Simulation And Experimental Study On Performance Of Two-stage Throttling Transcritical CO2 Ejector Refrigeration System

As a natural refrigerant the ODP and GWP of CO2 are 0 and 1,respectively,that conforms to our requirements for environmental protection.However,the traditional transcritical carbon dioxide refrigeration system exists a large throttling losses resulting from its high pressure.Therefore,it is of great significance to improve the system COP.In this paper,two kinds of throttling devices,needle valve and two-phase flow ejector,are used as the two stage throttling devices.The first throttling device is used to control the high pressure of the system,and the second throttling device to control the evaporation temperature and to recover part of the expansion work.In this paper numerical simulation and experimental research of the effects of the geometric parameters of the second throttling device,two-phase flow ejector,on the performance of the system were carry out.The system performance under different operating conditions was tested and the affecting factors of the system performance was analyzed.The experiment compared system performance of the second throttling device using ejector and conventional throttling device.This paper uses ANSYS CFX to perform numerical simulations of internal flow of ejectors of different sizes.The velocity,temperature,pressure in the ejector and entrainment ratio of the ejector were simulated under different inlet pressure conditions of the ejector.The simulation results show that the velocity of carbon dioxide in the ejector first increases and then decreases.The pressure and temperature rapidly declines at the throat of the second nozzle.The velocity,pressure,temperature in the ejector and entrainment ratio of the ejector with nozzle distances of 0mm,9mm,and 15 mm were simulated.The simulation results show that the ejector entrainment ratio is the largest when the nozzle distance is15 mm.Simultaneously,for the ejector with the length of 92 mm and 124 mm of the mixing chamber,the simulation results show that as the length of the mixing chamber is 124 mm,the performance of the ejector is higher than that of the ejector performance with the length of the mixing chamber of 92 mm.The experimental data show that under the fixed evaporation pressure and the outlet temperature of the gas cooler,the compressor power consumption increases with the increase of the exhaust pressure,and the cooling capacity and system COP increase first and then decrease with the increase of the exhaust pressure.The system performance reaches its maximum value at the exhaust pressure of 9MPa.For different working conditions,the system COP increases first and then decreases with increasing nozzle distance.At the nozzle distance is 9mm,the maximum COP is achieved.The experimental data of the ejector with the mixing chamber lengths of 92 mm and 124 mm,respectively,show that when the length of the mixing chamber is 124 mm,the system COP is higher than that of the mixing chamber length of 92 mm.These conclusions are consistent with the numerical simulation results,but the entrainment ratio of ejector is higher than the experimental value.The experimental results for comparison between ejector throttling and traditional throttling systems show that under the same experimental conditions,the ejector throttling device reduces the carbon dioxide throttling loss and improves the performance of system.At the optimal exhaust pressure of the system,the cooling pressure of the gas cooler of 9MPa,the COP of both systems reaches a maximum value.and the COP of the ejector refrigeration system is 37.8% higher than that of the conventional refrigeration system.