最新刊期
卷 39 ， 期 1 ， 2018
- 摘要：Elastocaloric cooling is a solid-state cooling technology based on the latent heat associated with the stress-induced phase transformation in elastocaloric materials. It is harmless to the environment, with remarkable theoretical system efficiency and cooling power density. Recently, rapid developments in elastocaloric cooling technology have been reported at research institutes in Europe and the USA. Some first-generation prototypes have been successfully developed and are properly functioning. As a guide for future related studies in China, this paper introduces the state-of-the-art and major challenges of this technology, including thermodynamic fundamentals, elastocaloric materials, prototype developments, and numerical simulations. Based on the literature, current elastocaloric refrigerators offer up to 14% Carnot cycle efficiency, a figure expected to exceed 30% in the near future; there is still significant potential for improvement when compared with the 80% Carnot efficiency offered by elastocaloric materials. Consequently, we can optimistically expect that elastocaloric cooling technology will experience faster growth and wider applicability in the future.关键词：not-in-kind cooling technologies;solid-state cooling;shape memory alloys;refrigeration cycle;prototypes3407|4828|8发布时间：2024-07-18
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Theoretical Performance Analysis of CO2 Transcritical Refrigeration Cycle with Mechanical Subcooling
摘要：The performance of the CO2 transcritical refrigeration cycle can be improved, and the throttling irreversibility loss can be reduced, by subcooling CO2 at the outlet of the gas cooler with a vapor compression refrigeration cycle (auxiliary cycle). A thermodynamical analysis is performed to study the operation characteristics of the CO2 transcritical refrigeration cycle with mechanical subcooling, and the results indicate that the maximum coefficient of performance (COP) is achieved at the optimum discharge pressure and optimum subcooling temperature. Compared with the traditional CO2 transcritical cycle, the improvement in COP is more significant at higher ambient temperatures and lower evaporation temperatures, and the discharge pressure and temperature can be notably reduced by the auxiliary cycle. The energy consumption of the auxiliary cycle compressor is much lower than that of the CO2 compressor. Among the 11 auxiliary cycle refrigerants, the differences between the COP improvements of the other 10 auxiliary working fluids are not obvious, except for R41. The CO2 transcritical cascade refrigeration cycle is more applicable to working conditions with higher ambient temperatures and lower evaporation temperatures.关键词：CO2;transcritical;mechanical subcooling;refrigeration cycle2501|2594|8发布时间：2024-07-18 - 摘要：The optimization and control of discharge pressure is important in transcritical CO2 heat pump water heating systems. The traditional high-pressure control equation simply takes the exit temperature of the gas cooler into account, neglecting the impacts of the pinch point and the outlet temperature of water, which might lead to a huge coefficient of performance (COP) loss in applications. In this paper, we report the development of a constrained gas cooler model by considering the influence of the pinch point and water inlet and outlet temperatures. The main impact factors of the optimum high pressure are determined through the design of experiments and statistical methods. Consequently, a new constrained optimal high-pressure control equation using least-squares regression is proposed. Based on the new control equation, the system COP loss is 1.8% in average (maximum 8.7%), providing better reliability for engineering practices.关键词：CO2 heat pump water heater;high pressure control;constrained model;design of experiment2154|2392|0发布时间：2024-07-18
- 摘要：High-temperature steam is in great demand for many industrial applications. The electric boilers used to replace traditional boilers in rural areas are not efficient enough to be employed in large-scale steam production. Furthermore, a large amount of heat appears to be wasted in related industries, so the employment of high-temperature heat pumps is quite promising in producing high-temperature steam. In this paper, an R245fa high-temperature heat pump system is applied to the production of steam. A mathematical model is developed to predict the performance of each component and the heat pump system, and an experimental investigation is also conducted. The results illustrate that, with an increase in evaporation temperature, the coefficient of performance (COP) and heat capacity increase, whereas the discharge temperature decreases. The relative COP ranges from 0.55 to 1.3 in the target operating conditions. The heat exchange efficiency is observed to increase from 1.7 to a maximum of 2.8 kW/(m2?K) as the water flow rate increases.关键词：heat pump;steam;evaporation temperature;experimental investigation1957|2264|4发布时间：2024-07-18
- 摘要：To solve the problem of heating in electric vehicle air-conditioning systems and suppressing the high discharge temperature of the compressor in winter, a quasi-two-stage compression heat pump system for electric vehicles has been designed using flash tank vapor injection. Additionally, an experimental platform for testing the performance of a quasi-two-stage compression scroll compressor has been built. Using this platform, single-stage and quasi-two-stage compression scroll compressors have been tested under five different outdoor temperature conditions. The experimental results show that the discharge temperature of the compressor increases with the decrease of ambient temperature. Under all five conditions, the discharge temperature of the single-stage compression scroll compressor is higher than that of the quasi-two-stage scroll compressor. In particular, when the ambient temperature reaches ﹣7℃, the discharge temperature of the quasi-two-stage compression scroll compressor is 10 ℃, less than that of the single-stage compressor. In low-temperature conditions, compared with the single-stage compression scroll compressor, the mass flow rate of the quasi-two-stage compression scroll compressor, the system heat capacity and heating coefficient of performance (COPh) improve by 12.9%–17.4%, 7.3%–8.3% and 7.6%–8.2% respectively.关键词：scroll compressor;electric vehicle air-conditioning;quasi two-stage compression;heat pump;performance experiment2583|3043|13发布时间：2024-07-18
- 摘要：The particular characteristics of confined space mean that it is easy for harmful substances to be produced and enriched. Moreover, the concentration of carbon dioxide in an enclosed space is strongly related with human health. In this paper, a new method is proposed in which carbon dioxide is removed from the air by reducing the air temperature to freeze and separate the carbon dioxide. Compared with traditional methods of removing carbon dioxide, the theoretical feasibility and advantages of the freezing method are analyzed. To verify the results of this theoretical analysis, an air purification system using liquid nitrogen as the cold source was designed. Experimental results show that the freezing method can remove carbon dioxide quickly. When the volume concentration of carbon dioxide is 1% and the volume flow of air is 100 L/min, it takes just 18 min to reduce the carbon dioxide concentration to less than 0.1%. The experiments also show that the freezing method of removing carbon dioxide can reach and maintain an extremely low carbon dioxide concentration of just 0.03%. This low concentration is hard to achieve by traditional methods.关键词：air purification;carbon dioxide removal;frozen and separated;confined space2149|2230|0发布时间：2024-07-18
- 摘要：The flow boiling heat transfer process of R404A in small-diameter tubes is a very complex physical phenomenon. At present, research on the heat transfer characteristics of R404A is mostly focused on large-diameter tubes, and there has been little research on small-diameter tubes, despite the different mechanisms underlying the experimental phenomena. Therefore, there is a strong need for specific experimental data to support the theoretical study of the heat transfer characteristics of R404A in small-diameter tubes. An experimental platform for evaporation in small-diameter micro-fin copper tubes was constructed, and the effects of heat flux, evaporation quality, mass flux, and saturation temperature on the heat transfer coefficient in the process of R404A flow boiling heat transfer was investigated. Each factor was studied independently, with all variables other than the particular one under consideration held constant throughout the analysis. The research results shows that the heat flux, quality, mass flux, and saturation temperature all have a significant influence on the heat transfer characteristics of R404A in small-diameter tubes, with the effects of these factors varying before and after dryout. Furthermore, these factors have a direct influence on the initial quality of dryout, the main form of heat transfer, and the occurrence of dryout.关键词：small diameter tube;boiling heat transfer characteristics;dryout;R404A2178|2323|1发布时间：2024-07-18
- 摘要：Thermoacoustic-driven pulse tube refrigerators are usually coupled directly or by long tubes called acoustic amplifiers. With these coupling methods, the efficiency of the whole system is quite low because of the nonideal acoustic field and flow losses. This paper describes a new thermoacoustic-driven pulse tube refrigerator that can reduce the transmission loss of sound power and improve efficiency using a mechanical resonator to couple the thermoacoustic engine and pulse tube refrigerator. Theoretical analysis is carried out for a heating temperature of 900 K and cooling temperature of 80 K. First, the coupling mechanism of the resonator is analyzed and the parameters of the mechanical resonator are optimized. Second, the influence of the heating temperature, cooling temperature, and mechanical resistance are investigated. Furthermore, the proposed method is compared with the traditional coupling configuration. The results show that the thermoacoustic-driven refrigerator with mechanical resonance attains an exergy efficiency of 22.5%, whereas the long-tube-coupled thermoacoustic-driven pulse tube refrigerator only reaches an exergy efficiency of 11.6%.关键词：thermoacoustic;pulse tube refrigerator;mechanical resonator2012|2811|1发布时间：2024-07-18
- 摘要：To optimize the operation of air conditioning systems, the extremum entransy increase principle is proposed, and an entransy loss model for each component and entransy increase model for the whole system are established for air conditioning systems based on entransy theory. The entransy increase rate of the air conditioning system under different operation conditions is experimentally studied when the system cooling load and total power consumption are fixed separately. The results prove that the extremum entransy increase principle holds and determine the optimal operation conditions. When the system cooling load is 5 kW, the optimal compressor frequency is 70 Hz, optimal chilled water flow rate is 0.6 kg/s, and optimal cooling water flow rate is 0.9 kg/s. These parameter values produce the minimum system entransy increase rate, minimum power consumption rate, and maximum system efficiency. When the system power consumption is 2.8 kW, the optimal compressor frequency, chilled water flow rate, and cooling water flow rate are 97.9 Hz, 0.41 kg/s, and 1.00 kg/s, leading to the maximum system entransy increase rate, maximum system cooling load, and maximum system efficiency.关键词：air conditioning system;operation optimization;entransy;work2075|2019|0发布时间：2024-07-18
- 摘要：At present, the international community pays more attention on global warming issue caused by the increased emissions of greenhouse gases. In this study, we establish a set of simulated atmospheric experimental devices to investigate the reactions of greenhouse gases with OH radicals or other atmospheric oxidants and evaluate their atmospheric chemistry implications, e.g., global warming potential (GWP). To estimate the accuracy of the experimental devices, the reaction rate constant of 1,1-difluoroethane (R152a) with OH radicals is measured using the relative rate method, and the infrared spectrum is obtained. The ks of R152a with OH radicals are found to be (3.3 ± 0.1) × 10–14 cm3 /(molecule?s) at 298 K and (2.27 ± 0.04) × 10–14 cm3 /(molecule?s) at 272 K. The atmospheric lifetime and radiative efficiency (RE) are then estimated as 1.4 years and 0.099 W/(m2?ppb), respectively. The GWPs (time horizons of 20, 50, and 100 years) are estimated at 474, 129, and 37, respectively. Our GWP values are in good agreement with those from IPCC-AR5 (discrepancy is less than 7%), which indicates the high reliability of our experimental apparatus.关键词：atmospheric chemistry;global warming potential;refrigerant;R152a2157|2097|1发布时间：2024-07-18
- 摘要：To investigate the heat and mass transfer effects on the regeneration process using condensation heat, experiments were performed in a hybrid air-conditioning system using low-concentration LiCl solution as the liquid desiccant. The regeneration rate and utilization ratio of condensation heat were regarded as evaluation criteria of regeneration performance. The effects of the important operating parameters on the system performance were analyzed experimentally. The results show that the air and solution mass flow rate and inlet temperature have a positive impact on the performance of regeneration. Under typical summer conditions, as the solution concentration increases from 21.20% to 24.91%, the utilization rate of condensation heat fluctuates between 0.416 and 0.507; the utilization rate of condensation heat would increase if the solution concentration was reduced. Finally, empirical correlations coupling the heat and mass transfer coefficients are presented. The experimental results provide reliable data for further analysis of using condensation heat efficiently for the solution regeneration process.关键词：heat and mass transfer;condensation heat;low concentration liquid desiccant;regeneration experimental study2046|2556|3发布时间：2024-07-18
- 摘要：Nanoparticles are likely to aggregate in the oil-rich layer during the boiling of nanorefrigerant–oil mixtures, leading to a low mixture-to-oil migration ratio and making it difficult for nanoparticles to circulate with working fluid in a refrigeration system. Surface modification is a promising approach to raising the mixture-to-oil migration ratio, as it offers the ability to suppress aggregation. In this study, the mixture-to-oil migration ratio of nanoparticles modified by different alkyl chain lengths and grafting mechanisms was measured based on the absorbance method and the effects of the alkyl chain length of the modifier and the grafting mechanisms on the migration characteristics were examined. TiO2 nanoparticles, R141b refrigerant, and NM56 oil were used to prepare the nanorefrigerant–oil mixture. The surface modifiers included C1TMS, C3TMS, C8TMS, C16TMS, and CTAB. The experimental results show that the mixture-to-oil migration ratio increases after the application of these modifiers. The C16TMS exhibits the best enhancement (increasing the migration ratio by 131.2%). It is also found that the mixture-to-oil migration ratio increases by 5.48%–20.11% as the alkyl chain length increases from 1 to 16. The modified nanoparticles grafted by covalent bonds exhibit better migration performance than those that rely on electronic attraction.关键词：refrigerant;nanoparticle;surface modification;boiling;migration;lubricating oil2095|2539|0发布时间：2024-07-18
- 摘要：Water is a frequently used cool storage material in the field of refrigeration and food preservation. Because of the large supercooling degree during the phase change period and the low thermal conductivity of water, a composite water-based phase change material containing nanoparticles and a dispersing agent was developed. The mass ratio of the material is H2O+0.7% nano-TiO2+1.0% sodium dodecyl benzene sulfonate (SDBS). The phase change temperature of the composite phase change material is 0.216℃ and the latent heat is 353.1 kJ/kg. By adding nano-TiO2, the supercooling degree of water can be reduced by 5–6℃, while the thermal conductivity increases by 62.7%, from 0.5988 to 0.9745 W/(m?K). Moreover, adding the dispersing agent SDBS improves the sedimentation problems of water-based nano-TiO2, enhances the stability of the material, and prevents the phase separation phenomenon. Through theoretical and experimental analyses of the thermal properties of the composite phase change material with different nano-TiO2 and SDBS ratios, the optimal mass ratio is determined to be 7:10. The color and thermal conductivity of the optimal material after the installation indicate that nano-TiO2 has good dispersion stability in water.关键词：composite phase change material;supercooling degree;thermal conductivity;nanomaterial;thermal property1956|2327|2发布时间：2024-07-18
- 摘要：Experiments were carried out to study the thermal performance of cylindrical and annular units. The results show that the annular structure effectively reduces the heat storage time and improves the heat storage rate of phase change materials. A novel phase change heat storage device is designed using annular units, and a phase change heat storage experiment system is set up to analyze the operation characteristics of the heat storage device. The results show that the rate of heat transfer is significantly improved when the flow rate and temperature of the heat transfer fluid increase. Under the discharge conditions, the rate of heat transfer is significantly improved when the flow rate of the cold fluid increases within a reasonable range. The heat storage device provides more than 44 L of hot water at a cold water flow rate of 4 L/min. According to the water usage conditions, the performance that water was supplied intermittently every 5 min and 10 min were investigated experimentally, and the results show that the water demand could be satisfied.关键词：heat charging/discharging performance;thermal storage device;annular unit2261|3095|6发布时间：2024-07-18
- 摘要：Under actual operating conditions, a closed cooling tower was simulated and the descaling characteristics of an induction-type electronic descaling device and a high-frequency electronic descaling device on the spray water side of an evaporative cooler was investigated experimentally. The results show that both kinds of electronic descaling instruments can accelerate the dissolution of dirt and reduce fouling resistance on the outside of the heat transfer tubes, but the effects are different. During the 150 h of the experiment, the greatest decline in the fouling resistance using the high-frequency and induction-type electronic descaling devices was 17.81% and 20.35%, respectively. With the induction-type electronic descaling device, the fouling resistance reduced several times. The results of scanning electron microscopy show that the structure of dirt crystals is rod-like under the action of the high-frequency electronic descaling device, but exhibits a block structure with the induction-type electronic descaling device. The results of independent experiments show that the two electronic descaling instruments can effectively reduce the hardness of circulating water. After a 12 h experiment, the circulating water hardness was observed to decrease by 40.35% and 37.04%, respectively.关键词：electronic descaling;evaporative cooling equipment;spray water system;fouling resistance2030|3193|1发布时间：2024-07-18
- 摘要：The refrigeration and heating performance of an air conditioner were tested by changing the outdoor environment temperature and relative humidity. The relative refrigerating capacity percentage (RRCP), relative heating capacity percentage (RHCP), relative power percentage (RPP), and relative energy consumption percentage (RECP) are introduced to establish the relationship between the measured performance and rated parameters. The results show that the variation in outdoor relative humidity has little effect on the performance of the air conditioner. However, under the cooling conditions, the outdoor dry-bulb temperature increased from 24 to 43 ℃, RRCP decreased from 16.5% to ﹣13.2%, RPP increased from ﹣4.8% to 24%, and RECP decreased from 22.2% to ﹣30%. When the outdoor dry-bulb temperature is lower than the corresponding temperature of rated case, RECP exhibits a positive deviation, and the performance of air conditioner is excellent. When the outdoor dry-bulb temperature is higher than the corresponding temperature of rated case, RECP exhibits a negative deviation, and the performance of the air conditioner is bad, indicating wasted energy. Under the heating condition, when the outdoor dry-bulb temperature increased from ﹣6 to 16 ℃, RHCP increased from ﹣34% to 14%, RRP increased from ﹣24% to ﹣2%, and RECP increased from ﹣13% to 16.9%. When the outdoor dry-bulb temperature is lower than the corresponding temperature of rated case, RECP exhibits a negative deviation, and the performance of the air conditioner is poor; when the dry-bulb outdoor temperature is higher than the corresponding temperature of rated case, RECP exhibits a positive deviation, and the performance of the air conditioner is excellent, which is conducive to saving energy.关键词：air conditioner;test conditions;rated parameter;energy efficiency ratio;coefficient of performance2094|2486|0发布时间：2024-07-18
- 摘要：Experiments were performed over the steady running period of assembly cold storage, and the working and stopping time of the refrigerating unit was tested under different environmental temperatures. A calculation model for the working and stopping time and the working time coefficient were established. MATLAB was used to calculate the working time coefficient of the assembly cold storage under different insulation thicknesses and different refrigeration units. The results show that the maximum working time error decreased from 11.6% to 2.78% after the mathematical model was revised, and the stopping time decreased from 7.91% to 1.46%. The maximum error between the calculated and experimental values of the working time coefficient was 3.47%, which indicates high precision. As the refrigerating capacity increased, the rate of decrease in the working time coefficient became smaller, and it was found to be appropriate to limit the refrigerant flow to 0.012 kg/s. Similarly, as the insulation thickness increased, the rate of decrease in the working time coefficient became smaller, and the insulation thickness was increased by 100 mm according to the working time of the cold storage.关键词：assembly cold storage;working time coefficient;energy efficiency;working time;stopping time2293|2306|1发布时间：2024-07-18
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