Numerical Simulation And Experimental Study On The Performance Of The Transcritical CO₂Two-Phase Ejector Refrigeration System

The pressure differential and throttling loss is large in the conventional CO₂refrigeration cycle （CCRC）, because of the refrigerating fluid before and after theexpansion valve in supercritical and subcritical state respectively based on the low CO₂critical point. Therefore, it is very promising to improve the performance of the system byrecovering the expansion work of the CCRC. Due to its advantages of simple construction,no moving part and high reliability, the ejector is used to instead of the traditionalexpansion device to recover some potential pressure energy during the expansion process inthe CCRC. In this paper, the numerical simulation and experimental studies on thetranscritical CO₂two-phase ejector refrigeration cycle （TPERC） with internal heatexchanger （IHX） were carried out, which is of great significance to improve theperformance of the transcritical CO₂refrigeration cycle in theory and practice.The experiment apparatus were rebuilded based on the previous studies and thecontinuous adjustable ejector was designed and manufactured. The mathematical models ofthe ejector, the shock wave in the ejector and the TPERC were built based on the rebuildedTPERC experimental system. The numerical simulation on the TPERC with and withoutinternal heat exchanger （IHX） was carried out. In order to verify the reasonableness andcorrectness of the numerical simulation result, the experimental studies on the TPERC withand without IHX were carried out under varying working conditions include evaporatingtemperature, outlet temperature of the gas cooler and the high-side pressure. Moreover, thecomparison between the experimental and the simulation results was carried out. At thesame time, the comparison between the experimental results of the TPERC and the CCRCunder the same working conditions was carried out. The effects of the IHX, evaporatingtemperature, cooling temperature and high-side pressure on the performance of the systemwere analyzed.The numerical simulation results show that:（1） The system performance is improvedwith the decrease in the gas cooler outlet temperature and the increase in the evaporatingtemperature. Under the simulated conditions, the COP of the system with increases by23.3%²5.43%compared with the system without IHX, moreover, the lower the outlettemperature of the gas cooler, the more obvious the improvement of the systemperformance becomes.（2）The simulation results of the shock wave in the ejector indicatethat the normal shock wave occurs in the divergent part of the Laval nozzle. The sectionwhere the shock wave occurred moves to the throat of the nozzle gradually with the raise ofthe evaporating pressure, while it goes far away from the throat of the nozzle graduallywith the increase in the total pressure of the ejector inlet or the area ratioAn out/At.（3）The performance of the TPERC system is higher than that of the conventional system under thesame working conditions. Moreover, the improvement of the system performanceincreases with the raise of the evaporating temperature, the COP of the TPERC increasedby19.47%-60.9%compared with that of the CCRC under the simulation conditions of theevaporation temperature ranges from-3℃to5℃。The experimental results show that:（1） The COP of the TPERC with and without IHXreduces with the increase in the gas cooler outlet temperature, while increase with the raiseof the evaporation temperature. The COP of the TPERC with IHX is higher than that of theTPERC without IHX under the same working conditions, and the improvement decreasewith the increase in the evaporating temperature. The use of the IHX can make the COPincrease by11.41%³6.11%under the working conditions of the fixed gas cooler outlettemperature40℃and the evaporation temperature ranges from-3℃to5℃.（2）The COPof the TPERC firstly increases then decreases with the increase in the high-side pressureunder the fixed evaporating temperature and outlet temperature of the gas cooler.Moreover, the improvement of the IHX on the system performance decreases with theincrease in the high-side pressure gradually. While the high-side pressure increases to somevalues, the use of IHX reduces the performance of the system instead. This suggests thatonly in reasonable working conditions can the IHX improve the system performance. Theeffect of IHX on the COP of the TPERC is more remarkable in lower high-side pressurePg c.（3）The comparison between the COP of the TPERC and the CCRC indicate that the COP ofthe TPERC increases by2.61%³5%, which suggests that the use of ejector has a positiveeffect on the system performance.The comparison between the experimental and simulation results indicate that: All ofthe simulation results are higher than the experimental results under the fixed workingconditions. It can be argued that because of the ideal assumptions and the neglection of thefactors, such as the pipe resistance and the pressure drop in the system, the simulationresults of the COP and the cooling capacity are higher than the experimental results. Thevariation trends of the two results are in good agreement with eachother.