Study On The Transcritical CO₂ Two-Phase Ejector And Double Throttle Refrigeration System

The conventional expansion valve used in transcritical CO₂ refrigeration systems can cause great throttle loss. The ejector used in transcritical CO₂ refrigeration systems can recover a part of expansion work. Therefore the ejector has got more and more attention due to its simple structure and low cost.Based on the previous work, the new ejector with the dual-throat nozzle was designed and the experimental setup of transcritical CO₂ refrigerating system was optimized and improved in this paper. The numerical simulation and experimental research on the performance of the ejector with the dual-throat nozzle and the transcritical CO₂ two-phase ejector refrigeration system with double expansion valve were carried out. The performance of the ejector and the transcritical CO₂ two-phase ejector refrigeration system with double expansion valve under different divergence angles of first nozzle, different equivalent diameters of first throat and different working conditions were analyzed. The conclusions as follow:（1） Both of the numerical simulation and experimental results indicate that under the fixed working condition, the entrainment ratio of the ejector increases and then decreases with the increases of the divergence angles of first nozzle, the entrainment ratio reached the maximum value as the divergence angle of first nozzle is about 5°.The numerical simulation results indicate that the entrainment ratio decreases with the increases of the equivalent diameter of first throat. The experimental results indicate that entrainment ratio increases and then decreases with the increases of the equivalent diameter of first throat, and the entrainment ratio reached the maximum value as the equivalent diameter of first throat is 2.0mm.（2） The numerical simulation results indicate that the entrainment ratio gradually increases with the increase of the outlet pressure of gas cooler, increases and then decreases with the increase of the outlet temperature of gas cooler under the conditions of 5°divergence angle of first nozzle and 1.2mm equivalent diameter of first throat. The entrainment ratio reached the maximum value as the outlet pressure and temperature of gas cooler are 9.2MPa and 42℃,respectively.（3） The experimental results indicate that under the fixed geometric parameters of the ejector with the dual-throat nozzle, the entrainment ratio increase with increases of the outlet pressure and temperature of gas cooler, increases and then decreases with the increases of the evaporating temperature, the entrainment ratio reached the maximum value as the outlet pressure and temperature of gas cooler and evaporating temperature are 9.2MPa,44℃and-3℃,respectively. Under the fixed working condition, the COP of the ejector refrigeration system increases and then decreases with the increases of the equivalent diameter of first throat, decreases and then increases with the increases of the divergence angles of first nozzle. The COP reached the maximum value as the the equivalent diameter of first throat and divergence angles of first nozzle are 2.0mm and 11°, respectively.（4） The COP of the transcritical CO₂ two-phase ejector refrigeration system with double expansion valve and traditional transcritical CO₂ refrigeration system with the expansion valve were compared. Under the fixed geometric parameters of the ejector with the dualthroat nozzle, the COP of the ejector and traditional refrigeration systems decreases with increases of the outlet pressure and temperature of gas cooler, increases with increases of the evaporating temperature. Under the fixed working condition, the COP of ejector refrigeration system is always higher than that of the traditional refrigeration system.