Pressure pulsation phenomena in a large Kaplan turbine are investigated by means of numerical simulations (CFD) and prototype measurements in order to study the dynamic behavior of flow due to the blade passage and its interaction with other components of the turbine. Numerical simulations are performed with the commercial software Ansys CFX code, solving the incompressible Unsteady Reynolds-Averaged-Navier Stokes equations under a finite volume scheme. The computational domain involves the entire machine at prototype scale. Special care is taken in the discretization of the wicket gate overhang and runner blade gap. Prototype measurements are performed using pressure transducers at different locations among the wicket gate outlet and the draft tube inlet. Then, CFD results are compared with temporary signals of prototype measurements at identical locations to validate the numerical model. A detailed analysis was focused on the tip gap flow and the pressure field at the discharge ring. From a rotating reference frame perspective, it is found that the mean pressure fluctuates accordingly the wicket gate passage. Moreover, in prototype measurements the pressure frequency that reveals the presence of modulated cavitation at the discharge ring is distinguished, as also verified from the shape of erosion patches in concordance with the number of wicket gates.
n this work, experimental research of a large Kaplan turbine at prototype scale for the minimum guaranteed head and high discharge is carried out with the goal of studying the influence of the guide vane opening in the flow pattern over the discharge ring. Measurements of wall pressure were made at different locations in the discharge ring wall between the guide vanes outlet to the draft tube inlet.The test was performed at constant head, while varying the opening of the guide vanes in discrete steps. The spectral analysis of these signals shows that main frequencies are the blade passage and their harmonics. One accelerometer was located at the man door entrance and, after a demodulated analysis of the signal, a good correlation was observed against pressure pulsation spectra. This shows that the vibrations of the turbine structure are modulated by its hydraulic components. Furthermore, a frequency corresponding to the first rotor-stator diametral mode appeared in the acceleration signal, showing the interaction between these two components. This frequency component appeared for guide vanes openings greater than 84%, reaching its peak at 100%.
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