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This paper is part of a two-part publication that aims to experimentally and numerically evaluate the aerodynamic and mechanical damping of a last stage ST blade at low load operation. A three-stage downscaled steam turbine with a snubbered last stage moving blade LSMB has been tested in the T10MW test facility of Doosan Skoda Power R&D Department in the context of the FLEXTURBINE European project (Flexible Fossil Power Plants for the Future Energy Market through new and advanced Turbine Technologies). Aerodynamic and flutter simulations of different low load conditions have been performed. The acquired data are used to validate the unsteady CFD approach for the prediction of the aerodynamic damping in terms of logarithmic decrement. Numerical results have been achieved through an upgraded version of the URANS CFD solver, selecting appropriate and robust numerical setups for the simulation of very low load conditions, such as increased condenser pressure at the exhaust hood outlet. The numerical methods for blade aerodamping estimation are based on the computation of the unsteady pressure response caused by the row vibration. They are usually classified in time-linearized, harmonic balance and non-linear approaches both in frequency and time domain. The validation of all these methods historically started in the field of aeronautical low-pressure turbines and has been gradually extended to compressor blades and steam turbine rows. For the analysis of a steam turbine last rotor blade operating at strong part load conditions, non-linear methods are recommended as these approaches are able to deal with strong nonlinear phenomena such as shock waves and massive flow separations inside the domain. Experimental data have been used to separate the contributions of mechanical and aerodynamic damping, extrapolating to zero mass flow the total measured damping. Finally, the comparisons between the aerodynamic damping coming from measurements and CFD results have been reported in order to highlight the capability to properly predict the last stage blade flutter stability at low load conditions. Such comparisons confirms the flutter free design of the new snubbered LSMB blade.

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Geometric uncertainties involved in the rotor blade manufacturing process are a major concern for designers. The deviation of the produced components from their nominal geometry have an impact on the Natural Frequencies (NF) that, under certain circumstances, may have negative effects on the dynamic forced response in operative conditions. Geometric defects are usually limited by imposing dimensional tolerances based on empirical considerations, simplified approach that may lead to costly manufacturing requirements that still may not guarantee safe results. This paper proposes a probabilistic representation of the geometric uncertainties for rotor blades and defines a procedure to evaluate their effects on the blade NFs. The deviation from nominal geometry is represented through the Principal Component Analysis (PCA) where it is expressed as a sum of characteristic geometric shapes (GUMs) modulated by mutually uncorrelated random variables (Principal Components, PC). The effect of each GUM is then linearly propagated on the blade NFs and a sensitivity matrix is finally defined. The procedure is applied to a case-study that concerns a set of 50 nominally identical compressor blades and the ability of GUMs to represent the effects of geometric uncertainties is tested.