Numerical and Experimental Damping Prediction of a Nonlinearly Coupled Low Pressure Steam Turbine Blading

Abstract

The rotor blades of a low pressure steam turbine stage are subjected to fluctuating steam forces during operation that cause blade vibrations. One of the main tasks in the design of low pressure steam turbine blading is the vibration amplitude reduction in order to avoid high dynamic stresses that could damage the blading. The vibration amplitudes of the blades in the last row of a low pressure steam turbine stage can be reduced significantly to a reasonable amount if adjacent blades are coupled by shroud and snubber contacts that reinforce the blading. Furthermore, in the case of blade vibrations, relative displacements between neighboring blades occur in the contacts and friction forces are generated that provide additional damping to the structure due to the energy dissipation caused by microslip effects. A three dimensional structural dynamics model including an appropriate spatial contact model is necessary to predict the generalized contact forces induced by the shroud and the snubber contacts and to describe the vibrational behavior of the blading with sufficient accuracy. To reduce the numerical effort to compute the vibration response, the Harmonic Balance Method (HBM) is applied to solve the resulting nonlinear equations of motion in the frequency domain.