Forced response of shrouded blades with variable operating points

Abstract

The variable operation of turbomachinery components turns out as a special challenge in the design process. Changing the turbines rotational speed leads to entirely new load conditions and, thus, to a range of different operating points. Beside structural mechanical dependencies (i.e. stiffening effects), the shroud contact situation is very sensitive to the systems rotational speed. In this paper a model of a real low pressure steam turbine blade is investigated numerically and experimentally. For the calculation, a three dimensional structural mechanical model including a spatial contact model is considered. The steady-state vibration response is calculated by the multi-harmonic balance method (MHBM) and an alternating frequency-time scheme (AFT). The test rig consists of a single blade clamped with two dummies at the shroud. The vibration response of the blade is measured by laser-doppler-vibrometry for various excitation levels and pressure distributions in the shroud contact. The comparison between measured and computed frequency response function (FRF) of the first edgewise bending mode (1E) shows a very good agreement. Both the frequency shift, as well as the reduction of the amplitude were detected by the MHBM and successfully verified experimentally. The obtained results of the single blade system are transferred to a rotating bladed disk assembly.