Study On Z-shaped Shrouds Collision-friction Damping And Its Vibration Absorption Mechanism Of Large Steam Turbines Rotor

Large turbine units have become the main trend of future energy industry equipment transformation and upgrading due to their advantages of reliability and environmental protection.Large units have long twisted blade bending and torsion deformation unit elastic shaft itself with the flexible blade vibration response of the coupling between overlap with double flexible structure characteristics,and are more likely to occur nonlinear severe vinration under wet steam flow complex excitation on the working condition of high parameters.Z-shaped shrouds damping structure is widely used to suppress the rotor system vibration.However,the lack of effective Z-shaped shrouds damping calculation methods,and the inability to accurately predict the influence of the Z-shaped shrouds on the vibration response and stability of the unit,have become the main bottleneck of large-scale application of large turbine units.Aiming at studying vibration absorption mechanism of Z-shaped shrouds collision-friction damping of rotor system,the Z-shaped shrouds collision-friction damping model and its calculation method are established,and the vibration characteristics and stability of the rotor system with Z-shaped shrouds are studied in depth by using theoretical analysis and numerical simulation.According to the contact motion characteristics of the Z-shaped shrouds during operation,the influences of Z-shaped shrouds collision and tangential friction between the shrouds contact surfaces are considered,and a Z-shaped shrouds collision-friction damping model is established,which considering restitution coefficient,initial collision velocity,friction coefficient and normal contact stiffness,etc.The calculation method of Z-shaped shrouds collision-friction damping under different working conditions is proposed with the correction coefficient method.The Z-shaped shrouds collision-friction damping model and its calculation method are verificated by using published experimental data.The rotor system dynamics model under different working conditions is established considering the non-linearity of the"double flexible" structure,complex excitation and Z-shaped shrouds collision-friction damping,etc.Z-shaped shrouds collision-friction damping under different working conditions is quantitatively calculated by using CM-ZCFD,and its sensitive variables are identified.The influences of restitution coefficient,initial collision velocity,friction coefficient and normal contact stiffness on the Z-shaped shrouds collision-friction damping and rotor system vibration response characteristics under different working conditions are explored.Meanwhile the vibration absorption mechanism of Z-shaped shrouds damping is elucidated:Z-shaped shrouds have a better suppression effect on the low order modal vibration of the system,but the suppression effect on the high order modal vibration of the system is limited;Z-shaped shrouds collision-friction damping has little effect on the frequency of the vibration response,but it can effectively suppress the response amplitude of each frequency component,and eliminate the system flutter phenomenon when only considering shrouds pure friction damping,and the effect on the vibration response of the system is more obvious;under the low rotating speeds conditions,the collision between the contact surfaces of the Z-shaped shrouds consumes the system energy more and plays the main damping role,and the influence of the Z-shaped shrouds damping can be effectively improved by increasing the restitution coefficient;under the medium and high rotating speeds conditions,the friction between the contact surfaces of the shrouds consumes the system energy and plays the main damping role,and the influence of the Z-shaped shrouds damping can be effectively improved by increasing the friction coefficient.The stability prediction method of large turbine rotor system is established based on the Z-shaped shrouds collision-friction damping,and the influences of the Z-shaped shrouds collision-friction damping and shrouds parameters on the system stability under different working conditions are analyzed.The results show that the instability of the rotor system with Z-shaped shrouds generally occurs in the range of higher rotating speed,smaller friction coefficient,larger normal contact stiffness,smaller restitution coefficient and smaller initial collision velocity,and the stability of the rotor system is more obviously affected by the restitution coefficient.On this basis,the influences of different Z-shaped shrouds damping models on the instability threshold of the rotor system are analyzed.The results show that using the Z-shaped shrouds collision-friction damping model proposed in this paper can predict a larger instability region of the rotor system than the instability region predicted by the shrouds pure frictional damping model commonly used in current design,which means that while using the commonly used shrouds pure friction damping model in the design process,there is a possibility of the instability of the whole system leading to insufficient damping and vibration suppression,which affects the safe system operation.With the optimal damping of the shrouds in the full-cycle operation as the optimization objective,and with the constraints that all components of the unit pass the safety check(allowable stress and allowable amplitude are not exceeded)and the service life is not reduced,and the Z-shaped shrouds sensitive variables as the optimization variables,a dynamic optimization method is established for the system stability.The optimized results shows that the optimized shrouds collision-friction damping increases significantly with the increase of rotating speed,especially it increases by 35%on the rated operating condition,which can effectively reduce the system vibration.