摘要:In order to investigate the influences of subcooling degree, mass flow rate and heat flux on the performance of the spray cooling system, a closed spray cooling system with R134a was built. The subcooling degree was adjusted by the subcooled section located before the nozzle inlet, the mass flow rate was controlled by the variable gear pump, and the heat flux was dominated by changing the voltage and current of the heating power. The experimental results indicated that, the variation of subcooling degree did not have significantly influence on the heat surface temperature and heat transfer coefficient as the heat flux and the mass flow rate were fixed. When the heat flux and the subcooling degree were constant, there existed a critical value of the mass flow rate, as the mass flow rate was smaller than the critical value the heat surface temperature decreased with the mass flow rate increasing, on the contrary, the heat surface temperature would increase with the mass flow rate increasing when the mass flow rate exceeded the critical value. When the mass flow rate and the subcooled temperature kept constant, there also existed a heat flux value which made the evaporation capacity equal to the supplement of droplets, therefore the heat transfer coefficient could reach the maximum value.
关键词:spray cooling;heat transfer coefficient;subcooling degree;heat flux;mass flow rate;R134a
摘要:In order to research the effects of the gas-cooler heat transfer area and the mass velocity in the gas-cooler on the system performance and the optimal discharge pressure of a transcritical CO2 heat pump water-heater, a gas-cooler mathematical model in which the heat transfer area and the mass velocity in it can be changed separately was built in this paper. The results show that the heating capacity and COP increase as the heat transfer area increases when the mass velocity is constant. Moreover, COP increases first and then declines with an increase in the mass velocity due to the influence of the pressure loss when the heat transfer area is constant. Additionally, the optimal discharge pressure declines as the heat transfer area increases, and it increases with an increase in the mass velocity. Thus, it should be considered properly to discuss the gas-cooler heat transfer area and the mass velocity in it to achieve superior COP and maintain the optimal discharge pressure when designing the transcritical CO2 heat pump.
摘要:This paper presents the main sources of conventional air-conditioning data, which can be used for product research and industry standard development. The sources and reasons of big data in air-conditioning field have been combed by both air-conditioning products and their enterprises. The big data in air-conditioning field are challengeable to be mined for its multi-metric, multi-dimension and regionalism. Applications of big data in air-conditioning field are generalized mainly on system optimization, fault detection and diagnosis, building energy prediction, allocation of enterprise human and capital resource, personalized customization etc. As user behaviors can be detected by mining the massive data, the protection of big data should be brought to the forefront.
关键词:data mining;refrigeration and air-conditioning;fault detection;energy analysis
摘要:In order to improve the performance of refrigerated display cabinet, decrease the food temperature and restrain the temperature fluctuation during defrosting, a kind of composite shelf is proposed in the food refrigerated display cabinet (RDC) based on the high-thermal conductivity of heat pipe and cool storage of phase change materials. The influence of three phase change materials of RT3, RT4, RT5 on the temperature of food packages, the characteristic of heat transfer and cool storage of composite shelf were investigated. Compared with the ordinary shelf, the food temperature on composite shelf of RT3 reduces by 13.7%-80%, temperature fluctuation decreases 53.3%-70%; the food temperature on composite shelf of RT4 reduces by 20%-83.3%, temperature fluctuation decreases 83.3%-87.5%; the food temperature fluctuation on composite shelf of RT5 decreases 66.7%-75%. Compared with the other two composite shelves, the average food temperature of each floor and each row on composite shelf of RT4 is the lowest and within 5℃, thus the cool storage performance of phase change material of RT4 is the best.
摘要:In the range of Re = 4800~10200, the experiment was made to investigate the heat transfer characteristics of two rows of the delta winglets arrayed in V shape at the back side of the round tube in the rectangular channel. The heat transfer enhancement was compared between the delta winglets with V-shaped array and the single-row large delta winglets, and the effects of attack angle of delta winglet, winglet height and spacing of winglet array were examined. Results show that Nu of the delta winglets with V-shaped array is 4.9% higher than that of the single-row large delta winglets, and the heat transfer in the channel has an increasing trend with the increase of the spacing in winglet array. For the delta winglets with V-shaped array and the single-row large delta winglets, with increasing the attack angle of the delta winglet, the heat transfer enhancement increases firstly and then decreases, and the optimal attack angle is α=60°, while the heat transfer enhancement in both layouts monotonically increases with the increase of the winglet height. The correlations of Nu for the delta winglets with V-shaped array and the single-row large delta winglets are respectively derived by the regression analysis of experimental data. These correlations can be referred for the arrangement of the delta winglets in the fin-and-tube heat exchanger.
关键词:fin-and-tube heat exchanger;heat transfer enhancement;longitudinal vortex;experimental study
摘要:In this study, an optimization method based on genetic algorithm and entransy dissipation theory is applied to the optimization design of the plate fin and tube heat exchangers in fan coil units. Two examples, i.e. the heat exchanger in a cooling dry fan coil unit and the heat exchanger in a heating fan coil unit, are studied and reported. Both of the two heat exchangers have plate fin and tube structure, but with different conditions and constraints. The optimization objective selected is the entransy dissipation based thermal resistance, and the design variables are the transverse tube pitch, longitudinal tube pitch, tube outside diameter, fin pitch and number of fins. To evaluate the performances of the heat exchanger, the heat transfer rate, air side and water side pressure drop, total pumping power and effectiveness are used as the evaluation index. The optimization results show that both the entransy dissipation based thermal resistance and other performances are improved, under the specific conditions in the current study.
关键词:optimization design;genetic algorithm;entransy dissipation;plate fin and tube heat exchanger;fan coil unit
摘要:As a refrigeration technology with wide-temperature district, auto-cascade refrigeration has a very broad application prospect in the refrigeration and cryogenic field. Compared with single stage compression, two stage compression and cascade refrigeration technology, the advantages and characteristics of auto-cascade refrigeration technology are pointed out after the principles and application of auto-cascade refrigeration technology are introduced. Mainly from auto-cascade refrigeration technology process and mixed refrigerant selection and proportion, the current research situation of auto- cascade refrigeration technology is analyzed, and the technology development trends of the design of system process and mixed refrigerant selection and proportion are pointed out. This paper provides a reference of the technology for further development and application.
关键词:auto-cascade refrigeration;development status;mixed refrigerants;cyclic process
摘要:A fractionation heat exchanger was employed to improve the separation efficiency of CO2 and propane in the small-sized auto-cascade refrigeration cycle. The effects of CO2 mass concentration, vapor quality at condenser outlet and the outlet temperature on the high-pressure side of the fractionation heat exchanger were analyzed. Results show that as CO2 mass concentration grows from 0.22 to 0.31, the evaporating temperatures of the experiment and theoretical calculation are raised by 6.8 K and 5.7 K, respectively. And the corresponding COP is increased by 68.3% and 24.9%, respectively. Simulation data also reveal that as the vapor quality increases from 0.5 to 0.9, the evaporating temperature reaches the minimum of 211.37 K and COP drops sharply by 98.8% at a vapor quality of 0.7. In addition, when the outlet temperature on the high-pressure side of the fractionation heat exchanger is raised by 8.21 K, the evaporating temperature and COP are increased by 1.41 K and 21.3%, respectively.
摘要:The boiling heat transfer coefficients of R410A are investigated in a single horizontal stainless steel microchannel with 2 mm inner diameter. The experiments are conducted by varying the mass flux from 200 to 600 kg/(m2?s), heat flux from 5 to 15 kW/m2, quality from 0.1 to 0.8, while maintaining the saturation temperature at 0 ℃ and 5 ℃. The results show that the boiling heat transfer coefficients rise sharply with the increase of the heat flux when the quality is higher than 0.5, and the average increase amplitudes are 4.6% and 7.7%. The influence of the heat flux on heat transfer coefficients is negligible when the quality is lower than 0.5. The heat transfer coefficients rise slightly with the increase of the mass flux, and the average increase amplitudes are 1.1% and 2%. But the saturation temperature almost has no influence on heat transfer coefficients. After that, the possible mechanisms are discussed. At last, the results are also compared with the results of Choi et al and Ebisu et al, whose inner diameters are 1.5 mm, 3 mm and 6.4 mm. It shows that the heat transfer coefficients will increase with the decrease of the inner diameter when the quality is lower than 0.5, and the average increase amplitudes are 18.4%, 23.6% and 19.5% respectively.
关键词:microchannel;boiling heat transfer coefficient;experiment study;R410A
摘要:An ejector expansion refrigeration cycle with two stage evaporation can not only regulate the exit quality of ejector but also improve the coefficient of performance of system. This experimental study presents the effect of the second stage evaporator on the transcritical CO2 ejector expansion refrigeration system with two stage evaporation. The system performances were investigated with the variation of chilled water flow rate of the second stage evaporator, and the effects were compared with that by the regulation of ejector area ratio, which is the mixing section area to nozzle throat area. The experiment results indicate that the pressure of gas cooler and the first evaporator as well as the compressor mass flow rate increase with the rise of the chilled water flow rate of the second stage evaporator. The larger the ejector area ratio, the higher the gas cooler pressure and the lower the first evaporator pressure and compressor mass flow rate. Furthermore, the entrainment ratio decreases with the increase of chilled water flow rate of the second stage evaporator, while the COP and cooling capacity of the system increase. Especially, the COP and cooling capacity increase largely at lower entrainment ratio. This study provides another excellent regulation approach for ejector expansion cycle.
关键词:ejector expansion refrigeration system;two-stage evaporation;performance of system;carbon dioxide
摘要:The condensation heat transfer performance outside horizontal enhanced tubes of near-azeotropic refrigerant R404A and zeotropic refrigerant R407C was investigated in this study. The experimental data were analyzed using Wilson graphical method. The results show that the coefficients of condensation heat transfer outside horizontal enhanced tubes of R404A and R407C increase with the increase of supercooling degree of tube wall, which is different from pure refrigerant. For near-azeotropic or zeotropic refrigerant, some components will encounter the gas film resistance caused by the condensation of other components. With the increase of supercooling degree of tube wall, the volatile components begin to condense and the gas film becomes thin, thus the coefficient of condensation heat transfer will increase. The coefficient of condensation heat transfer outside horizontal enhanced tubes of R407C is 70% lower than R404A, which is due to the fact that the temperature glide of R407C is larger than R404A, and the impact of gas film outside tubes is greater. The heat transfer coefficient of R407C is improved obviously at higer heat flux condition, which is suitable for practical application.
摘要:To analyze effects of dry-bulb and wet-bulb temperatures (D&WBTs) on unit performance, experimental research on cooling capacity and EER of ducted heat pump are studied by means of air-enthalpy method when D&WBTs change in ±1.0 ℃/±0.5 ℃ of variations set in national standard GB/T 17758—2010. Results show that the ducted heat pump is in best working condition when indoor and outside DBT reach lower limit of variation ﹣1 ℃ and WBT reach upper limit of variation +0.5 ℃. EER of unit is 2.8, which is 3.26% higher than the value measured in standard condition. The indoor WBT was found out to be the most significant factor with influence coefficient of 0.0464, followed by the outdoor DBT with influence coefficient of ﹣0.0340. If DBT ±0.3 ℃ and WBT ±0.1 ℃ is provided to judge the energy efficiency class of air conditioner unit, the measurement error of EER drops from ±3.5% to ±1%.
关键词:refrigeration performance test;energy efficiency ratio;test conditions;dry and wet-bulb temperature;variations
摘要:A novel frost-free air-source heat pump water heater applying dehumidification device and thermal storage on the basis of traditional system is studied in this paper. The operation mode of this system includes heating mode and regeneration mode. An experimental apparatus is built up to study this system, and according to the experiment, the frost-free operating time of this system is 32 min, 34 min and 36min respectively when the temperature of air is ﹣3 ℃, 0 ℃, 3 ℃ and the relative humidity is 85%. And the temperature is high enough for the regeneration of desiccant, which could keep the heat pump heating water continuously. Besides, the COP of this system at temperature of ﹣3 ℃ and 3 ℃ is 17.9% and 3.4% higher than the reverse cycle defrosting system in the same conditions, which proves that this new system has better performance under low environment temperature.
摘要:In order to improve the refrigerating capacity and coefficient of performance (COP) of system in summer and reduce the damping of heating capacity in winter, a novel variable refrigerant flow (VRF) system/unit with both cold and heat storage was developed in this work, and the performance of the system was experimentally investigated under the various operating modes, i.e. ice-storage, cold-release, heat-storage, heat-release. The experimental results showed that when the system worked continuously about 8 hours under the ice-storage mode (during off-peak period at night in summer), the cooling capacity stored in the ice layer was up to around 380 MJ, which could be used to increase the subcooling degree of the system running in the day, and in case of about 35 mm of the ice thickness outside the coil, the system could work incessantly about 8 hours under the cold-release mode (during peak period of electricity). Compared with the conventional air conditioning system, the cooling capacity of the system was increased by 29% and the COP was increased to 136.4%. In winter, hot water was stored in the energy-storage tank during off-peak period at night; under the heat-release operating mode in the daytime, the problems such as heating capacity deceleration and frosting of the unit were relieved by increasing the compressor suction and evaporating temperatures of the system.
关键词:air handling unit;degree of subcooling;VRF;energy-storage
摘要:The microchannel heat exchanger was used as the evaporator of a separated type heat pipe, and the working fluid filling rate of the separate type heat pipe was studied experimentally when the filling rate changed from 80% to 150%. The heat transfer, the wall temperature of microchannel evaporator and the EER of the system were measured. The heat transfer characteristics under different filling rate were analyzed respectively, and the heat transfer coefficient was calculated. The results show that the heat transfer increased with the temperature difference between the indoor air and the outdoor air. The optimal filling rate was about 120%. Moreover, the microchannel evaporator was compared experimentally with the finned tube evaporator under the condition of same heat transfer. The weight and the refrigerant charge of the microchannel evaporator is 45% and 15.9% lower than that of the finned tube evaporator, respectively. The EER of the system with the microchannel evaporator is 2.8% higher than that of the system with the finned tube evaporator.
关键词:eparated type heat pipe;microchannel heat exchanger;filling rate
摘要:The quality characteristics including liquid loss rate, TPA, chromatic aberration, pH, TVB-N, total bacteria of fresh beef after aging were evaluated during storage at ﹣2.5 ℃, 0 ℃ and 4 ℃, which would provide reference for accurate ice temperature storage of fresh beef. The results showed that the temperature of ice-temperature cold room was stable and accurate, and the storage period of the fresh beef at ﹣2.5 ℃, 0 ℃ and 4 ℃ were 24 days, 15 days and 8 days, respectively. After 24 days storage at ﹣2.5 ℃, pH value, TVB-N value and aerobic plate count of the beef were 5.59, 15.39 mg/100 g and 2.2×106 CFU/g, respectively. The color of beef was still bright red without peculiar smell. Compared to 4 ℃ and 0 ℃,the ice temperature (﹣2.5 ℃) could maintain the color of beef effectively, decrease liquid loss rate, retard the increase of pH and TVB-N, and inhibit the growth of microorganisms. In summary, fresh beef could maintain the good quality during 24 days storage at ice temperature (﹣2.5 ℃) .
摘要:Due to the poor heat stability, insulin is easy to lose its bioactivity during storage. Bioactive protection of LEA-motif (LEAM) and trehalose towards insulin was investigated respectively and synergistically. High performance liquid chromatography method was used to study the proficiency of insulin and differential scanning calorimetry was used to study glass transition temperature. According to the results, both LEAM and trehalose can protect the bioactivity of insulin. The protective effect of LEAM is better than that of trehalose. That is to say that the bioactivity proficiency and heat stability of insulin within LEAM are better than those of insulin within trehalose. It is interesting that adding a bit trehalose can enhance the bioactive protection effect of LEAM towards insulin. During lyophilization, synergistic bioactive protection of LEAM and trehalose can not only enhance the stability of insulin, but also protect insulin from thermal degeneration. So such synergistic bioprotectants can be used to maintain the bioactivity of heat sensitive protein during lyophilization.