| Internal combustion engine is still the primary power source of automobiles at present.However,more than one third of the fuel energy is dissipated in the form of the waste heat in exhaust gas.This huge amount of energy has large potentials to be recovered.Rankine cycle is a promising technology for the waste heat recovery,of which the working fluids become very important.As with zeotropic mixture,the temperature glide property can reduce irreversible loss in heat transfer process.The zeotropic mixture also has many other advantages so that it is widely used in waste heat recovery.Composition shift of the zeotropic mixture accompanied by phase change occurs in thermodynamic system,which affects system performance for the ratio difference between circulating composition and charge composition.This paper chooses two kinds of zeotropic mixture,light alkane/CO2 and heavy alkane/R123,based on the characteristic of the engine exhaust heat.The zeotropic mixtures can not only show the excellent thermodynamic properties of pure alkanes,but also reduce the flammability zone of alkane with the addition of retardants.The thermodynamic model of waste heat recovery system is built based on first law and second law of thermodynamics.The performance of transcritical Rankine cycle of light alkane/CO2 and subcritical Rankine cycle of heavy alkane/ R123 are analyzed.Rankine cycle configurations are divided into two kinds(C1 and C2)on the basis of phase change.The advantages and disadvantages of C1 and C2 are compared.The composition shift model of zeotropic mixture is developed based on void fraction model.Composition shift phenomenon that occur in evaporation process for heavy alkane/R123 and condensation process for light alkane/CO2 are discussed,as well as the connection between composition shift and temperature glide.Researches show that thermal performance of Rankine cycle can be improved by adding light alkane to pure CO2 system.The maximum thermal efficiency and net power output of thermodynamic system used CO2/n-Pentane are 16.4% and 16.06 kW,respectively.In comparison with those of pure CO2,the improvement of 34.4% and 13.3% can be obtained.Meanwhile,the operation pressure of the system can be effectively reduced.The operation pressure of CO2/Propane decreases 20.5% than that of pure CO2 at the same thermal efficiency of 12%.However,the advantage of heavy alkane/R123 is not significant compared with pure R123 in subcritical cycles,and only R123/Cyclohexane shows better performance.Since C2 cycle can effectively utilize the heat released in condensation process,it can perform better results than C1 cycle,but the operation pressure of C2 cycle is higher than that of C1 cycle,so C1 cycle can provide better safety options.As with composition shift,the circulating composition of low boiling component would rise,while the circulating composition of high boiling component would decrease.In the transcritical cycle using CO2/Propane as working fluid,the circulating composition translated into 66.52%/33.48%,when the charge composition was 65%/35%.The thermodynamic performance of the system would be poorer due to the increase of the circulation concentration of the low boiling point retardants.The corresponding relationship between the composition shift and temperature glide is clear shown in this study.A larger temperature glide is always accompanied with a larger composition shift.Therefore,the composition shift could be predicted by temperature glide. |
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