Theoretical Study On A Vapor-liquid Ejector Assisted Organic Rankine Cycle

Organic Rankine cycle(ORC)is a promising heat-to-power technology,and the small-scale becomes its trend in development.But the working fluid pump consumes a large amount of power,resulting low system efficiency.This has to be solved for the development of small-scale ORC.A vapor-liquid ejector is a flow device that allows a high pressure vapor to entrain a low pressure liquid to a pressure that is higher than the vapor pressure without consuming any external mechanical energy.Its application to ORC system can provide a solution for the problems of large energy consumption of working fluid pump and low system efficiency encountered by the small-scale ORC system.This thesis focuses on the working characteristics of the vapor-liquid ejector and the system performance of the vapor-liquid ejector assisted ORC.The study was carried out from the four perspectives as listed beblow:Firstly,a novel ORC with a vapor-liquid ejector(EORC)was proposed,in which a vapor-liquid ejector and a second-stage evaporator were introduced to conventional ORC.The EORC was compared to the conventional ORC and a regenerative ejector ORC(RORC).The results showed that the performance of EORC was always better than ORC,and it was also superior to RORC under the lower working fluid pump isentropic efficiency or higher evaporator temperature.Secondly,two mathematical models were proposed for the vapor-liquid ejector: one was for designing when the inlet parameters were given,and another was to evaluate the ejector performance under variable working conditions.Verification to the experimental results indicated that the maximum error was 8.9%.Thirdly,different organic working fluids were compared in the vapor-liquid ejector,and the effects of important parameters were studied.It was found that at the considered working condition R365 mfc had the largest entrainment ratio of 6.5 and highest pressure lifting,followed by R1336mzz(Z)、R245ca、R1233zd(E)、R245faand R236 ea.Moreover,an increasing in the subcooling of the secondary liquid flow and a decrease in the entrainment ratio,the area ratio and the pressures at the inlets led to the increasing of the pressure lifting,while the inlet superheating had insignificant effect.The pressures and temperatures inside the vapor-liquid ejector were more remarkably impacted by the secondary liquid flow conditions.Finally,the thermodynamic model of EORC was built,and the system performance was investigated.The results showed that the temperature of evaporator I,the pinch temperature difference and energy consumption of the working fluid pump were decided by the vapor-liquid ejector,while the ejector performance was closely related to its entrainment ratio,area ratio,the pinch temperature difference in the condenser and pressure lifting.Furthermore,the highest exergy destruction occurred in the expander,followed by the ejector,evaporator I,evaporator II,pump and condenser.An increase in the temperature of the evaporator I caused the increasing of system exergy destruction,whereas the condenser temperature had the opposite influence.The investigations of working characteristics of the vapor-liquid ejector and EORC benefit the extension of vapor-liquid ejector applications and lay a foundation for the research of vapor-liquid ejector combined with ORC.It is of great significance for the development of small-scale ORC...