| In view of two shortcomings of the low resource utilization rate and the shortage of per capita resources in our country,large-scale generator such as ultra-supercritical steam turbines and combined heat and power cycles have become the main development trend of core equipment in the energy industry due to their green and high efficiency.However,the length of the last stage rotor of large generator sets usually exceed one meter,and the continuous increase of national requirements for power plant peak shaving capabilities,so that the last stage long blades are often subjected to complex nonlinear fluid excitations in the wet steam zone,which induces blade vibration and in turn affects the operational safety.In this paper,the last stage of a steam turbine is taken as the research object.Based on the computational fluid dynamics and computational structural dynamics theory,the fluid excitation and vibration response of the last stage under variable load conditions,the aeroelastic stability of the rotor system and the influencing factors are studied.First of all,based on the inherent vibration characteristics of the long blade,the flow field simulations of each load condition is carried out,and the modal analysis of the blade in the three states of static,rated speed and variable load is carried out;subsequently,aerodynamic loads were extracted from the flow field results to explore the static vibration characteristics of long blades under variable loads.The study found that under rated speed conditions,the natural frequency of the low-order mode is much larger than the static state;after the aerodynamic load is superimposed,the overall change of the natural frequency is small,and it has a relatively large influence in the frequency on the torsional vibration mode or dominated by the torsional vibration mode.When subjected to centrifugal-aerodynamic composite loads,its static vibration response can be approximated as the superposition of centrifugal displacement and aerodynamic displacement in the axial direction.Then,through the two-ways coupling method,analyze the unsteady pressure fluctuations and summarize the effect of blade deformation on the fluid field.At the same time,the dynamic vibration characteristics of long blades excited by unsteady fluid are discussed.The study found that the areas where blade deformation has the greatest influence on the flow field parameters are mainly distributed on the blade leading edge.In addition,due to the two characteristic flow conditions,the blades are subjected to two exciting forces in opposite directions in the axial direction,the response of the subsequent blades vibrates in the downstream direction at 1.0 kg/s.Under the working condition of 0.1 kg/s,the blade vibrates in the upstream direction,and the vibration mode of 50% blade height in this working condition is the composite vibration of the first order mode and the second order mode,while the blade tip is the first-order mode vibration.Finally,based on the unsteady simulations of the first bending mode,the aeroelastic stability of the long blade under the variable load condition and the variable inter blade phase angle is discussed;from the perspective of wall work density,total wall work and accumulated work,the mechanism of induced vibration is described.The results show that under the variable load condition of IBPA=180 degree,the aerodynamic damping coefficient also gradually decreases with the decrease of the inlet mass flow;moreover,it is found that the influence of inlet parameters on aeroelasticity is mainly distributed in 70% to 90% span suction surface.Under the condition of variable inter blade phase angle in 1 kg/s,blade flutter occurred when the inter blade phase angle is 90°,and the aeroelastic stability is best at-90°.In addition,when the blade vibration is in the forward wave mode,the main difference of the aeroelasticity under different inter blade phase angle exists on the suction surface;while in the backward wave mode,both exists on the suction surface and the pressure surface. |
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