查看更多>>摘要：Vacuum freezing is an innovative rapid freezing technique with various advantages and huge potential for diverse applications. This study aimed at investigating variations in the moisture, main quality, microstructure, and mechanical properties of vacuum-frozen and thawed apple slices resulting from the use of different amounts of external water added prior to freezing. Increased external water content (30%) contributed to lower frozen temperature and less water loss in vacuum-frozen apple samples, and higher drip loss, equivalent vitamin C content, lower relative electrical conductivity, lower soluble solids content and lower total color difference in thawed samples. Slices of thawed samples with more added external water before vacuum freezing displayed higher density of moisture distribution and reduced microstructure destruction. They also displayed higher elastomeric and less viscous behaviors, as assessed with stress-relaxation analysis, and increased hardness and chewiness and reduced cohesiveness, as assessed with texture profile analysis. The quality of thawed apple slices with 30% pre-added external water was closest to that of the fresh apples. The traditional contact plate freezing treatment produced the worst quality. Thus, vacuum freezing with 30% pre-added external water is recommended for the rapid freezing processing of apple slices.
查看更多>>摘要：The suction valve is an essential one-way valve inside the reciprocating compressor controlling the suction gas flow. It is usually installed on the cylinder head of the compressor. However, the suction valve in oil-free mini compressors was preferred to be installed on the piston to cool down the bearing inside the piston by the suction flow. In this case, the motion of the piston will act varying accelerated speed on the valve plate. During its opening process, the valve plate would be pushed by resultant force of the inertia force generated by the accelerated speed and the flow thrust together. It would result in too high valve lift and deformation, and decrease the reliability of the valve plate extremely. For this reason, the stiffness of the valve on the piston should be greater than that on the cylinder head. However, stiffness strengthened valve plate moving with the piston would behaves oscillation motion during its opening and closing process. In this paper, a three-dimensional fluid-structure interaction (FSI) model validated by experiments was established using ADINA to study the oscillation motion of the valve installed on the piston. Obtained results showed that the lift integral of the valve on the piston was 4.5% less than that of the valve on the cylinder head, and the impact speed on the valve seat of the valve on the piston was smaller than that of the valve on the cylinder head, which indicated that the suction valve installed on the piston had higher reliability.
查看更多>>摘要：This work analyses the influence of the Internal Heat Exchan ger (IHX) on the energy performance of a vapor compression cycle associated with a freezing cabinet using different R404A low-GWP alternatives. Among them, the refrigerants R454C (GPW100=146), R455A (GPW100=146), R468A (GPW100=146), R290 (GPW100=5) and R1270 (GPW100=1.8) have been tested in a horizontal freezing cabinet maintaining a product target temperature of -20°C at the heat rejection temperatures of 20, 30 and 40°C. The results from tests show that the use of the IHX reduces energy consumption in all scenarios without significantly increasing the compressor's discharge temperature. For a test period of 16 h, the refrigerants that offer better reductions are R404A and R1270, followed by R455A, R290, R454C and R468A. The maximum energy saving is rated to 9.2% at the heat rejection temperature of 40°C, demonstrating that the enhancement of the IHX is better as higher the heat rejection conditions are.
查看更多>>摘要：The two-phase flow behaviors in T-junction are quite complex in energy transport systems. In this paper, the two-phase flow pressure drop of refrigerants in a horizontal branching T-junction was analyzed numerically and predicted using artificial neural network. Firstly, the distribution of static and total pressure was obtained based on Eulerian method, and the parametric studies on the local pressure drop were conducted. It is observed that the vortexes in the entrance of branch pipe lead to the pressure fluctuation and irreversible pressure losses, and the “descend-ascend” of static and total pressure happens under high mass flow split ratio in run pipe. Then, the ANN predicting model of local pressure drop coefficients was established. It shows that GA-BPNN and PSO-BPNN has the best predicting ability for K12J and K13J respectively, and the relative errors are within 10% for most cases. Finally, the sensitivity analysis was conducted, indicating that the effect of mass flow split ratio (F) and inlet quality (x1) is the most significant for K12J and K13J respectively.
查看更多>>摘要：Flow boiling heat transfer coefficient of R410A in horizontal annular channels were evaluated with a 279 K saturation temperature over a mass flux range of 50–80 kg/(m2s) and a vapor quality range of 0.2–0.8. All the four tested annulus have a 19 mm outer diameter of the flow channel containing three types of enhanced tubes (a micro-porous tube, a ring-shaped finned tube, and an independent small-bosses surface tube) and a smooth tube. The micro-porous tube benefitted by its surface shows the highest heat transfer coefficient (the enhancement factor exceeds 2.52) in flow boiling. The smooth tube shows a higher heat transfer coefficient than the independent small bosses tube due to its special surface structure. For changing average vapor quality of outlet and inlet, the heat transfer coefficients of the micro-porous tube and smooth tube increase with the increase of the average vapor quality. Considering the heat transfer coefficient and the data of pressure drop in all of these annular channels reveals that the micro-porous tube can be a great choice for flow boiling on the annular side for the best performance. The results from correlation analysis of smooth tube predict experimental data with a ± 10% error band. The improved correlations for enhanced tubes were presented, and the experimental values are predicted within a ± 2% error band.
查看更多>>摘要：As a natural substance, HCs (hydrocarbons) refrigerant have no destructive effect on ozone and has little impact on greenhouse effect. It is an ideal new refrigerant. R290 and R600a have been used to a certain extent, but hydrocarbon refrigerant has a high risk of flammability, which hinders its engineering application. This paper focuses on using hydrocarbon refrigerants as alternative refrigerants, taking R290 and R600a as the main research objects, adding refrigerant R134a to hydrocarbon refrigerants to study the flammability change of mixtures. The flammability limits of R290/R134a, R600a/R134a and R600a/R290 mixture refrigerants were measured under different components. In comparison, R134a had more obvious flammability inhibition effect on R290 than R600a. The influencing factors of the flammability limits of HCs and the estimation method of the flammability limits of mixtures with the average MAPEs ranged from 0.62% to 2.94% was discussed. In addition, the refrigeration cycle of HCs/R134a mixtures with various concentrations was simulated. The COP, volume refrigeration capacity per unit and condensation pressure of the mixtures had local maximum near the azeotropic point, respectively.
查看更多>>摘要：To extend the working time of battery of the electric refrigerated truck, the optimization method of the refrigeration system of a certain electric refrigerated truck is researched in this paper. The compartment of the electric refrigerated truck and the working conditions of refrigeration is simulated by TRNSYS in a whole day. The genetic algorithm (GA) is used to optimize the operating parameters of the refrigeration system. Firstly, the appropriate optimization variables were selected. Secondly, the objective function was established in order to minimize energy consumption, then the constraints were set according to the actual working conditions. Finally, the parameters such as refrigerant flow rate, refrigerant transfer pump flow rate, and air supply flow rate are computed by genetic algorithm. The results show that with the reasonable parameters, the genetic algorithm can be effectively applied to the optimization of refrigerating truck refrigeration system, which can reduce the energy consumption of refrigerating truck. After optimization, the total energy consumption of refrigerating system can be reduced by average 0.5 kW, and the coefficient of performance (COP) can be increased by 0.15 equally. In addition, the endurance of the electric refrigerated truck will be greatly improved, and the cost of electricity consumption will be reduced.
查看更多>>摘要：Pressure drop during flow boiling of R410A was experimentally investigated at a saturation temperature of 279 K over a mass flux range of 50–80 kg/(m2s) and a vapor quality range of 0.2–0.8. The inner tubes with the same outside diameter of 19 mm contain a smooth tube and three enhanced tubes: a micro-porous (No. 1) tube, a ring-shaped finned (No. 2) tube, and an independent small-bosses surface (No. 3) tube. It was observed that the pressure drop in single-phase and flow boiling conditions increased with the increase of the mass flux, and the ring-shaped finned tube showed the highest pressure drop (more than 1.35 times and 1.31 times that of the smooth tube, respectively) due to the effect of the ring-shaped fins. The pressure drop in the micro-porous tube and the smooth tube increased with the increase of outlet vapor quality in the range of the mass fluxes tested. Results from correlation analysis showed that two existing correlations can be implemented to predict the frictional pressure drop in a smooth tube within a deviation of ± 10%. Improved correlations to predict pressure drop in the enhanced tubes were presented, and 94% of the data points of the micro-porous tube were predicted within a ± 10% error band.
查看更多>>摘要：Stimulated by the fast development of cold chain commodity market and a large amount of demand in cold storages, the application of CO2 refrigeration system is to enhance remarkably. Owing to the low critical temperature of CO2, the dedicated subcooling technology is widely-used in the CO2 transcritical refrigeration cycle to increase the system performance. In this regard, reducing the compressor power of subcooling subcycle and extending the working range of heat source temperature is highly important for the mechanical subcooling technology and absorption one, respectively. Motivated by such purpose, the CO2 transcritical cycle with dedicated subcooling and cascade recooling is presented and the thorough investigation is performed thermodynamically. The proposed system is compared with layouts with dedicated mechanical subcooling and dedicated absorption one at first. Subsequently, the effect of recooling capacity ratio, the critical parameter for the system performance, is analyzed in-depth. Finally, the optimum recooling capacity ratio for different conditions is fitted by the neural network approach. Compared to layouts with dedicated mechanical subcooling and dedicated absorption one, the secondary compressor work goes down by 28.5% and the lower limit of heat source temperature is extended by 4 °C, respectively in the proposed system. The paper is favorable for the development and performance improvement of CO2 transcritical refrigeration systems.
NSTL
Elsevier