最新刊期
卷 38 ， 期 3 ， 2017
- 摘要：Two controllers (single channel optimal controller (SCOC) and multivariable linear quadratic Gaussian controller (LQG)) are built to improve the performance of a transcritical CO2 refrigeration cycle. The SCOC is first proposed to search the optimal gas cooler pressure online by adjusting the nozzle throat area. Considering the uncontrollable cooling capacity by the SCOC, the second controller LQG is designed to achieve the controllable cooling capacity. Experimental results show that the optimal gas cooler pressure is actually achieved by the SCOC with a maximal COPh of about 3.15 under the given operating conditions. However, with the SCOC, the cooling capacity is changed passively and uncontrollably. Moreover, experiments with LQG indicate that the cooling capacity and gas cooler pressure are accurately tracked independently. However, LQG cannot ensure the maximal operating performance when the system becomes steady. This work points out that the two controllers have their own advantages and disadvantages. More work needs to be done to combine the two algorithms into one. This will achieve maximal efficiency under the desired cooling capacity.关键词：refrigeration;ejector;CO2;optimal control;multivariable controller1974|2286|1发布时间：2024-07-18
- 摘要：The performance of a new 100-kW single-stage R744 piston compressor developed by Norwegian University of Science and Technology and Sintef is investigated in this paper. The oil circulation, volumetric efficiency, and overall efficiency are tested as an overview of the compressor performance. Experimental results with a pressure-ratio range from 1.9 to 4 under compressor revolutions of 1000 to 2500 r/min are reported. The results show that the highest oil circulation rate is 2.84%, acquired with a pressure ratio of 2.1 and a revolution of 2500 r/min, while the volumetric efficiency reaches a maximum value of 85.8% as a pressure ratio of 1.88 and revolution of 1000 r/min are reached. Optimum overall efficiencies of this compressor can be achieved in a pressure-ratio range from 1.5 to 2.5 and a compressor revolution near 1500 r/min. Specifically, the maximum value is obtained up to 79.1% with a revolution of 1500 r/min and a pressure ratio of 2.0. The fitted overall efficiency results are compared with the efficiency data from two commensurate products in a catalog. This illustrates that the overall efficiency of the new compressor is about 11.5% higher than the others when the pressure ratio ranges from 1.5 to 3.2.关键词：compressor;R744;efficiency;pressure ratio;revolution1913|2738|2发布时间：2024-07-18
- 摘要：This study sets up an experimental platform to test the performance of a CO2 air cooler. The direct expansion system and the pump-driven system were examined by changing parameters such as evaporating temperature, storage temperature, circulation ratio, and face velocity. Experiments indicate that in the direct expansion system, the heat transfer coefficient and refrigerating capacity exhibit a decreasing trend as the evaporating temperature decreases. The heat transfer coefficient dropped from 20.2 W/(m2?K) to 16.6 W/(m2?K), and the refrigerating capacity dropped from 7.5 kW to 6 kW as the evaporating temperature dropped from ?22 ℃ to ?47 ℃ during the experiment. A peak of the heat transfer coefficient exists for different circulation ratios in the pump-driven system. (The best circulation ratio is 3 in this paper.) For example, when the storage temperature was at ?20 ℃, the heat transfer coefficient increased by about 13.2% as the circulation ratio increased from 1 to 3. However, the heat transfer coefficient began to decrease as the circulation ratio continued to increase. The heat transfer coefficient dropped by about 2% when the circulation ratio increased to 5. When the face velocity was changed from 2.2 m/s to 2.5 m/s, the heat transfer coefficient increased by just 2.12%. However, it increased by 11.4% when the face velocity was changed from 2.5 m/s to 3.2 m/s. Next, the increase rate of the heat transfer coefficient became slow as the face velocity increased from 3.2 m/s to 3.25 m/s, which is only 0.88% in this case.关键词：CO2 air-cooler;heat transfer coefficient;refrigerating capacity;circulation ratio;face velocity2113|2116|0发布时间：2024-07-18
- 摘要：In order to achieve energy conservation and emissions reductions, an air source/sewage source heat pump hot water system was designed. Field tests were conducted in a demonstration project on Tunxi Road at Hefei University of Technology. The results show that at an ambient temperature of 8 ± 1.2 ℃ and a water supply temperature of 50 ℃ condition, the wastewater temperature is decreased from 30.6 ℃ to 15.5 ℃ through preheater and waste heat recovery. Heat recovery energy accounts for about 1/3 of the total energy for hot water heating. At an ambient temperature of 10 ± 1 ℃ and a water supply temperature of 45 ℃ condition, the energy efficiency of the composite heat pump system was 3.45. Compared with a separate air source heat pump, the energy efficiency is increased by 18%. Compared with a gas water heater and an electric water heater, the composite heat pump system can reduce operating costs by 53.3 and 72.4%, respectively. The results show obvious advantages of composite heat pump system on energy saving, environmental protection, and cost reduction.关键词：air source heat pump;sewage source heat pump;design;field test;waste heat recovery1905|2355|3发布时间：2024-07-18
- 摘要：The performance of a spiral-tube heat storage tank added to a traditional cascade air source heat pump was experimentally investigated. The water temperature of different measuring points and the refrigerant temperature at the inlet and outlet of the heat storage tank were measured. The dry bulb temperature for indoor simulated conditions was 22 ℃ ± 0.1 ℃, the relative humidity was 50% ± 3%, and the dry bulb temperature for outdoor simulated conditions was ?12 ℃ ± 0.1 ℃. The characteristics of the heat storage tank in the thermal storage mode, thermal storage defrosting mode with intermittent heating, and thermal storage defrosting mode with continuous heating were experimentally studied. The experiment results indicated that the spiral-tube heat storage tank was effective for heat storage and heat release for low-grade heat sources at different temperatures. During a thermal storage defrosting mode with intermittent heating, and thermal storage defrosting mode with continuous heating, the amounts of heat release in the heat storage tank were 1642.7 kJ and 1892.4 kJ, respectively. These results can meet the requirements for defrosting and for part of indoor heating.关键词：air source heat pump;cascade cycle;heat storage tank;charging and discharging characteristics;defrosting1826|2113|3发布时间：2024-07-18
- 摘要：Nowadays, more and more attention is being paid to the new low Global Warming Potential (GWP) refrigerant R1234ze(E). Mixing this refrigerant with R32 has been studied in recent years. We focused on contrasting the actual performance of the R1234ze(E)/R32 mixture (mass ratio: 27%/73%, named L-41b, with a GWP value of 493) with R410A in a heat pump system. A test system of an air source heat pump was designed and built in an environmental chamber in order to compare the differences between L-41b and R410A under a wide range of working conditions. The compressor energy consumption (COP), heating capacity, refrigeration cycle pressure ratio, and discharge temperature of the compressor were discussed and compared with R410A under similar working conditions. The results shows that, in the variable-condition experiments, the COP deviation decreased from 8.6% to 2.8% when the evaporation temperature increased from 5℃ to 13℃ at a constant condensation temperature of 38℃. The COP deviation was minimized when the condensation temperature increased from 30℃ to 42℃ at a constant evaporation temperature of 5℃. The results further demonstrate that the L-41b mixture can be used to replace R410A in a heat pump system, particularly at relatively high cycle temperatures.关键词：R1234ze(E)/R32;mixed refrigerants;air-source heat pump;operating condition2175|2269|1发布时间：2024-07-18
- 摘要：This paper studies the evaporation heat transfer of R32 inside two different diameters of horizontal tubes under saturation temperatures of 0 ℃, 5 ℃, and 10 ℃. The evaporation heat transfer coefficient of the refrigerant was obtained by using a heat resistance separating method. Taking mass flux and saturation temperature as the influencing factors, the experiment results include two parts：analysis of single-tube thermal resistance and evaluation of single-tube comprehensive performance. The experimental results indicate that the heat transfer coefficient and pressure drop rise as the mass flow rate increases. The diameter has a great influence on the heat transfer coefficient, and the heat transfer coefficient of No.1 is 1.1~1.3 times as that of No.2. The effect of temperature on the heat transfer coefficient and the pressure drop differs under different mass flow rate. As the mass flow rate increases, the proportion of the external water side-tube resistance to the total thermal resistance increases, while that of the inside gradually decreases. The heat transfer coefficient per pressure drop of two single tubes decreases as the mass flow rate increases. The saturation temperature shows various effects on the parameters at different mass flow rate.关键词：evaporating experiment;mass flux;saturation temperature;thermal resistance analysis;evaluation of comprehensive performance2081|2356|4发布时间：2024-07-18
- 摘要：With implementation of the Maximum Allowable Values of The Energy Consumption And Energy Efficiency Grade For Household Refrigerators（GB 12021.2—2015）, this study experimentally investigated the application of 21 kinds of microchannel heat exchangers (MPE) on domestic refrigerator system. The investigation was conducted from component to system, and heat exchanger performance was compared with original condenser. The results show that for the best MPE sample, the heat transfer performance can increase by 13%, the air-side pressure drop is only 28%, and the refrigerant-side pressure drop is only 54%. The best MPE sample was introduced into a refrigerator system, and the charge of refrigerant for the system was optimized. The optimized system energy consumption was tested under national standard GB/T8059. The results show that the energy consumption of the new system was 2.8% lower than the original under a test condition of 25 ℃, and the calculated energy consumption based on the new national standard was 2.6% lower than the original. This can meet the first-class requirements of the new national standard.关键词：domestic refrigerator;GB 12021.2—2015;microchannel heat exchanger;energy conservation1963|2198|3发布时间：2024-07-18
- 摘要：The instability of a linear compressor is studied by an experiment on a developed linear compressor in a refrigerator. The experimental results show that there are three types of unstable phenomena: the start-up process, external disturbance process, and solenoid valve adjustment process. During the start-up process, an unstable status is caused by the adjustment of the supply power and changes in the refrigerant suction/discharge pressure. This finally settles down as the supply power and suction/discharge pressure are unchanged. Some external disturbance of the refrigeration system causes an unstable oscillation with a fluctuation of about 0.3 mm of the piston displacement. Accordingly, the current and the input power fluctuate at about 10% of their average values. This oscillation gradually becomes stable as there is no more disturbances. The solenoid valve adjustment process has a great influence on the stability of a linear compressor. In this experiment, the maximal fluctuation amplitude of the displacement is about 2.2 mm, and the piston is apt to strike the cylinder. Therefore, the design optimization for a linear compressor structure, and a control strategy based on the change characteristics of the working conditions, is necessary to ensure operational reliability and high efficiency.关键词：linear compressor;control;stability;start-up feature1895|2454|1发布时间：2024-07-18
- 摘要：Fluid flow maldistribution in each tube of a dry-expansion shell and tube evaporator can cause inefficient heat exchange. In this paper, a numerical simulation of flow distribution in a dry-expansion shell and tube evaporator is carried out using CFD commercial software. A new type of dispenser is proposed to balance the flow distribution between different tubes. A numerical simulation and visualization experiment are compared to verify the reliability of the simulation results. An experimental platform was set up, and a conical liquid distributor was designed and manufactured. The liquid distributor was installed in a chiller. The performance of the chiller under different conditions was tested. The results show that the inlet and out?let pressure drops in the evaporator decreased from about 15 kPa to about 3 kPa, and the entire COP increased by about 5%.关键词：dry-expansion evaporator;discharge distribution;cone distributor;COP1836|2169|1发布时间：2024-07-18
- 摘要：Because the total gross of cold stores is increasing, the energy consumption of cold stores is attracting increasing attention. Infiltration through the doorway, which accounts for a large part of the total heat load, has become an important research focus with regard to the energy consumption of the cold store. In this paper, by using an unsteady RANS model, a transient infiltration simulation model is established. By utilizing a cold store with height ratio between the cold store and the door 2:1, experimentally measured data under conditions with different temperature differences and operation modes of the cooling fans are used to validate the model. The results show that the predicted value and change trends of the infiltration air volume, local wind speed, and temperature of the established model agree well with the experimental data. The predicted error for the infiltration air volume before 40 s of door-open time is ±10%. By using this model, the characteristics and mechanism of the infiltration are analyzed. The results show that, owing to the effect of the airflow made by cooling fans, the temperature in the cold store when the fans are on is lower than that when the fans are off. There is little density difference between the warm air and the cold air. The gravity imposed on the cold air has little variation. Thus, the infiltration air volume changes linearly with the door-open time.关键词：cold store;infiltration;temperature field;simulation1759|1880|2发布时间：2024-07-18
- 摘要：In the present paper, the cold loss of a fridge when unloaded was studied through an experiment, and an improvement was proposed. First, the heat transfer process between the fridge interior and the external environment was simplified into a one-dimensional heat conduction process. On an all-weather simulation experiment platform, to investigate the effect of ambient temperature on cooling capacity loss, the ambient temperature was set to 5, 10, 15, 20, 25, 30, 35, and 40 ℃ with a fixed ambient humidity of 80%. To investigate the effect of ambient humidity on cooling capacity loss, the ambient humidity was set to 55, 65, 75, 85, and 95% with a fixed ambient temperature of 25 ℃. The temperature of the inner and outer walls for every combination of the above two series of parameters was measured when the refrigerator was unloaded. According to the Fourier heat conduction law, the heat flux of each wall can be obtained, and then the cold loss is calculated. The results show that the cold loss of the refrigerator increases with ambient temperature, and the ambient humidity has no effect on cooling capacity loss. The cold loss of the refrigerator back wall is obviously greater than that of the other walls, while that of the top wall is smaller. The insulation of the refrigerator back wall is thickened to decrease the entire cold loss, and the insulation of the refrigerator top wall is made thinner to guarantee the refrigerator storage capacity. This leads to an approximately uniform cold loss distribution of the refrigerator walls.关键词：refrigerator;cooling capacity loss;heat conduction;improve2000|2428|0发布时间：2024-07-18
- 摘要：As one of the multivariate data analysis methods, principal component analysis (PCA) is widely used for sensor fault diagnosis in refrigeration and air conditioning systems. First, the 18 sensors commonly used in a variable refrigerant flow (VRF) system are selected to establish sensor fault detection and diagnosis (FDD) models according to the thermal equilibrium principles and control logics of the system. Then, the process of sensor FDD is presented with the Q statistic and Q contribution as test standards, combined with the principles of a PCA algorithm. Next, validation is conducted using the measured data after introducing sensor faults of different types and degrees. Finally, the characteristics of sensor FDD are obtained under different fault conditions. As a whole, the results prove the reliability of applying a PCA to the sensor FDD process for VRF systems. Specific performance characteristics are as follows: fault detection efficiency has big differences for different sensors under different types and extents of faults; the fault detection efficiency of the PCA-based sensor fault detection method under the conditions with small deviation faults is low; and for individual sensors, the fault detection efficiency is integrally low. Since fault diagnosis is based on fault detection, the above-mentioned fault detection method may play important role in the FDD process.关键词：principal component analysis;fault detection and diagnosis;Q statistic;Q contribution;sensor;variable refrigerant flow air conditioning system1839|2053|4发布时间：2024-07-18
- 摘要：The anti-icing performance of a super-hydrophobic surface, which was aluminum-based surface prepared by an etching method, at different substrate temperatures of ﹣15, ﹣20, ﹣25, ﹣30 ℃ for both static and dynamic cryogenic liquid droplets were experimentally investigated under an ambient temperature of 20 ℃ and relative humidity of 60%. Results showed that the super-hydrophobic surface presented good anti-icing performance under both static and dynamic conditions of liquid droplets. In static liquid droplet anti-icing experiments, the freezing time of the super-hydrophobic surface decreased rapidly as the cold surface temperature was decreased. When the substrate temperature was ﹣25 ℃, the anti-icing performance of the super-hydrophobic surface changed abruptly. The anti-icing performance of the super-hydrophobic surface worsened as the temperature of the cold surface decreased. In dynamic liquid droplet anti-icing experiments, at surface temperatures of ﹣15 and ﹣20 ℃, liquid drops were able to rapidly rebound off the low-temperature surface. When the surface temperature was ﹣25 and ﹣30 ℃, respectively, low-temperature liquid droplets froze quickly and were incapable of rebounding off the super-hydrophobic surface, i.e., the super-hydrophobic surface lost the ability of anti-icing. Anti-icing mechanisms were analyzed according to the phase change nucleation theory. The research results are expected to provide a reference for the application of super-hydrophobic surfaces in outside heat exchangers for winter air conditioning.关键词：aluminum-based super-hydrophobic surface;anti-icing1915|2165|6发布时间：2024-07-18
- 摘要：Based on the models of Campbell and de V-1, a new mathematic model for predicting soil thermal conductivity is developed in this paper. Using the parameters shape factors ( ) and soil solid thermal conductivity ( ), the Campbell model is calculated by the method of the de V-1 model. According to this method, the soil solid thermal conductivity and shape factors are considered to be soil solid components, and include the effect of temperature on the soil solid thermal conductivity. By comparing the Campbell and de V-1 models, the measured results are in good accordance with the numerical ones, and the model shows better performance when predicting soil thermal conductivity. In addition, the effects of soil temperature and moisture on different soil thermal conductivities are studied. The results indicate that under the same soil temperature and moisture, the thermal conductivity of sand is greater than that of loam, and the thermal conductivity of loam is greater than that of clay. This will provide new ideas for establishing a coupled heat and moisture transfer model in unsaturated soil for heat exchangers.关键词：unsaturated soil;thermal conductivity;model study;optimization1798|2127|1发布时间：2024-07-18
- 摘要：In the hollow-fiber-membrane-based liquid desiccant air dehumidification process, water vapor can permeate through the membrane effectively, while the liquid desiccant droplets are prevented from crossover. Hence, the traditional problems of desiccant droplet crossover and indoor environmental pollution are solved. In this research, a test rig of the membrane module is built, and the performance of the system is investigated experimentally. The results show that the dehumidification efficiency is in the range of 20%–60%. This is comparable to a direct-contact packed bed dehumidifier. The largest dehumidification rate is about 800 g/h under very hot and humid weather conditions. Moreover, the cooling capacity of the dehumidification is as high as 700 W. Therefore, it performs well even under hot, and humid South China weather conditions.关键词：heat and mass transfer;experimental study;dehumidification;liquid desiccant1769|2805|2发布时间：2024-07-18
- 摘要：Because the outdoor air temperature in cold regions is severely low in winter, the combustion-supporting air will cause deflagration and vibration while directly supplied to the boiler and the combustion efficiency is also decreased. At the same time, the temperature of the return water in a large heating system, and the exhaust gas from the gas boiler, are both high. Thus, the exhaust gas could not be cooled down to a relatively low temperature by the return water during the waste heat utilization process. This paper presents a new way to heat the air by using flue gas waste heat, which is smaller, requires less space, and has less resistance compared with conventional heat exchange equipment. The proposed method can be applied to the flue gas waste heat deep-utilization of a large gas heating boiler. Field test results show that the flue gas waste heat deep-utilization and the combustion-supporting air heating system can cool the flue gas temperature near to or even lower than the return water temperature of heating network. The utilization efficiency of gas combustion is increased by 13.2 %, and the recovery rate of the flue gas waste heat is 66.7 %. Then, flue gas waste heat deep recovery is realized, and the problem in which the combustion-supporting air causes deflagration and vibration while directly supplied to the boiler is solved. The paper provides a reference for flue gas waste heat deep-utilization in cold regions, and for combustion-supporting air heating system optimization.关键词：cold regions;large gas-fired boiler;flue gas waste heat recovery;air heating;engineering test1905|2525|0发布时间：2024-07-18
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Analysis of Thermal Cycling Stability of Nanocopper-erythritol based on Increase in Material Entropy
摘要：This paper briefly introduces the preparation of nanocopper-erythritol. According to the internal mechanism of the change of thermal property in phase change material (PCM), the causes of material changes in the circulation process are explored. Using test data from differential scanning calorimetry and a thermal conductivity tester (Hotdisk), the heat conductivity coefficient, supercooling degree, and latent heat during 0, 20, 40, 60, 80, and 100 thermal cycles are analyzed. The change process of PCMs is summarized based on the point of growth of material entropy which is introduced through the thermodynamic principle of thermodynamic entropy. Experimental results show that after 100 cycles, the performance parameters of the phase change material have been decreased differently. The declining degree of the thermal conductivity coefficient is lower when the amount of nanocopper increases. The overall cooling degree of erythritol with nanocopper added is less than that of erythritol without nanocopper. In the latter stages, the declining degree of the latent heat value increases as the number of cycles increases.关键词：nanocopper-erythritol;thermal cycle;material entropy;heat conductivity coefficient;super-cooling degree;latent heat1941|1932|2发布时间：2024-07-18 - 摘要：Oocyte cryopreservation has become a practical tool in assisted reproductive technology and fertility preservation. Cryoprotective agents (CPAs) addition and removal are essential steps during oocytes cryopreservation, however, the processes of CPAs addition and removal may cause fatal osmotic and toxic injuries to oocytes. In order to study the injuries of porcine MII-stage oocytes during CPAs addition and removal combined processes and minimize osmotic and toxic injuries to oocytes, primarily, a microfluidic device for continuous loading and unloading of CPAs was designed and fabricated in this study. CPAs were loaded and unloaded with the microfluidic linear method and the conventional step-wise method, respectively. Then, the cell volume changes and the effects on the survival rate and developmental potential of oocytes were investigated. The results showed that the oocyte volume changes with the microfluidic device were obviously less than step-wise method. The survival, cleavage and blastocyst rate of oocytes were 95.3%, 64.4%, and 19.4%, respectively, which were significantly higher than the traditional step-wise method (79.4%, 43.6%, and 9.7%) (P<0.05). In conclusion, microfluidic device can be used efficiently to load and unload of CPAs, and significantly reduce the osmotic shock to oocytes, which may open up a new path for oocyte cryopreservation.关键词：cryopreservation;cryoprotective agents(CPAs);osmotic injury;oocyte1879|2255|4发布时间：2024-07-18
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