Numerical and Experimental Investigation on Unsteady Inflow Conditions of a Pump-Turbine at Low Load

This paper examines the occurrence of prerotation and reversal flow in the conical draft tube of a pump-turbine by using different turbulence models and compares the results to experiments. The computational domain consists of the entire geometry of a reduced scale pump-turbine. The results based on time-dependent computational fluid dynamics (CFD) are compared to laser doppler velocimetry (LDV) and wall-pressure-measurements in the conical part of the draft tube. Beside the LDV measurements, pressure fluctuations induced by complex flow patterns are also recorded and analyzed. The capability of simulations is assessed by an evaluation of the global integral values of the pump-turbine. The velocity profiles in axial and circumferential directions are compared at two measurement planes for two part-load operating points. The increased wall pressure distribution caused by swirling inflow is compared to the time averaged wall static-pressure from experiments. When operating at unstable pump conditions, an unsteady flow behavior arises in form of co-rotating vortices upstream of the impeller inlet. Analysis of the inlet flow shows continuously appearing and decaying vortex ropes in the conical draft tube. On the basis of these observations, discrete fourier transformation (DFT) analysis provides the power spectrum of the simulated time dependent pressure signal in the draft tube cone, where significant peaks below the runner rotational frequency are observed. The spectral analysis applied to transient pressure measurements at the draft tube wall shows dominant peaks in the low frequency region, which may indicate weak vortex structures rotating at low frequency.