Simulation And Experimental Study On Performance Of An Improved Transcritical CO₂ Ejector Refrigeration System

For the transcritical CO₂ refrigeration systems,using ejector as a throttling element can recover the expansion work and greatly improve the performance of the system.In the system,the refrigerant is throttled by the first throttle valve into a two-phase flow state,and the large velocity difference between vapor and liquid will affect the performance of the ejector and system.The experimental setup of CO₂ ejector refrigeration system with double expansion value is optimized and improved in this paper.A gas-liquid separator is added after the first throttle valve.The upper gas working fluids is used as the primary flow of ejector.The lower liquid working fluids enters the second throttle valve and then flows into the evaporator for heat transfer.This can improve the working fluid state of the ejector inlet and reduce the vapor volume into the evaporator.In this paper,the numerical simulation and experimental research on the performance of the improved CO₂ two-phase ejector refrigeration system with double expansion valve are carried out under different operating conditions and different geometry of the ejector nozzles.The entrainment ratio of the ejector and the system COP of the ejector refrigeration system and the improved ejector refrigeration system were compared.The follow conclusions are obtained.?1?The experimental results show that under the experimental operating conditions,the gas-liquid separator in front of the ejector can separate the gas-liquid mixture flow in the system,and the primary flow of the ejector is approach to the single-phase flow,and thus improves the entrainment ratio and the system COP.Under the different evaporation temperature,the entrainment ratio of the ejector is increased by 22.4%⁶6.4%,and the system COP is increased by 2%²6.9%in the evaporation temperature range of-2?³?.Under the different outlet temperature of the gas cooler,the system COP is increased by 1%¹9%.Under the different equivalent diameter of the first throat,the system COP is increased by 22.8%³6.2%.?2?The experimental results of the improved ejector refrigeration system indicate that for the fixed geometric parameters of the ejector,under the conditions of the outlet temperature of gas cooler 37?and the evaporation temperature 3?,with the increase of the outlet pressure of the gas cooler,the system COP reaches the maximum value at the outlet pressure of gas cooler 8.752MPa,and the entrainment ratio of ejector reaches the maximum value when the outlet pressure of gas cooler is 9.1MPa.Under the fixed operating conditions,the entrainment ratio of the ejector increases first and then decreases slightly with the increase of the equivalent diameter of the first throat,and reaches its maximum value when the equivalent diameter of first throat is 2.0mm,while the system COP is on the rise.?3?The numerical simulation results indicate that the gas-liquid separator in front of the ejector can make the main fluid closes to the single-phase flow.The refrigerant achieves supersonic in the nozzle expansion section of the ejector.The results show that under the fixed geometric parameters of the ejector,the maximum velocity of the working fluids in the ejector and the entrainment ratio increases gradually with the increase of the outlet pressure of the gas cooler.And the entrainment ratio increases gradually with the increase of the outlet temperature of the gas cooler.The entrainment ratio increases and then decreases with the increase of the equivalent diameter of first throat.The comparison between the numerical simulation and experimental results shows that the simulated entrainment ratio is higher than the experimental entrainment ratio,which may be due to the deviation between the assumed boundary conditions in the simulation process and the experiment.