Pressure pulsations and stress in a high head Francis model turbine
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Model tests on a high head francis turbine has been conducted at the waterpower laboratory at NTNU. Simultaneous measurements of the pressure were performed on different parts of the waterway. Compact RIO hardware and Labview software from National Instruments was used for data aquisition. Pressure sensors were placed at the inlet, vaneless space, runner channel, draft tube cone and draft tube outlet in the rig. Generator torque and the rotational speed of the runner was also measured. Spectral analysis was performed on all measurements to map out frequencies and corresponding amplitues at various load conditions. The RSI induced pulsations dominates the vaneless space and runner channels. Pressure pulsations are dampened across the runner and sensors at the trailing edge display values that are significantly larger than sensors positioned at the outlet. RSI pulsations are dependent on the flow and relative amplitudes in the vaneless space is reduced with over 90% when the load is lowered from 100% to 50%. Vortex rope frequency is detected in the runner channels and the generator torque. This suggests that the low frequency oscillations causes a mass oscillation through the system that affects the output of the generator. An air lekeage through the shaft was detected during testing. The incoming air resulted in an excessive draft tube vortex, thus the results obtained in this work are not directly comparable to field test conditions.