The Role of Operating Conditions on Flutter Instability of a Low-Pressure Turbine Rotor

Modern trends in low pressure gas turbine design lead to lighter designs with fewer stages, thinner sections and highly loaded components. Blades are thus more prone to flutter induced vibrations that can compromise their structural integrity. Accurate predictions of flutter inception and evolution remains a challenging task. Several numerical methods have been developed for a long time and dedicated experimental campaigns have been carried out to have a better insight of flutter problems. The ARIAS EU project provided the opportunity to investigate flutter occurrences of turbine and compressor rotors with unique test benches and to test state of the art numerical techniques. In this context, the paper discusses the impact of the turbine operating conditions on flutter inception of an unstable low pressure turbine rotor extensively tested during the EU ARIAS project. The operating conditions were changed by modifying the rotor rotational speed, and the first bending and torsion mode families of a cantilever configuration have been analyzed. Numerical results reveal that an increase of the rotational speed leads to more unstable configurations for the bending mode-shape, while the torsional mode-shape moves from stable to unstable condition when lowering the rotational speed. Such numerical evidence has been confirmed by the blade tip timing acquisition technique also in terms of most unstable nodal diameter. Finally the limit cycle amplitudes measured with the tip timing are compared with the results of a vibration amplitude estimation method based on a combination of acoustic measurements and computational results.