Investigations of unsteady pressure loading in a Francis turbine during variable-speed operation

Current study was aimed to investigate the unsteady pressure loading in a model Francis turbine under variable-speed configurations. Focus was to investigate the time-dependent characteristic frequencies and the pressure amplitudes. Detailed analysis of both stochastic and deterministic pressure loading in the vaneless space, runner and draft tube was conducted. Total 12 pressure sensors were integrated in the turbine, including four sensors in the runner. The runner rotational speed was changed by ±30% of the rated speed, and the guide vanes were at a fixed aperture. Total four operating conditions were investigated. The measurements showed that, in the vaneless space and runner, amplitudes of unsteady pressure fluctuations increase with the runner angular speed. Pressure field at the blade trailing edge is strongly influenced by the draft tube flow at part load and low load. The variable-speed configuration allowed power generation under the stable condition, where the vortex rope effect is low. However, this led to high-amplitude stochastic frequencies in the runner and draft tube. Overall, the pressure measurements indicated that not only efficiency but also detailed study on pressure fluctuations inside the turbine is vital before designing a runner for the variable-speed configurations.