Numerical and experimental analysis of instability phenomena in pump turbines

Today, utilities operating pump turbines require fast and frequent changes between pumping and generating modes as well as extended operation at off-design conditions. Operation of the units in unstable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. Two main features of unstable behaviour of pump turbines are known. One sometimes occurs in generating mode at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic) and the other one shows in pump operation as a drop in head as the flow is reduced (saddle-type pump instability of head curve). If the stability characteristics of a pump turbine need to be known already at the design stage of the runner, numerical flow simulation (computational fluid dynamics, CFD) is the most promising tool. As the characteristics of the flow near the stability limit are highly unsteady, steady state CFD, as it is usually applied for the runner design, does not deliver the necessary insight into the flow field. In order to analyse the flow field in runner and diffuser of a pump turbine in the unstable areas of operation, ANDRITZ HYDRO has evaluated several CFD procedures with different approaches for the calculation of the described instabilities in pump and turbine operation. The results of the unsteady flow calculation are compared with model test results.