Investigations of time dependent flow phenomena in a turbine and a pump-turbine of Francis type: rotor-stator interactions and precessing vortex rope

The present work deals with time dependent phenomena, which develop in turbines and pump turbines of Francis type. These phenomena may cause serious damages to hydropower plant. The overall objective of this study is to bring a better understanding of two kinds of time dependent phenomena such as rotor-stator interaction (RSI) and precessing vortex rope (PVR) by numerical simulations of fluid flows and measurements. Two case-studies appropriate to the investigated unsteady phenomena are selected. The analysis of RSI is performed on the results of a pump-turbine scaled model. A Francis turbine model is selected to investigate PVR in the draft tube. For the flow numerical simulations, CFD package, ANSYS-CFX, is used, which is a finite-volume based flow solver. For the both case studies, the fluid is assumed to be incompressible, and hence the effects of the hydro-acoustic waves are neglected. Prior to performing the numerical simulations, the effects of grid resolutions, time increments, configurations of the computational domains and turbulence model are studied, and thereby, the optimum parameter sets for each of the test cases are found. With respect to rotor-stator interaction, several operating points from part load to full load conditions are investigated. At full load operating point corresponding to a strong rotor-stator interaction, the pressure fluctuations resulting from numerical simulations and measurements are analyzed. The computed results of static pressure fluctuations are validated with the measurements on the casing wall of the spiral casing, the bottom wall of a stay-vane and guide-vane channels for the RSI case and on the wall of the draft tube cone for the PVR case. The predicted fluctuations in the static pressure fit well with the measured counterparts in terms of both magnitude and frequency for RSI and PVR cases, except for several points such as on the wall in the spiral casing. These results confirm that with the exception of vibration condition, cavitation or fluid-structure interaction, the incompressible flow hypothesis is valid. For the RSI simulations, the diametrical mode of the pressure fluctuation is clearly identified by decomposing the results obtained from the numerical simulations. The hydro dynamical reason why two peaks in the pressure fluctuation appear during one passage of the blade pitch is also described. Moreover, a non-dimensional number based on discharge, the period of runner revolution and the time during which, flow passes through the vaneless gap between the guide vanes and the runner blades is proposed. Accordingly, it is found that the magnitudes of the pressure fluctuations well correlate with the proposed function for the three operating conditions investigated from the part load through the full load conditions.

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