Large Eddy Simulations of Francis Turbine Operating at Ultra-Low Loads

Abstract

High-fidelity large eddy simulations (LES) were conducted to characterize the spatial and temporal structure of turbulent flows in an industrial-sized Francis turbine. The unit operated at 50% and 40% of the best efficiency design flowrate. Contours of vorticity, velocity, pressure, and iso-surfaces of Q-Criterion were presented to characterize the effects on the draft tube. Probes placed alongside the draft tube measure the pressure signal to investigate the flow-induced pressure fluctuations inside the turbine unit. The maximum intensity of pressure fluctuations at 50% partial load was 22.66% of the turbine head, while the strength of the pressure fluctuations was 26.36% at 40% partial load. A large number of unorganized smaller vortices observed in the draft tube contribute to the creation of pressure fluctuations. Two pressure modes can be easily recognized (1) high frequency with low amplitude pressure fluctuations and (2) low frequency with high amplitude fluctuations. These pressure fluctuations could be harmful to the structural integrity of the unit and also have undesirable influences on the operational stability of the hydro-turbines.