Flow-Induced Vibration And Stability Of Large Steam Turbine Rotor System

In view of the low energy utilization rate and low per capita resources in China,large-scale generator sets such as nuclear steam turbines,ultra-supercritical steam turbines,and cogeneration steam turbines have become the main development trends of the core equipment of the energy industry due to their outstanding advantages such as high efficiency and environmental sustainability.However,the large generator usually contains complex structural features,such as ultra-long shafting(>10m),large flexible final blades(>1.3m),complicated shrouds and etc.And the vibration generated on the rotor system from low-frequency rotors by the complex nonlinear dynamic fluid excitation in the wet steam zone is complicated.All of these become the key limiting factors for the safe operation,development and application of large steam turbines.In this paper,a 1000MW steam turbine is studied,and various vibration modes of large steam turbine rotors under different working conditions are studied to improve energy supply/thermal efficiency and ensure operational stability using theoretical research and numerical simulation methods.The relationship of system stability of large steam turbines 'under various operating conditions to fluid excitation and its induced vibration mechanism,to vibration mode,and to instability mechanism and its influencing factors are deeply studied in a systematic way.Based on the classical nucleation theory of non-isothermal correction and the droplet growth model of low pressure correction,a three-dimensional wet steam non-equilibrium condensation flow control method was established,and the main variables and influence mechanism of wet steam flow field at rated flow and small flow was investigated respectively,considering the pressure pulsation induced ty the Phase Temperature Difference(PTD)and the Small Flow Disturbance(SFD)during the non-equilibrium condensation process of wet steam.A numerical solution method for fluid excitation in complex conditions based on flow field simulation is established for large steam turbines,which can solve problem of difficult to identify complex excitation in rotating steam wet steam environment.Based on fluid excitation modeling and numerical solution method,the fluid excitation under different working conditions is calculated.The influence of phase temperature difference between gas and liquid phase(PTD)and its induced pressure pulsation on non-equilibrium condensation under rated flow conditions is analyzed.The effect of small-flow disturbance(SFD)and its induced pressure pulsation on fluid excitation is studied.Combing with flow field phase change heat transfer(nucleation zone),static entropy mutation(shock distribution),and de-flow vortex(streamline distribution),the generation mechanism and amplitude-frequency characteristics of fluid excitation under different intake conditions are explained.The results show that under the rated flow condition,the main excitation source of the flow field is the non-equilibrium condensate flow excitation.The major generation mechanism is that the temperature difference between the gas and liquid phases in the nucleation area of the blade surface changes the position and type of the shock wave in the flow channel,and the reconstruction and exacerbation the pressure pulsations distribution.The frequency components of the induced rotor response are more complex and larger than the equilibrium condensation.Under the small-flow cogeneration condition,as the flow rate decreases,the wet steam leaves the surface of the blade before reach the nucleation condition.The de-flow,the vortex and the separation of boundary layer exacerbates the pressure pulsation on the blade surface,resulting in more multiple excitation frequency division and frequency band widening,which is more likely to cause forced vibration,resonance or instability of the system.Considering the geometric nonlinearity of flexible blades,the complex fluid excitation under different working conditions and the nonlinearity of induced shroud frictional damping excitation,the nonlinear dynamic model of large steam turbine rotor system is established.The relative dynamic flexibility compliance method and the CN group theory are used to simplify the model.Depending on the analysis target,the rotor structure is dispersed into a three-dimensional solid element and Euler-Bernouli space warping beam element.The multi-harmonic balance method is introduced to establish an equivalent model and a quantitative solution method for the complex multi-frequency excited shroud frictional damping.On this basis,the transient vibration response of the rotor system under different operating conditions is solved,and the influences of the intake parameters(intake velocity V,intake angle ?)and different rotational speeds on the vibration response of the system under different flow rates are analyzed.The damping effect of the shroud frictional damping on the vibration response of the system is studied and verified by the online monitoring vibration response data of a 1000MW turbine unit.Considering the influence of excitation and nonlinear factors on the stability of the system,based on the relationship between the characteristic multipliers of the single-valued matrix and the unit circle from Froquet's theory,the stability criterion under multi?frequency excitation is derived;using the Runge-Kutta method,the system motion equation is integral solved.The influence law of the intake parameter(V,?)and the rotational speed Q on the nonlinear bifurcation behavior of the system is studied.The instability mechanism and sensitive variables of the system under different working conditions are discussed.Flow-induced three-dimensional(?-?-V)stability limit surface is derived.Based on three critical instability flow thresholds for low-flow cogeneration condition(enthalpy blast,reverse pressure stall and flow induced instability),a multi-criteria composite stability prediction method considering multi-field coupled excitation is established.By using the relationship between the zero-damping plane and the shroud parameters,a rapid prediction method of limit threshold of system stability in the design stage is proposed.With target of the maximum stability of energy supply/thermal efficiency and optimal shroud damping under the corresponding working conditions,constrained by operation stability and non-resonant,a stability dynamic optimization method is carried out with variables of intake parameter and shroud parameter.The example analysis shows that the optimized intake parameters and shroud frictional damping expand the stable running range of the system.It provides engineering application methods for stability prediction and dynamic optimization in the design stage,which can effectively avoid the"congenital deficiency" of empirical design.