最新刊期
卷 44 ， 期 4 ， 2023
- 摘要：In the context of the "3060 carbon peaking and carbon neutrality goals", promoting energy-efficient transportation and rational utilization is one of the important paths to achieve carbon neutrality, where heat exchangers are known to play a crucial role. Compared with traditional heat exchangers, microchannel heat exchangers significantly reduce the volume while maintaining the same heat exchange capacity. They also significantly improve the heat exchange efficiency and have applications in many important fields. For example, supercritical CO2 (S-CO2) power generation systems, transcritical heat pumps, and refrigeration systems have important application prospects as they are environmentally friendly and offer high-efficiency advantages in the context of the "dual carbon target." Drastic changes in the properties of S-CO2 and the high-temperature and high-pressure requirements of the system pose significant challenges to heat exchanger design, high-temperature and high-pressure resistance, compactness, and efficiency. Therefore, S-CO2 heat exchangers have become a hot topic in scientific and industrial research. In recent decades, significant progress has been made in related research. This paper comprehensively reviews the research progress of microchannel heat exchangers in S-CO2 systems. The paper focuses on different structural forms and design optimization methods of printed circuit heat exchangers (PCHE) and discusses the impact of PCHE optimization on the performance improvement of the entire S-CO2 system. This review provides a comprehensive discussion of S-CO2 microchannel heat exchangers and provides an important reference for the selection, design, and optimization of heat exchangers in systems using CO2 or other supercritical working fluids as the working media for power generation, heat pumps, and refrigeration.关键词：microchannel heat exchanger;printed circuit heat exchanger;optimization design;heat transfer enhancement;supercritical CO21210|1218|1发布时间：2024-12-12
- 摘要：With rapid development of electronic devices, modern electronic devices require higher power and exhibit more heat flow density. Consequently, their thermal management requirements have increased drastically. Manifold microchannel heat sinks are considered as the optimal cooling technology for electronic devices due to their high heat dissipation capability, simplicity, and reliability. This paper describes the heat dissipation principles and advantages of manifold microchannel heat sinks and reviews relevant research about their geometrical parameters and optimal structures. It also provides an outlook on their future development.关键词：electronics cooling;liquid cooling technology;manifold microchannel;optimization1040|1087|0发布时间：2024-07-18
- 摘要：In this study, based on fins of four conventional shapes that are applied in microchannels, four novel fins are proposed using compound fins with cavities. Numerical simulations were conducted to investigate the flow and heat transfer characteristics of microchannels with conventional fins and those integrating fins with cavities. The mechanism by which the cavities influence the flow and heat transfer performance was analyzed, and performance evaluation criteria (PEC) was adopted as a criterion to compare the overall performance. The results showed that as the fin was compounded with cavities, the width of the fin decreased; thus, the separation zone downstream of the fin was reduced. In addition, the main flow induced a cyclic flow in the cavities. The velocity gradient at the liquid-liquid interface between the main flow and the cyclic flow was lower than that at the liquid-solid interface between the main flow and the fin surface, resulting in lower flow friction. Therefore, the pressure drop of the flow associated with fins with cavities was lower than that for conventional fins. However, the cavities reduced the disturbance of the main flow, leading to a deteriorated heat transfer enhancement performance. The influence of the cavities on the flow and heat transfer depended on the fin shape. Compared with conventional fins, the overall performance of fins compounded with cavities was reduced owing to heat-transfer deterioration.关键词：microchannel heat exchanger;heat flux;surface coefficient of heat transfer;fin;cavity698|981|0发布时间：2024-07-18
- 摘要：Microfluidics is of great importance in investigating the various physicochemical processes of carbon dioxide, the latter of which plays a fundamental role in the carbon capture, utilization, and storage (CCUS) process, as well as in achieving the goals of carbon peaking and neutralization. Here, the current status of carbon dioxide microfluidic technology, that is, the CO2-on-a-chip technology, is reviewed. CCUS technology and some basic aspects of microfluidics are briefly introduced. In particular, the advantages and impacts of microfluidic technology in advancing CO2-related studies are discussed. Examples of applications and research progress of microfluidics in screening solvents for CO2 capture, supercritical CO2-based extraction, enhanced oil recovery by CO2, biochemical conversion of CO2, and CO2 storage are specifically introduced. Finally, the success of applying microfluidics to CO2 is summarized, and the future development of this area is envisioned.关键词：carbon dioxide;CCUS;microfluidics;bubble;mass transfer1087|869|0发布时间：2024-07-18
- 摘要：To investigate the effects of spray cooling on air-side heat transfer and pressure drop, and to determine the water spray flow rate when spray droplets evaporate completely on the hot surface of a compact heat exchanger, we built an experimental plant to analyze the air-side heat transfer and pressure drop of an automotive compact heat exchanger with and without spray cooling. The results indicate that the j factor and f factor obtained from experiments without spray were in good agreement with those calculated from Chang’s correlations, within an error of 10%. Compared with that without spray, the j factor with spray increased significantly, but the increasing rate declined with a decrease in the spray flow rate and substrate temperature. The f factor with spray changed slightly compared with that without spray. Using the substrate temperature and air velocity as variables, we proposed a correlation of the critical value of the spray flow rate (where spray droplets can evaporate completely). Based on Chang’s correlation, the j factor with spray can be obtained using a correction factor. These results support the prediction of air-side heat transfer with spray and also facilitate future application of spray cooling in automotive air conditioners.关键词：automotive air conditioner;compact heat exchanger;spay cooling;j factor;f factor707|1108|0发布时间：2024-07-18
- 摘要：Experiments were conducted to study the condensation heat transfer and pressure drop characteristics of refrigerant R32 in aluminum herringbone tubes, ripple tubes, and smooth tubes. The experimental refrigerant mass flow rate is 100-350 kg/(m2·s). The saturation temperature is 308 K, 313 K, and 318 K, respectively, while the vapor quality is 0.2-0.8. The experimental results show that the herringbone tube has the highest coefficient of heat transfer, followed by the ripple tube, while the smooth tube is the worst. The friction pressure drop is highest for the ripple tube, and the pressure drop of the herringbone tube is higher than that of the smooth tube. The surface coefficient of heat transfer and friction pressure drop decrease with an increasing saturation temperature and increase with an increasing mass flow rate. The performance evaluation factor, PEF, was introduced to evaluate the heat transfer performance of the heat transfer tube. The herringbone tube has the best heat transfer performance with PEF values ranging from 2.07 to 2.72, while the ripple tube has PEF values ranging from 0.68 to 0.90. A new heat transfer correlation was proposed following the form of classical heat transfer correlations, and the error of the new correlation formula was within ±20%.关键词：condensation heat exchange;surface coefficient of heat transfer;frictional pressure drop;heat transfer correlation747|924|0发布时间：2024-07-18
- 摘要：The pursuit of fresh food quality has promoted the rapid development of the cold chain industry. The increase in cold chain equipment and facilities causes excessive energy consumption and refrigerant leakage, which has a negative impact on the environment. The development of the cold chain industry is therefore under pressure to reduce emissions under the current "two-carbon" goal. Considering the carbon footprint of the entire cold chain, this study calculates carbon emissions from three aspects, including refrigerant leakage, energy consumption, and food loss and waste. Emission reduction using environmental-friendly refrigerants, high-efficiency technologies, and low-carbon energy is also analyzed. The calculation results show that the current carbon emissions related to the cold chain are up to 402 million tons and will increase to 436 million tons in 2030 if the cold chain industry is developed with existing technology. However, emissions can decrease to 272 million tons, if environmental-friendly refrigerants, high-efficiency technologies, and low-carbon energy are applied.关键词：cold chain;carbon emission;dual carbon targets;low-carbon technologies683|1116|0发布时间：2024-07-18
- 摘要：The replacement of internal combustion engine vehicles with electric vehicles is becoming an important developmental direction in the automotive industry because of increasingly serious environmental problems and climate change. The high energy consumption for heating from heat pump air-conditioning systems leads to a reduction in the driving range of electric vehicles in winter. In this study, a simulation model of a heat-pump air-conditioning system for electric vehicles was established and its operating performance was analyzed in various air circulation modes under different ambient conditions. The results showed that the energy consumption in the outside air circulation mode was 34.4% higher than that of the recirculated air circulation mode under typical winter conditions in the cold zone. Furthermore, the energy consumption of the outside air circulation mode was reduced by 17.1% compared with the recirculated air circulation mode under typical winter conditions in the hot summer and warm winter zone. The energy consumption of heat pump air-conditioning systems for electric vehicles can be reduced by reasonably controlling the air circulation mode based on ambient conditions on the premise of meeting the requirements of temperature and humidity in the cabin. A control strategy for the energy-saving operation of a heat pump air-conditioning system for electric vehicles in winter is proposed and presented based on the ambient air psychrometric chart through regional division. In addition, the energy-saving operation mode of the heat pump air-conditioning system for electric vehicles in winter for different climatic zones and the number of passengers varying in the cabin are discussed.关键词：electric vehicle;heat pump;dehumidification;operation strategy724|1081|0发布时间：2024-07-18
- 摘要：In heat pumps, the use of environmentally friendly refrigerants can reduce damage to the ozone layer. Under the condition of low ambient temperature, based on the existing heat pump system, the CO2 transcritical cycle and mechanical subcooling were adopted. An R1234yf/CO2 mechanical subcooling heat pump system (MSHPS), a CO2/R1234yf double air cooler mechanical subcooling heat pump system (DMSHPS), and a CO2 two-stage compression triple air cooler heat pump system (TSCHPS) are proposed in this study. By establishing a thermodynamic model, the heating performance of the system using CO2 and R1234yf was analyzed. Furthermore, the performance of different systems was analyzed through the optimal discharge pressure, evaporating temperature, condensing temperature, and flow rate ratio of the auxiliary cycle and main cycle. In this study, R1234yf was replaced with R134a; the heating performances of R1234yf and R134a in the MSHPS and DMSHPS were compared and found to be similar.关键词：heat pump;CO2 transcritical;mechanical subcooling;R1234yf;heating performance;thermodynamic analysis788|954|0发布时间：2024-07-18
- 摘要：In existing research, 90 °C is reported as the highest temperature of hot water supplied by a traditional CO2 heat pump. To supply pressurized hot water over 100 °C with the CO2 transcritical heat pump, a tube-in-tube heat exchanger was redesigned and optimized in this study. The internal heat exchanger system was adopted to verify the feasibility of a CO2 heat pump with an optimized tube-in-tube gas cooler to supply pressurized hot water over 100 °C. The influences of ambient temperature and inlet and outlet water temperatures on the heating capacity and system performance were studied by simulations and experiments. Results show that the error of experiment and simulation is less than 7.59%, 100 °C hot water can be obtained through the redesign of the gas cooler under high-temperature conditions. When the ambient temperature was 40 °C and the inlet water temperature was 9 °C, the highest COP (coefficient of performance) reached 3.64 in the experiment and 3.87 in the simulation. The highest COP in this study was 4.47 when the ambient temperature was 40 °C and the inlet and outlet water temperatures were 9 °C and 85 °C, respectively. With an increase in ambient temperature, the heating capacity increased and the COP exhibited the same variation trend as the heating capacity.关键词：CO2;heat pump;high temperature heat pump;heat exchanger727|848|0发布时间：2024-07-18
- 摘要：Multi-ejector is an effective device for improving the operation and regulation of transcritical carbon dioxide (CO2) refrigeration systems. This paper presents a dynamic model of a transcritical CO2 refrigeration system. The air cooler and low-temperature evaporator models are established using the finite volume method and the flooded evaporator model is established by lumped parameter method. A gas-liquid separator and accumulator model is established according to the conservation of mass and energy. A two-phase ejector model is established based on the physical properties of CO2, considering the phenomena of internal transonic flow, phase change, and congestion. The model predictions had an error within 10% compared with experimental data reported in the literature. The transient response of the system was studied by adjusting the multi-ejector and speeds of the compressor as disturbance conditions. The results show that the developed dynamic model has characteristics similar to that of the experimental data, which provides high-precision simulation conditions for studying the performance and regulation features of a multi-ejector refrigeration system.关键词：refrigeration system;multi-ejector;system dynamic model;transcritical CO2656|879|0发布时间：2024-07-18
- 摘要：In this study, a novel defrosting method was developed for an air-source heat pump (ASHP) with a multi-circuit outdoor coil operating in a low-temperature and high-humidity environment. The outdoor heat exchanger of the ASHP is divided into three independent circuits for segmental defrosting. When a defrost operation is required, one of the circuits uses part of the high-temperature gas at the compressor outlet to defrost. At this time, part of the high-temperature gas still enters the indoor heat exchanger for heating, while the other two circuits of the outdoor heat exchanger are used as evaporators. The novel segmental defrosting method was compared with the traditional reverse cycle defrosting (RCD), and the results indicated that the overall performance of the system was better than that of RCD. Under the conditions of ambient temperature of 1 °C, relative humidity of 80%, and condensing temperature of 40 °C, the reverse cycle defrosting time is 246 s, which is shorter than the segmental defrosting time of 330 s. Further, the energy consumption of reverse circulation is 522.1 kJ, which is lower than that of 735.8 kJ for segmental defrosting. However, reverse cycle defrosting needs to absorb heat from the water-side heat exchanger. An average heating performance of 2.23 kW can be maintained during the defrosting process, with small fluctuations in suction and exhaust pressure and a slow decline in COP, and a continuous heat supply can be achieved during the defrosting process.关键词：air-source heat pump;thermodynamics;segmental defrosting;defrosting characteristics779|964|0发布时间：2024-07-18
- 摘要：This study analyzed the effects of compressor speed, expansion valve opening, and drying temperature on the thermal performance of a closed-loop heat pump drying system through single-factor experiments. Subsequently, system performance variation characteristics were obtained. The results show that when the compressor speed was increased from 2 000 r/min to 3 000 r/min, the power consumption increased by 89.6%, the heating capacity increased by 11.4%, and the COP（coefficient of performance） decreased by 41.3%. When the expansion valve was opened from 30% to 80%, the power consumption decreased by 5.2%, heating capacity increased by 73.5%, and COP decreased by 82.9%. When the drying temperature was increased from 40 °C to 60 °C, the power consumption increased by 38.1%, the heating capacity increased by 3.1%, and the COP decreased by 25.3%. Based on the analysis of the research results, it can be concluded that increasing the opening of the expansion valve and reducing the drying temperature and compressor speed can effectively reduce the system power consumption and improve the system performance.关键词：heat pump drying;thermal performance;compressor;expansion valve;drying temperature684|983|0发布时间：2024-07-18
- 摘要：To evaluate the APF of an air conditioner according to GB 21455—2019, the performance parameters of low-temperature intermediate cooling conditions can be measured or converted, resulting in two different APF values using these two different methods in the same air conditioner. Considering the difference in results obtained by the different methods in the national standard, it is necessary to analyze the rationality of the difference and propose an improvement plan. Based on the definition formula of APF, this study presents the reason for the difference in the APF value determined by the test method and the conversion method at the low-temperature intermediate refrigeration condition. The test method given in the national standard causes a variation in energy efficiency under rated intermediate cooling conditions, contrary to the variation in the calculated APF value, which is not reasonable. One possible way to solve the problem of non-uniqueness of APF values is to change the method of obtaining the performance parameters of the low-temperature intermediate refrigeration operating condition stipulated in the national standard from "obtained by testing or conversion according to the rated intermediate refrigeration operating condition" to "obtained only by conversion according to the rated intermediate refrigeration operating condition."关键词：APF;air conditioner;low-temperature intermediate refrigeration condition;energy efficiency standard for air conditioners646|832|0发布时间：2024-07-18
- 摘要：Refrigerated tank containers are currently the primary method of transporting cryogenic liquids. The air outlet of the refrigeration system is blocked by the liquid tank, resulting in low energy efficiency of the refrigeration system. This study aims to improve the energy efficiency of the refrigeration system for a tank container with a limited installation space. The resistance loss of the air duct is reduced and the air volume of the condenser is increased by optimizing the condenser air duct structure and pipeline arrangement, thereby improving energy efficiency. By optimizing the structure of the air outlet, sufficient air volumetric flow rate can be achieved under blocked conditions. By optimizing the pipeline arrangement, the resistance loss of the condenser is reduced, and the air volumetric flow rate is increased. The test results of the above system show that air duct structure optimization and condenser flow path optimization can improve the energy efficiency of the refrigeration system by 25.0%, indicating that the proposed optimization scheme can effectively improve the energy efficiency of the refrigeration system for tank containers.关键词：refrigerated tank container;condenser;air duct resistance reduction;flow path arrangement;energy efficiency700|841|0发布时间：2024-07-18
- 摘要：Based on a self-built indirect evaporative cooler experimental prototype, high-precision temperature and humidity sensors were placed inside a narrow airflow channel for dynamic data acquisition. An enthalpy difference chamber was used to create a stable external environment and hence inlet parameters for the channel. The influence of different inlet air velocities, primary to secondary air ratios, and environmental heat and humidity parameters on the heat and humidity exchange process of the airflow inside the channel was studied. The results showed that the relative humidity of the secondary air changed rapidly. When the relative humidity exceeds 95%, the evaporation process is saturated. A wet-channel length of 40 cm satisfies the design requirements of most evaporative coolers. At low air velocity and primary to secondary air ratios, the saturation evaporation length of the evaporative cooler was shorter, while the cooling capacity was higher. For the standard initial conditions of dry bulb temperature 37.8 °C and wet bulb temperature 21.1 °C, the channel air velocity was constant at 0.5 m/s, and the relative humidity rose to 95.7% after an evaporation distance of 10 cm. The relative humidity of the ambient air had a significant impact on the cooling performance. Under the same dry bulb temperature of 33.5 °C, the average temperature of the secondary air in Urumqi with a wet bulb temperature of 18.2 °C is 7 °C lower than that in Beijing with a wet bulb temperature of 26.4 °C.关键词：evaporative cooling;heat regenerative cycle;plate heat exchanger;relative humidity;latent heat627|875|0发布时间：2024-07-18
- 摘要：Backplane air conditioner cooling solves the heat dissipation problem inside a rack, which can reduce the waste of cooling capacity and significantly improve refrigeration efficiency. To explore the cooling capacity of a backplane air-cooled rack under different working conditions, a rack model was established using CFD simulation software to simulate the thermal environment of the rack. The results showed that the air distribution inside the rack was good, and there was no mixing of cold and hot air. The server temperature decreased with an increase in the circulating air volumetric flow rate, and the decreasing trend saturated at higher flow rates. As the inlet air temperature increased, the server temperature increased linearly. When the inlet air temperature was 18, 20, 22, 24, and 26 °C, by changing the total circulating air volumetric flow rate changed within the range of 2 400–3 200 m3/h, the maximum load power range of a 42U rack was 11.7–15.0 kW, 11.2–14.5 kW, 10.8–14.0 kW, 10.3–13.4 kW and 9.8–12.7 kW.关键词：data center;backplane rack;cooling capacity;simulation study674|903|0发布时间：2024-07-18
- 摘要：Spacecraft use surface tension to transport liquid fuel and continuously provide gas-free propellants to engines. To ensure the normal operation of spacecraft, it is important to study the capillary flow behavior of liquid propellants. In this study, the capillary rise process was simulated using a single capillary tube, and the gas-liquid interface was captured using the phase-field method. A two-dimensional axisymmetric model of capillary rise was built and solved using the finite element method. To validate the model, numerical results were compared with those calculated using Jurin’s law and the Lucas-Washburn model. The relative errors were 4.44% and 5.21%, respectively, which confirmed the feasibility of the model for simulating the dynamic capillary rise process. Based on the verified model, the height and velocity of the capillary flow were investigated using different gravitational accelerations and cryogenic fluids. The results showed that when the effective gravity was small, the capillary rise process could be divided into three stages: purely inertial, inertial-viscous, and purely viscous. When the effective gravity is high, the capillary rise process can be divided into four stages, because the gravity of the liquid in the tube cannot be ignored. The four stages are purely inertial, inertial-viscous, viscous-gravitational, and purely gravitational. In addition, the height and velocity of the capillary flow decreased as the effective gravity increased. In the initial stage, the higher the surface tension of the cryogenic fluid, the greater the rising height and velocity. As the liquid level increases, the viscous force gradually increases. The velocity of the capillary flow with liquid hydrogen exceeded that with water, liquid nitrogen, and liquid oxygen.关键词：capillary flow;gravity effect;phase-field method;cryogenic fluids;rising height640|883|0发布时间：2024-07-18
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