Detection of Draft Tube Surge and Erosive Blade Cavitation in a Full Scale Francis Turbine

Abstract

A fullscale Francis turbine has been experimentally investigated over its full range of operation to detect draft tube swirling flows and cavitation. The unit is of interest due to the presence of severe pressure fluctuations at part load and of advanced blade suctionside cavitation erosion. Moreover, the turbine has a particular combination of guide vanes (20) to runner blades (15) that makes it prone to significant rotorstator interaction (RSI). For that, a complete measurement system of dynamic pressures, temperatures, vibrations, and acoustic emissions has been setup with the corresponding transducers mounted at selected sensitive locations. The experiments have comprised an efficiency measurement, a signal transmissibility evaluation, and the recording of the raw signals at high sampling rates. Signal processing methods for demodulation, peak power estimation, and cross correlation have also been applied. As a result, draft tube pressure fluctuations have been detected around the Rheingans frequency for low loads and at 4% of the rotating frequency for high loads. Moreover, maximum turbine guide bearing acoustic emissions have been measured at full load with amplitude modulations at both the guide vane passing frequency and the draft tube surge frequency.