Unsteady computational fluid dynamics analysis of the hydrodynamic instabilities in a reversible Francis turbine used in a storage plant

The hydraulic storage plants play a strategic role in the management of large electrical networks. Besides the widely known capability to store and provide great amount of energy, in a green and efficient way, these plants are of utmost importance to guarantee the perfect balance of the Italian electricity grid. For this reason the reversible turbomachineries, used in these plants, are subjected to a particularly intensive use which strongly affects their reliability. The present work deals with a real issue of the ENEL Green Power "Anapo" storage hydro plant, located in Sicily (Italy). In this plant strong and increasing vibrations were detected in the Fink's guide vane, particularly during the pumping operating conditions. In order to find the sources of these vibrations and for a quantitative evaluation of the structural loads on the Fink's guide vane blades, an unsteady CFD 3D model of the reversible turbine was implemented and validated using data provided by ENEL. The model accurately reproduced the hydrodynamic behavior of the water flow within the entire machinery, between the Kaplan's diffuser and the spiral case. This allowed for the calculation of the time dependent trend of the structural loads over the Fink's vane blades. The frequencies and the amplitudes of the loads were analyzed through the use of a Fast Fourier Transformation. Moreover, the CFD model allowed for a sensitivity study of the guide vane opening angle effects. This study provides an interesting insight into the hydrodynamic behavior of the machine.