Research On Thermal Power Conversion Characteristics Of Ejector Large-Temperature-Drop Cogeneration System

In response to global warming caused by greenhouse gas emissions,China has proposed"carbon peak"and"carbon neutral"strategic goals,which put forward higher requirements for system development and technological innovation in the field of energy applications.The large-temperature-drop cogeneration system has the characteristics of two thermal mediums,various forms of available heat sources,simultaneous output of thermoelectricity,and an adjustable output ratio.It can be widely used in industrial waste heat recovery and clean energy utilization.Focusing on the energy conversion mechanism and high-efficiency construction of the ejector large-temperature-drop cogeneration system,a thermodynamic model of the two thermal mediums thermal-power conversion system is constructed in this thesis.The thermal-power conversion characteristics and the influence of irreversible factors are analyzed.The thesis also researches the structural design and simulation of the high-performance ejector,the construction and experimental verification of ejector large-temperature-drop cogeneration system.Aiming at the complex thermodynamic cycle with the characteristics of two thermal mediums,a thermodynamic model of the two thermal mediums thermal-power conversion system is constructed in this thesis.It is classified qualitatively and quantitatively according to the power balance relationship between the heat engine cycle and the heat pump cycle.Mathematical models of continuous downstream and continuous countercurrent ideal thermal-power conversion systems are established.The results of thermal-power conversion characteristics study show that the temperature crossover between the exothermic medium and the endothermic medium is more obvious in the ideal continuous downstream thermal-power conversion system,and a larger dimensionless process power can be obtained.The equivalent temperature rise of the endothermic medium can be used to judge the type of ideal thermal-power conversion system,and its maximum value represents the output power limit.The influence of irreversible factors in semi ideal model is analyzed.There is an optimal high heat source temperature in the heat engine cycle to optimize the thermal performance of the system.And the low irreversible coefficient will lead to the failure of the thermodynamic process of the semi ideal model.Taking the ideal two thermal mediums thermal-power conversion system as the evaluation benchmark,the evaluation indexes such as heat exchange perfection,equivalent temperature rise ratio of the endothermic medium,and net power output efficiency ratio are put forward,which provides a theoretical basis for the construction and optimization of the actual system.The high-performance ejector is a key component to reduce the irreversible effects,improve the heat pump thermal coefficient,and realize the large-temperature-drop thermal process.In this thesis,the modified constant rate of momentum change method is combined with the CFD simulation method to optimize the design of the ejector structural profile.The shock position and shock intensity in different ejector structures are studied.In addition,the instability characteristics of three types of ejectors are compared and analyzed.The simulation results show that the modified constant rate of momentum change method ejector has the best performance in the double choking zone,while the traditional constant pressure mixing model ejector has a wider range of working conditions.The recommended diffuser outlet angle of the modified constant rate of momentum change method is 5°,the recommended nozzle exit position is 1.25D₄.Under unstable conditions,the shock effect in the ejector suction chamber and the mixing chamber is significantly enhanced,and the shock effect at the critical section is reduced or even disappeared.Considering the irreversible dissipation effects,a dimensionless semi-empirical lumped parameter model of the ejector is derived,which provides a basis for the overall design and operation characteristics of the actual two thermal mediums thermal-power conversion system.Based on the thermodynamic model of the two thermal mediums thermal-power conversion system and the design of the high-performance ejector,this thesis combines the ejector heat pump system with the organic Rankine cyc le system to construct a series type and parallel type ejector large-temperature-drop cogeneration system.The mathematical model of state parameters,evaluation index model,and exergy analysis model are established,and the optimization principle of organic working fluid is formulated.The comparative study of thermal performance of different system structures shows that the parallel type system has higher heat source utilization efficiency and adjustable thermoelectric output ratio,which is suitable for preparing radiant heating water with higher temperature.The parametric analysis results show that there is an optimal generating temperature to maximize the net power output efficiency of the system.Changing the evaporation temperature has a significant impact on the thermal performance of the heat pump subsystem.The parallel type system can adjust the thermoelectric output ratio by changing the working fluid flow ratio.Based on the above research contents,a parallel type large-temperature-drop cogeneration experimental platform with a total heat exchange of 15k W is designed and manufactured.To verify the correctness of the mathematical model and the design method of the main components of the ejector large-temperature-drop cogeneration system,the experimental verification and the research on the operation characteristics are carried out.The experimental results show that the startup and shutdown response of the experimental platform is rapid,and the startup and shutdown time is less than12min.The measured entrainment ratios of ejector 1 and ejector 2 are 0.571 and0.645 respectively,which verifies the correctness of the design method of the high-performance ejector.The relative error of the dimensionless semi-empirical entrainment ratio lumped parameter model is less than 0.68%,which has high reliability.The measured isentropic efficiency of the scroll expander varies from20.95%to 38.54%.In the thermal output mode,ejector 2 can realize large-temperature-drop radiant heating.Compared with ejector 1,the heat supply coefficient is increased by 3.03%,the heat exchange perfection is improved by2.44%,the exergy efficiency is improved by 3.41%,and the evaporation temperature has a wider application range.In the power output mode,the measured maximum output power of the generator is 382.81W,the net power output efficiency is 2.60%,the isentropic efficiency of the expander is 34.11%,the comprehensive power output efficiency of the generator is 57.37%,and the heat exchange perfection is 26.77%,which is in good agreement with the design conditions.The mathematical model of the system is verified by the experimental data,and the operating characteristics of the system under variable working conditions are studied.The results show that reducing the mass flow of cold source is conducive to expanding the adjustable range of working fluid flow ratio.Increasing the outlet temperature of the cold source is conducive to the realization of a large-temperature-drop thermal process,and the exergy efficiency is significantly improved.When the organic working fluid flow rate increases from 0.023kg/s to0.04kg/s,the adjustable range of working fluid flow ratio is expanded from 0～0.30to 0.05～0.60,the maximum net power output efficiency is 6.03%,and the maximum exergy efficiency is 33.72%.The research on operating characteristics improves the experimental research content and provides a control basis for the actual system.This thesis deeply studied the basic theory and the technical application of the ejector large-temperature-drop cogeneration system,which is conducive to promoting clean energy utilization and industrial energy saving and emission reducing.More importantly,it is of great significance to implement China’s"double carbon"strategic goals.