Prediction of High Cycle Fatigue Life of Steam Turbine Blading Based on Unsteady CFD and FEM Forced Response Calculation

This paper presents the high cycle fatigue life prediction of a shrouded packet blade row for a large high pressure steam turbine. The fatigue life assessment was based on a loosely coupled CFD and FEM calculations supported by the direct cyclic mechanical testing of a full scale model of the blade root attachment. In the first stage, potentially dangerous vibration modes were identified from the linear finite element analyses and verified by testing. Aerodynamic forces acting on rotating blades were obtained from an unsteady sliding mesh CFD calculation based on a viscous unsteady compressible turbulent flow. A forced vibration analysis was then performed for critical resonance frequencies close to nozzle passing frequency considering only the inherent material damping. The calculated alternating stress in the blade root attachment was used to estimate the safety against the high cycle fatigue failure. Fatigue life was evaluated for current and new design of stationary vanes. In the new design the number of vanes was increased and their full 3D shape was optimized. Both changes reduced the excitation forces by more then an order of magnitude. The final evaluation showed that the vibratory stresses for the new design were well below the actual fatigue limit of the blade root attachment.