Investigation of the unsteady pressure pulsations in the prototype Francis turbines during load variation and startup

Abstract

This work investigates the unsteady pressure fluctuations in two prototype Francis turbines during load variation and start-up. Although hydraulic turbines are expected to experience such events over their lifetime, the resulting pressure amplitudes are so significant that they take a toll on a machine's operating life. The interest of the present study is to experimentally measure and numerically characterize time-dependent pressure pulsations. Specific focus is on (1) how pressure pulsations of both synchronous and asynchronous types in vertical- and horizontal-axis turbines change when the load of a turbine changes from steady conditions, (2) what the pressure amplitudes during load change are, and (3) how quickly pressure amplitudes vary when a generator is synchronized to the power grid (load) during start-up. To this end, four pressure sensors were integrated in the draft tube cone. The results are quite interesting, especially during transition from the steady state to the transient load change. In the vertical-axis turbine, amplitudes of asynchronous pressure pulsations are 20 times larger than those of the synchronous component; whereas, in the horizontal-axis turbine, amplitudes of asynchronous pressure pulsations are two times smaller than those of the synchronous component. During a load change, amplitudes of synchronous pressure pulsations are nearly double compared with the asynchronous component. For the turbine startup, only synchronous-type pressure pulsations are found and the flow was asymmetrical over the draft tube circumference.