Study On Thermoacoustic Coupling Characteristics And Its Instability Suppression Of Duct Cavity With Arbitrary Impdedance Boundary Conditions

Thermoacoustic coupling phenomenon exists in combustion systems such as ship / power station gas turbine,aeroengine and industrial boiler,which seriously affects the quietness and operation reliability of equipment.At the same time,thermoacoustic coupling oscillation will affect the combustion process and reduce the efficiency of equipment.Attention should be paid to avoid thermoacoustic coupling oscillation phenomenon in the design stage of equipment.Therefore,the research on thermoacoustic coupling oscillation mechanism and suppression methods has attracted the attention of scholars.In order to reveal the physical mechanism of thermoacoustic coupling oscillation,the duct cavity structure is usually taken as the research object.The research shows that the geometric structure and boundary conditions of the duct cavity have a significant impact on the thermoacoustic coupling oscillation phenomenon and characteristics.However,the existing research is mostly limited to the simple duct cavity structure,and the influence of complex boundary conditions such as end impedance condition,cross-section gradual expansion,Helmholtz resonator and wall micro perforation structure on the thermoacoustic coupling characteristics is not considered,and lack of mechanism analysis of thermoacoustic coupling oscillation of complex duct cavity structure.Therefore,It is of great significance to study the thermoacoustic coupling characteristics and instability suppression of the duct cavity with impedance boundary conditions.The main research work of this thesis is as follows:The analysis model of thermoacoustic coupling system of Rijke tube with end impedance condition is established.The sound pressure distribution function in the tube is constructed by one-dimensional improved Fourier series.The first-order heat release model is used to describe the interaction relationship between the heat source and the sound field.The modal frequency and stability growth rate of the thermoacoustic coupling system are solved by Helmholtz differential equation and Galerkin discrete method.The quantitative relationship between the end impedance condition,the position of heat source and the stability of thermoacoustic coupling system is revealed by using the method in this thesis.The experimental device of Rijke tube is built to verify the accuracy of the theoretical analysis results.Through the analysis of the amplitude of sound pressure response,the highly sensitive thermoacoustic coupling characteristics of Rijke tube are verified.Based on the Rijke tube with end impedance condition,the control element is introduced,and the analysis model of cavity control-element thermoacoustic coupling system is established by using the energy principle and Rayleigh Ritz method.The model can flexibly change the end impedance condition and control element.The suppression effect of control elements,such as Helmholtz resonator(HR),on the instability of thermoacoustic coupling system is analyzed in detail,and the relationship between HR structural parameters,resonance frequency and system stability is obtained.It is proved that the introduction of improper HR will enhance the instability of the system or introduce new unstable modes.For a given thermoacoustic coupling system,the effective frequency range of HR and the effective parameter range of HR structure are determined.It is verified that the thermoacoustic coupling system is in an unstable state when the input energy of the heat source to the system is greater than the energy dissipation of the control element and the impedance boundary.The dynamic characteristic analysis model of variable cross-section duct cavity thermoacoustic coupling system with end impedance condition is established.The variable cross-section duct cavity is treated by substructure method and is divided into regular acoustic cavity and irregular acoustic cavity substructure.The adjacent acoustic cavity substructures are connected by auxiliary air plate,and the irregular acoustic cavity substructure is mapped by spatial coordinate transformation method.The energy principle is extended to two-dimensional space,and the modal characteristics of variable cross-section duct cavity thermoacoustic coupling system are obtained by combining Rayleigh Ritz method.The effects of section change position,change proportion and gradient angle on thermoacoustic coupling characteristics are explored.The results show that properly changing the duct cavity section is helpful to improve the stability of the system,but when the heat source is at its critical position,the instability of the system increases.At the same time,the influence law of the gradient section angle on the thermoacoustic coupling system is obtained.The reasonable gradient angle setting can improve the stability of the system to a certain extent.The micro perforated plate(MPP)structure is introduced into the duct cavity wall.The energy principle and Rayleigh Ritz method are extended to three-dimensional space.The acoustic effect of MPP is characterized by the form of wall distributed impedance.The analysis model of three-dimensional variable cross-section duct cavity thermoacoustic coupling system containing MPP is established,which can introduce MPP at any position on the duct cavity wall.The thermoacoustic coupling characteristics of the system are obtained by solving the matrix characteristic equation.The results show that MPP can suppress multi-order modal instability and effectively improve the stability of thermoacoustic coupling system.Through the analysis of sound intensity distribution and divergence characteristics,the sound field energy transmission path and the distribution law of energy source and trap of thermoacoustic coupling system are revealed.Whether the thermoacoustic coupling system is stable depends on the energy transmitted from the heat source to the acoustic system.Blocking the transmission path of energy source or reducing energy transmission can significantly improve the stability of the system.In addition,thermoacoustic instability can be controlled by adjusting the installation position of MPP.According to the relationship between the established thermoacoustic oscillation mode and the effective frequency range of HR,the suppression of thermoacoustic coupling oscillation by tuned HR is carried out based on the secondary regulation strategy.Firstly,the thermoacoustic coupling oscillation frequency is used as the target frequency for preliminary regulation,so as to reduce the regulation time and realize rapid positioning;Then,the Newton Raphson iterative principle is used to adjust the sound pressure amplitude twice until the sound pressure is less than the oscillation threshold and the system reaches a stable state.The experimental results show that the control algorithm can effectively track the changes of thermoacoustic coupling oscillation characteristics of the system,and adjust HR parameters(such as volume)in real time,so as to achieve the control purpose of restraining thermoacoustic coupling oscillation.In this thesis,the dynamic analysis model of cavity thermoacoustic coupling system with arbitrary impedance boundary conditions is established.Considering the impedance boundary conditions such as end impedance condition,HR,variable cross-section and MPP,the law of cavity thermoacoustic coupling characteristics is studied,and the suppression schemes of thermoacoustic instability are proposed according to different situations.The relevant numerical results provide an effective basis for the structural design of combustion system,It provides a reference for the research of thermoacoustic coupling oscillation control.