Experimental Investigation of a Francis Turbine during Exigent Ramping and Transition into Total Load Rejection

This study investigates the unsteady pressure fluctuations in a hydraulic turbine observed during a dangerous case of steep ramping interrupted by an unexpected transition into total load rejection. Although hydraulic turbines are expected to experience such events only a few times over their lifetime, the resulting pressure amplitudes are so significant that they take a toll on a machine’s operating life. The focus of the present study is to experimentally measure and numerically characterize time-dependent pressure amplitudes in the vaneless space, runner, and draft tube of a model Francis turbine. To this end, 12 pressure sensors were integrated into a turbine, including four miniature sensors mounted in the runner. Steep ramping was performed by changing the rotational speed using a frequency controller. After a few seconds, as the load increased, total load rejection was initiated. This resulted in the generation of strong vibrations throughout the entire structure and strong pressure fluctuations in the turbine. The data analysis shows that pressure amplitudes are in the order of 10–20% of hydraulic energy in the vaneless space and runner, with high-amplitude fluctuations occurring at expected characteristic frequencies, including those associated with rotor-stator interactions, water hammer travel times, and standing waves in the turbine. Various stochastic frequencies were also observed, especially at the runner outlet.