| As a new trend in the central and district heating technology field,the large-temperature-drop heat exchange technology can decrease the return temperature of the primary water and increase the heat transfer capacity of the heating networks.It has a high heat exchange efficiency and obvious ecomomic advantages.The research of this thesis focuses on the scientific issues of energy conversion laws and mechanisms involved in the large-temperature-drop heat exchange technology,and the construction principles of the high-efficient system are explored.Starting from the theoretical basis of the two-medium heat exchange system,the characterisitcs of the ideal generalized heat exchange process,the energy conversion law and construction principles of the system are researche.Moreover,the construction of the practical system and the experimental studies are conducted.The analysis of ideal model is an essential way to master the basic laws of system.In this thesis,the concepts of generalized heat exchange process,large-temperature-drop heat exchange process and ideal generalized heat exchange process are proposed.The ideal generalized heat exchange process is defined as the real limit in the two-medium heat exchange system.By analyzing the analytical model of this ideal process,the relationship between the outlet temperatures of the two heat exchange media is derived.It is found that the heat exchange limit is only related to the inlet temperature and heat capacity ratio of the two heat exchange medium.The thermodynamic characteristics of the ideal generalized heat exchange process are studied and the influences of key parameters on the heat exchange characteristics of the process is clarified.It indicates that the greater the gap between the value of heat capacity and 1,the greater the ultimate heat exchange amount of the ideal process than that of the conventional heat exchanger system.The evaluation indexes such as heat exchange perfection,generalized temperature efficiency,temperature cross factor,which is based on the ideal process are proposed for evaluating the practical generalized heat exchange system,which provides a theoretical basis for the construction of the high-efficient large-temperature-drop heat exchange system.Based on the ideal generalized heat exchange process,the realization criterion of the large-temperature-drop heat exchange process in the generalied heat exchange system is proposed.Depending on the analysis on the heat exchange characteristics of the cycle-combined heat exchange process,it indicates that with the appropriate thermal cycle temperatures of heat engine,higher temperature efficiency of the heat exchagners and lower endo-irrevesibility degree in the thermal cycles would lead to the promotion of the system heat exchange effect.Based on the process characteristics of the ideal generalized heat exchange process,the cycle-combined heat exchange system with counter flow temperature regulation heat exchanger in parallel is comfirmed as the high-efficient construction.Deconstructing the ejector large-temperature-drop heat exchange system,the energy conversion and transmission laws of the actual large-temperature-drop heat exchange system are analyzed.The possibility of constructing a ejector large-temperature-drop heat exchange system by ejector heat pump is confirmed.It is pointed out that the higher thermal coefficient of heat-driven heat pump is the key to realize high-efficiency heat transfer of large-temperature-drop heat exchange system.Furthermore,the construction principle of the large-temperature-drop heat exchange system is determined.A new design method for realizing the high efficiency of the ejector,which is the key component to ensure the realization of large-temperature-drop heat exchange system,is proposed in this thesis.The new method fully considers the difference between the actual working gas and the ideal gas,and add a mixing chamber to the deveice which is designed using constant pressure mixing model.According to the relationship between dissipative charactersistics of working fluid flow and adiabatic efficiency,this method determines the average adiabatic efficiency of the diffuser and the optimal angle between the diffuser oulet and the horizontal axis.The entrainment ratio of the ejector designed under the new method is 2.5 times larger than that of normal constant mixing method and 2.1 times larger than that of constant rate of momentum change method under the condition of large temperature difference heat exchange system.The new ejector has the advantages of uniform internal working fluid mixing,gentle static pressure change,low degree of shock wave impact,high ultimate back pressure value and strong working stability,which is the technical guarantee for the construction of high efficiency ejector large-temperature-drop heat exchanger unit.Based on the research results of the cycle-combined heat exchange system,the basic construction of the ejector large-temperature-drop heat exchange system are presented.On the basis of technical constraints,aiming at the better heat exchange effect,the R141 b is selected as the working fluid of the ejector,and the system form with external temperature control heat exchanger in parallel is confirmed as the optimal system construction.For the defects of the basic structural system,according to the construction principels of the large-temperature-drop heat exchange system,a new type systm is proposed in this thesis,which is configured based on the two principles of thermal efficiency development and energy cascade utilization.This double-section system has high-efficient regenerator,pre-heater and reheater.With this system,the primary water with temperature of 130℃ can be cooled to 28.3℃,which is 6℃ lower than the outlet temperature of primary water in the single-section system.The double-section system has higher heat exchange performance and application scope of application.In addition,the influence of internal design parameters on thermal characteristics and performance is also studied.The decisive role of different parameters in design results is analyzed,providing a basis for system optimization.This thesis first designed and manufactured a double-section large-temperature-drop heat exchange experimental prototype(with rated heat exchange load of 200kW),and established a large-temperature-drop heat exchange system performance test experimental platform.The researches conducted via this platform is as follows:(1)Analysis and summary of the start and stop characteristics of this double section system.It indicates that the double-section large-temperature-drop heat exchange unit has a fast and stable start and stop response.(2)Determination and comparison of rated performance of single and double-section systems.Compared with tthe single-section system,the double-section system increases the heat exchange by 13.6%.(3)Determining the performance of different structural ejectors and their impact on the overall performance of the unit.Compared with the normal CPM ejector,the entrainment ratio of the ejectors designed with new method are increased to 1.86 and 0.5 times more than the normal ejectors,separately.The heat exchange effect of the system is enhanced by 25%.(4)The characteristics of the variable operating conditions of the unit under the influence of water inlet temperature,flow rate,secondary water inlet temperature and flow distribution ratio are studied,which lays the foundation for debugging,control and optimization in practical engineering application sThe research of this paper is of great significance to promote the engineering application of large-temperature-drop heat exchange technology,promote the theoretical development and technological innovation of large temperature drop heat exchange process,improve energy utilization efficiency,and guide the rapid progress of China’s heating technology. |
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