Current operational considerations require steam turbines to operate in a more flexible way, with an increasing part-load operation. For low flow rates, the interaction of highly separated flow with the high rotational speed causes windage flow. This type of flow is characterized by increased temperature and highly unsteady flow, which forms vortex structures that rotate at a fraction of the rotor speed. If their magnitude is sufficiently high and the frequency is close to the blade eigenfrequency, non-synchronous vibration (NSV) is induced. In this paper, low-flow turbine operation is investigated using a three-stage turbine rig, featuring an instrumentation concept focused on capturing aerodynamic and aeroelastic phenomena. Extensive steady probe, unsteady pressure, and tip-timing measurements are utilized. It covers a wide range of operation in terms of rotational speed and mass flow rates. Low-flow regimes are detected by a reversed torque and increase in temperature. Unsteady measurements during transient operation identified large-scale vortical flow structures rotating along the circumference, so called rotating instabilities (RI). The onset, growth, and breakdown regimes of RI are characterized for different low-flow conditions. The quantitative characteristics of RI are derived by a cross-correlation of multiple unsteady sensors. The blade vibration measurements show a moderate structural response from unsteady aerodynamic excitation. Later in the study, an acoustic excitation system has been applied to trigger a locked-in NSV. From that, significant blade response has been observed, revealing a high degree of mistuning and damping of the rotor blading.

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