for a 4 Ah 21700 type cylindrical single cell and 3x3 battery pack and the model is validated by experiment on a single cell. Thermal behavior on a single cell is first analyzed, the results show that the heat generated in the charge is smaller than the discharge, and the polarization heat contributes the most to total heat, especially under higher rate. It can also be concluded from the battery pack that the temperature of the cell inside the battery pack is significantly greater than the external battery, while the temperature difference exists the opposite regular due to the worst heat dissipation of the central cell. Finally, after taking the enhanced liquid cooling strategy, the maximum temperature is 320.6 K that is reduced by 9.38%, and the maximum temperature difference is 4.9 K which is reduced by 69.6% at 2C, meeting the requirements of battery thermal management system.

Effective recovery of engine waste heat is a key strategy to alleviate energy and environmental issues. In this study, an improved transcritical-subcritical parallel organic Rankine cycle based on zeotropic mixtures is proposed to recover engine waste heat. Firstly, the proposed system is compared with other systems and shows the best performance. Then energy and exergy analyses are conducted, and three design parameters are analyzed. According to the results, for each turbine inlet temperature of high-pressure branch, the system performance initially increases then decreases with turbine inlet pressure of high-pressure branch, and the net power output firstly increases then decreases with evaporation temperature of low-pressure branch. Furthermore, the effect of composition of zeotropic mixtures on system performance is investigated, and zeotropic mixtures R600a/R601a and R600/R601 are compared. The results indicate that adopting zeotropic mixture can improve system performance significantly, and system based on R600/R601 performs better than based on R600a/R601a. Finally, performance optimization of system is conducted. The results show that system based on R600/R601 (0.5/0.5) achieves the maximum net power output of 114.47 kW which is higher 10.68% and 6.42% than based on pure R600 and R601 respectively, and the engine power output is improved by 11.49%.

采用增强壁面函数的标准k-ε模型对超临界R134a水平圆管内冷却换热进行了模拟研究.分析了管内不同截面上流体温度、速度和湍动能的分布情况及对应关系。研究了质量流量和浮升力对换热系数的影响。结果表明,流体速度随着温度的降低而减小

In this study, to promote the performance of engine waste heat recovery, for the first time, an innovative transcritical CO2 parallel Rankine cycle driven by engine waste heat and liquefied natural gas cold is proposed and studied. Firstly, the thermodynamic analysis including energy and exergy analysis is conducted, and heat transfer requirement analysis for fluids in different working state is presented. Then parametric analysis is conducted, the effects of different parameters on system performance are investigated. According to the results, by adjusting the temperature and pressure of cycle, system performance can be maximized, and with the utilization of liquefied natural gas cold, the entire system achieves the increase of power output of 20%, improvement of thermal and exergy efficiency of 2%. When condensation temperature changes from 20 degrees C to -10 degrees C, system power output has an improvement of 86% (from 103.37 kW to 192.37 kW) while heat transfer area only increases by 25% (from 162.83 m(2) to 218.72 m(2)). Moreover, a performance comparison of system based on CO2 and six widely-used organic fluids is carried out, and results demonstrate that CO2 has better performance than organic fluids. Furtherly, the optimization analysis of system power output is conducted. The results indicate that the proposed system achieves the maximum power output of 205.60 kW, at the same time, the engine power output can be improved by 21.42%. Finally, an exergy destruction analysis is presented, and the results show that the exergy destruction rate of condenser reaches to 70.94% of total exergy destruction due to the large temperature difference between CO2 and liquefied natural gas.

Developing countries such as Pakistan are experiencing severe energy crises because of an increasing difference between energy demand and energy production. Since buildings are the major consumers of energy in Pakistan, immediate steps must be taken to reduce energy consumption by devising energy efficient measures for buildings. The majority of buildings in the country use distributed heating/cooling devices, i.e:, air conditioners (A/Cs), to regulate building temperature. These A/Cs consume most of the energy in the buildings and are often subjected to problems such as heating or cooling unoccupied spaces and overheating/overcooling some areas in the building. These problems arise mainly because of independent manual controls of A/Cs and can be addressed by installing centralized heating, ventilation, and air-conditioning (HVAC) systems. However, the cost of deploying an actual HVAC system in these buildings remains high. Costs are associated with disruption to occupants as well as the dollar cost of installing the HVAC infrastructure. Moreover, the cost associated with removal or replacement of the existing A/C units is also substantial. Therefore, in this work, we abstract the functionality of the HVAC system by using the existing distributed heating/cooling infrastructure in buildings to provide a low-cost centralized command and control mechanism namely Retrofitting low-cost Heating Ventilation and Air Conditioning (RetroHVAC) system. We designed an occupancy detection system and incorporated it into the RetroHVAC system to enhance its energy-saving potential. Additionally, a desktop application was designed to enable user interaction and policy enforcement. Experimental evaluation comparing different RetroHVAC control strategies based on time scheduling, temperature threshold monitoring, and real-time occupancy information show that the RetroHVAC system can substantially reduce the building energy consumption without compromising user comfort.