Forced Response Prediction for Steam Turbine Last Stage Blade Subject to Low Engine Order Excitation

Low Engine Order (LEO) excitations on a steam turbine Last Stage low-pressure (LP) Bucket (or Blade) (LSB) are largely the result of flow unsteadiness (e.g. flow circulation and reversal) due to low steam exit velocity (Vax) off the LSB at the off-design conditions. These excitations at low frequencies impose major constraints on LP bucket aeromechanical design. In this study, bucket forced response under typical LEO excitation was analytically predicted and correlated to experimental measurements. First, transient CFD analyses were performed at typical low flow, low Vax operating conditions that had been previously tested in a subscale low pressure turbine test rig. The unsteady pressure distribution on the bucket was derived from the transient CFD analyses at frequencies corresponding to the bucket’s modes of vibration. Subsequently, these computed unsteady pressure were mapped onto a LSB finite element model, and forced response analyses were performed to estimate the bucket dynamic response, i.e. the alternating stresses and strains. The analytically predicted bucket response was compared against measured data from airfoil mounted strain gages and good correlation was found between the analytical prediction and the test data. Despite uncertainty associated with various parameters such as damping and unsteady steam forcing etc., the developed methodology provides a viable approach for predicting bucket forced response and in turn High Cycle Fatigue (HCF) capability during early phases of steam turbine LSB design.