Research On Vibration Characteristics Of Cascade-shaft System In Nuclear Steam Turbines

With China's energy consumption structure continuing to upgrade,the proportion of nuclear power will gradually increase.Among the nuclear power units that have been put into operation and in construction in currently China,and most of which are more than 1000 MW.The application of the super-long blades in the low-pressure cylinder makes the interaction between the cascade vibration and the shaft vibration more and more significant.This paper dealt with the interaction between the cascade and the shaft,simulated modal characteristics of the system and its variation with the flexibility and speed of the cascade with numerical method,and calculated the response of the system under typical electrical fault torque or wake force.A method named 'Judging the Cascade' Relative Flexibility + the Cyclic Method'was put forward to simplify the structure of the cascade-shaft,and a model of the system with cascade' flexibility.rotating speed,typical electrical fault torque and wake force included was established.Firstly.the cascades with little flexibility were equivalent to hollow cylinders,according to the principle of equal mass and equal moment of inertia.Secondly,the analysis of the whole system was reduced to a single elementary sector,using the system's cyclic symmetry and the C_N group theory.Thirdly,the 3D solid elements were used to discretize the basic sector,and the differential motion equations and their solving methods of the basic sector were derived based on the energy method.The modal characteristics of the system and its variation with the flexibility and speed of the cascade were simulated,and the influence of cascade flexibility on frequency doubling resonance was discussed,with the interaction between high flexibility cascades and shaft.The QR damping method was used to solve the modal characteristics of the system,and the results show that the model above can identify considerable interaction between the cascade and the shaft.Compared with the modal frequency of the system which was not simplified,the correctness of the simplifying method was verified.By changing the relative flexibility between the cascade and the shaft,it was concluded that the flexible change of the cascade has no effect on the bending vibration of the shaft.The modal characteristics of the system in each target rotation zone during the start-up period shows that change of the rotational speed has no effect on the first bending vibration of the shaft,and the natural frequencies corresponding to the other modes increase with the increase of the rotational speed.Furthermore,the doubling frequency excitation causes resonance of the cascade,which won't occur while ignoring flexibility of the cascades.The response of the cascade-shaft under typical electrical fault torque shows that all the 4 faults are all able to cause resonance of the system,and the two-phase short-circuit fault is more harmful to the system than the other 3 faults.Then,the transient flow of the vapor between cascades was simulated based on the homogeneous model to get wake force,and the harmonic response due to the wake force shows that the steam load has a considerable effect on the deformation caused by rotation.Furthermore,the feasibility of boundary processing method of cascade root was verified.