Simulation and measurement of unstable pump characteristics: comparison with PIV

Pump turbines are mainly designed in pump mode since its retarding flow is more complex and more sensitive to flow detachment. In order to reduce the size of the machine and to gain a better turbine performance, the layout of the pump targets at high pressure coefficient as possible. The operation at high pressure coefficients is limited by the sudden drop of the pump characteristics which typically evolves at a certain head. Due to its shape the unstable pump curve is called saddle curve.
The operation in the saddle curve must be avoided at all since there is no possibility to leave the dropped curve. During the start-up procedure, even a safety margin to the dropped curve must be respected.
Pump turbine designers are faced with the difficulty that the proper prediction of the pump saddle curve with standard simulation tools is very demanding. Therefor it is essential to better understand the detailed flow features during this unstable regime. ANDRITZ HYDRO and the Graz University of Technology investigate in a cooperative research project the key mechanisms leading to the saddle curve in pump mode of pump turbines by means of 2D laser particle image velocimetry (PIV) test rig measurements and various CFD simulations. Furthermore, ANDRITZ HYDRO continuously validates and improves its simulation tools during pump-turbine developments.
In recent pump-turbine projects, ANDRITZ HYDRO succeeded to considerably improve the stability behaviour in pump mode. This success proves that the main mechanisms leading to the unstable pump curve are understood and that the simulation tools have been improved. The paper will demonstrate various CFD tools approaching the proper prediction of the saddle curve. The comparison of CFD results with recent model tests show that the flow effects are manageable and can be specifically influenced during the design phase. The flow features of the instability are outlined with PIV and CFD.