ICOPE-15-C102 Coupled Vibration Mechanism between Long Blade Bending and Shaft Torsion of Large Capacity Turbines

Abstract

This paper focuses on the coupled vibration mechanism between long blade bending and shaft torsion of large capacity turbines. The double-flow low-pressure rotor is equivalent into a lumped model of 4 degrees of freedom. A method is given to calculate the parameter of the lumped system. The natural frequency and mode shape of this simplified coupled system is conducted. The ratio of shaft torsion frequency to the blade bending frequency is defined as frequency ratio. The ratio of bladed disk equivalent inertia moment to shaft inertia moment is defined as inertia moment ratio. The natural frequency of the simplified model is mathematically expressed as the function of the frequency ratio and the inertia moment ratio. The influence of frequency ratio and inertia moment ratio on the coupled vibration is revealed. The study shows that the variation of the modal characteristic of the coupled system with frequency ratio can be divided into two uncoupled regions and one coupled region. When the frequency ratio falls in the coupled region, the couple between blade bending and shaft torsion generates three coupling frequencies, two of them are higher than the frequency of the blade-disk, the left one is smaller than the frequency of the bladed disk.