Unsteady Flow Analysis of Pump Mode Small Discharge Condition for a Francis Pump-turbine

Abstract

Unsteady flow phenomena, including vortex flow at runner inlet, helical backflow in the draft tube and numerous vortexes inside the guide vanes, can occur in pump-turbines under off design conditions at pump mode and can impact normal operation of pump-turbines. All of these phenomena cause serious pressure pulsation, which is quite different from cases in normal pump mode. There is also a difference of pressure pulsation frequency and amplitude in different place through the runner. This paper builds a whole flow passage of a model pump-turbine, simulates flow characteristics in runner by CFD technology, analyses pressure pulsation in the runner and explores the origin and mechanism of pressure pulsations. The SST-CC turbulence model is adopted to perform unsteady simulations of the pump-turbine under 0.46QBEP small discharge condition at pump mode. Unsteady flow structures are proceeded combined with hydraulic loss and pressure amplitude spectra. The results indicates that there is complicated disordered flow inside the runner under 0.46QBEP small discharge condition at pump mode, shows the amplitude and frequency characteristic of pressure pulsations through runner flow passage.