Abstract. When Francis-turbines and pump-turbines operate at off-design conditions, typically a vortex rope develops. The vortex rope causes pressure oscillations leading to fluctuations of the forces affecting the runner. The presence of dynamic runner forces over a long period of time might damage the bearings and possibly the runner. In this experimental investigation, the fluctuating part of the runner forces and the pressure oscillations on the draft tube wall were measured on a model pump-turbine with a simplified straight cone draft tube in different operating conditions. The investigation focuses on the correlation of the pressure fluctuations frequency measured at the draft tube wall with the frequency of the fluctuating forces on the runner. The comparison between pressure fluctuations and dynamic forces shows a significant correlation in all operating points. For the comparison of different components in the spatial directions of the forces, the pressure fluctuations were separated in a synchronous part and a rotating part for operating points with higher amplitudes. The rotating pressure fluctuations correlate with the radial forces especially in the operating points with a rotating vortex rope. At frequencies with higher amplitudes in the pressure fluctuations caused by the vortex rope movement, there are also higher amplitudes in the radial forces at the same frequencies.
The energy market imposes new requirements in hydraulic turbines operation. Usually, the old hydraulic turbines are not designed to meet these new requirements. Therefore, the refurbished solutions for hydraulic turbines are expected to be robust and flexible in operation in order to regulate the grid. A methodology is developed for a hydropower plant equipped with Francis turbines. Firstly, the solution available in the hydropower plant is examined. Secondly, two new solutions are designed for the hydraulic passage available in situ. Next, several scenarios from peak load operation to wide range operation are investigated in order to asses the performance of each technical solution. Consequently, the performances are compared proving the best solution for hydropower plant refurbishment.
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