Numerical analysis of the S-shape characteristic in a pump-turbine

The paper presents a numerical analysis of the unstable behaviour of a pump-turbine operating in turbine mode near the no-load condition. The entire machine, including the adduction and draft tube, was modelled and leakage flow rates were also considered in the numerical simulations. To validate the numerical model, transient simulations were carried out by means of AnsysCFX 14 on the pump-turbine model in stable operating conditions and the resulting performance was compared with the experimental ones. Then, to study in-depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, an unsteady numerical analysis, running through the flow-speed characteristic up to the pump-turbine brake zone during load rejection, was carried out. The flow rate was decreased at each time step, defined according to an impeller rotation of one degree, and about 20 impeller revolutions were necessary to completely define the unstable operating area.