Investigation of steady and unsteady cavitating flows through a small Francis turbine

Abstract

The evaluation of cavitation characteristics is one of the most significant tasks whilst properly designing a water turbine. This work investigates the steady and unsteady cavitating flows through a small Francis turbine, based on the two-phase mixture model. Detailed flow field analysis with and without cavitation is performed for three operating regimes namely the Best Efficiency Point (BEP), part-load (OP1), and over-load (OP2). The effect of reducing the cavitation number on the hydraulic performance is assessed, and the range of safe operation in respect of cavitation-free is revealed. Under specific operating conditions the structure of cavitation in the runner and the evolution of the vortex rope in the draft cone are shown. The part-load operation gives rise to undesired phenomena of pressure pulsations due to the vortex rope generated at the runner outlet. Moreover, the pressure fluctuations and the torque oscillation seem to be more pronounced at part-load and over-load operating conditions. The obtained results are encouraging and may help in designing and testing the small Francis turbines which behave differently compared with the large-scale models.