Study On Performance Analysis Of A Two-stage Ejector Refrigeration System With Two Evaporators And Zeotropic Mixtures

The ejector refrigeration system can make full use of the low-grade heat source to obtain the cooling capacity,which can not only improve the efficiency of energy recovery and utilization,but also effectively alleviate the environmental pollution caused by the waste gas being directly discharged into the atmosphere.In order to improve the comprehensive cooling efficiency of ejector refrigeration,the performance analysis of a two-stage ejector refrigeration system with two evaporators?TSERS?is done by energy,conventional exergy and advanced exergy analyses in this thesis.The effect of evaporator load distribution,operating temperatures and working fluid on the performance of TSERS is theoretically analyzed.In addition,the performance of subcritical zeotropic mixtures and CO₂ mixture for single-stage ejector refrigeration system and TSERS were studied.Under the same refrigeration output,compared with Single-stage ejector refrigeration system with two evaporators?SERS?and Two-ejector refrigeration system with two evaporators?TERS?,the COP of TSERS increased by 42.1%and19.7%,the system exergy efficiency increased by 72.8%and 24.6%,and the system exergy loss decreased by 44.0%and 20.8%,respectively.The three components with the greatest exergy loss in TSERS are ejector 2,generator and condenser.Approximately 44.8%of the total system exergy loss is avoidable.Among the three main exergy loss components,the ejector 2 has the largest avoidable exergy loss,and it also has the largest avoidable endogenous exergy loss.So,optimizing ejector is of a great help to improve system performance.The order of optimization of the three main exergy loss components of the TSERS should first be ejector 2,followed by the condenser,and finally the generator.When the ratio Q_e1/Q_e grows from 0.2 to 0.8,the COP reduction rate is 25%,but the exergy efficiency changes little,and exergy loss increased by 34.92%.The optimal compression ratio of the TSERS is around 1.3.The temperature of condenser has the greatest influence on cooling COP.When R152a is used as refrigerant,the system performance is slightly better than other working fluids.For the study of zeotropic mixtures in subcritical single-stage ejector refrigeration system,when three different mixture refrigerants?R601/R245fa,R245ca/R236ea and R245fa/R236ea?are used as the working fluid,with increasing of the mass fraction of the higher critical temperature refrigerant,the entrainment ratio of ejector,system COP,and system exergy efficiency are increased first and then gradually decreased,and the optimum ratio is 0.6/0.4.Under the same conditions,the mixture refrigerants R601/R245fa showed better performance than the other two mixtures.When the mass fraction of R601 was 0.6,the entrainment ratio of ejector,system COP and exergy efficiency increased 1.59 times,1.56 times and 1.62 times compared with pure working fluid,respectively.When the mixture refrigerant R601/R245fa is used as the working fluid of the single-stage system,the generator temperature rises by 15?,the entrainment ratio of ejector,system COP and exergy efficiency increase by 25.93%,20%and 10.57%,respectively.The performance of mixture refrigerant is always better than that of the pure working fluid.The analysis of subcritical TSERS with mixture showed that when the mass fraction of R601 is about 0.6,the performance of the system is the best,and the system COP is about twice higher than that of pure working fluid.The entrainment ratio of ejector 1 and ejector 2,and system COP increased by 10.74%,17.26%and 16.35%,respectively when the generator temperature increased by 10?.When the condenser temperature increased to 10?,the entrainment ratio of ejector 2 and system COP decreased by68.19%and 62.20%.With the increase of the temperature of the low-temperature evaporator and the high-temperature evaporator,the COP of the system increases.The mixture refrigerant CO₂/R290 is selected as the working fluid of the single-stage ejector refrigeration system.When the mass fraction of CO₂ is 60%,the performance is the best.With the increase of heater pressure,the entrainment ratio and COP both increased gradually.With the increase of the outlet temperature of the heater,the entrainment ratio increased,but the COP of the system decreased.With the decrease of condenser temperature,both the cooling COP and the entrainment ratio are increased,and the effects of evaporator temperature are opposite.The performance of CO₂ mixture is much higher than that of transcritical CO₂ pure working fluid system.Under the optimal ratio,the entrainment ratio and system COP are 2.36 times and 4.45 times higher than pure working fluid.The pressure of the condenser can be reduced to about 3.86 MPa.For the performance analysis of CO₂mixture used in TSERS,when the mass fraction of CO₂ is 0.6,the performance parameters of the system increase with the increase of heater pressure.Similarly,the increase of evaporator temperature is beneficial to the optimization of performance.