Numerical Investigation of Low Engine Order Excitations on the Last Rotor Blade of Steam Turbines

Low Engine Order (LEO) excitations of the last steam turbine rotors generally arise from high unsteady flows due to circulation and reversal caused by low steam exit velocity and non-uniform flow-paths. These low-frequency excitations impose significant constraints on the aero-mechanical design of the Low-Pressure (LP) rotor since they may induce substantial forced response vibration amplitudes leading to High Cycle Fatigue (HCF) failures. Therefore, it is crucial to accurately predict the effects of Low Engine Order forcing to ensure the reliability and durability of the LP blade design. In this work, two different numerical approaches are used and the results compared. The first approach consists of URANS CFD simulations with the outlet distortions coming from diffuser to assess LEO forcing for forced response assessment. The second approach is based on pressure distributions obtained from a frozen rotor full annulus simulation. Forced response analyses are finally performed with an in-house tool based on modal work computations.