Origin, Evolution, Decay and Break-Down of Rotating Instabilities in Low-Flow Turbine Operation – Part B: Unsteady Results

Abstract Whereas Part A of this paper focused on the time-mean effect of rotating instabilities in low-flow turbine operation, Part B details the unsteady aspects, which eventually triggered non-synchronous blade vibrations. Both experimental and numerical data obtained from full-annulus unsteady simulations are used to investigate the temporal and spatial evolution of rotating instabilities. Measurements are used to characterize the windage vortices, which show various modal structures depending on the operating point, which is solely defined by the flow coefficient. Mode decomposition and wavelet analysis are utilized to better understand the temporal evolution of rotating instabilities in low-flow operation. It can be demonstrated that coherent unsteady flow structures form at random intervals, travel across the circumference and decay over time. In previous measurements, the spatial extension and propagating speed of convective rotating instabilities could be divided into several characteristic flow regimes. In this paper, the numerical simulation is validated against the measurement, identifying the same characteristics of large-scale instabilities inside the diffuser. Furthermore, the simulation identifies additional unsteady structures beyond the measurement region, and enables the further analysis on formation and decaying mechanism of them. Rotating instabilities form in flow separation regions, such as at the rotor leading edge, which are identical to the high-swirl and separation regions identified in the time-mean analysis in Part A. Rotating instabilities originate from the interaction between the local pressure fluctuation and the incidence at the blade, resulting in a wavelike pattern around the circumference. These findings can help to design turbines that are less susceptible to non-synchronous vibrations in low-flow operations.