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.

IntroductionIncreasing the operational flexibility of a pumped storage power plant often involves an extension of the continuous operating range of the pump turbine. Also, large head variations are mostly associated with a wide off-design operating range. This makes it necessary to ensure off-design operating characteristics with low fluid dynamical effects at a high degree of reliability. Especially during pump mode, part-load flow conditions can cause periodically unsteady flow patterns in the distributor section of the pump turbine. The occurrence of such unsteady flow phenomena upstream of the runner like stalling flow or Rotor-Stator-Interaction were investigated by many authors in the past.Mesquita and Ciocan [1] are one of the few authors, who studied the flow in the stay vane channel of a pump turbine focusing on the flow between guide vanes and stay vanes.A very slow rotating stall in the diffuser channel rotating at 1% of the impeller rotational speed was observed in [2] by performing experimental analysis with a centrifugal pump (n q = 21rpm) at Q/Q n = 0.8. Similar observations at a flow rate Q/Q n = 0.8 are reported in [3] and also [4,5] who revealed the onset of rotating stall phenomena. Furthermore, at a partload flow rate Q/Q n = 0.77, Zhang [6] detected by means of instationary LDV-measurements a significant frequency peak at 1-2Hz, induced by periodically unsteady flow.In this work, several off-design operating points are analyzed to get information on the flow behavior in the stay vane channel, including zero-flow and full-load conditions. Measurements of a model pump turbine are used to assess the capability of the simulations to predict transient flow mechanisms. LDV technique is applied to the model pump turbine which provides optical accessibility in the spiral case.A detailed analysis of several quantities at operating point Q/Q n = 0.74 allows a better understanding of the substantially altering flow patterns close to the stay vane channel. This also makes it possible to draw conclusions about the influence of stalled flow on the global flow behavior in the distributor.The following work is based on the paper submitted at the CMFF15 [7].

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