Experimental and numerical investigation of the speed-no-load instability of a low specific speed pump-turbine with focus on the influence of turbulence models

Abstract

A reduced scale model of a low specific speed pump-turbine with 7 runner blades and 20 guide vanes is experimentally and numerically investigated. Main goal is to identify the onset, origin, and development of flow instabilities. The four-quadrant characteristic of the pump-turbine is experimentally determined. The focus of this paper is on the turbine mode operation at off-design conditions involving runaway and the “S-shape” turbine characteristic. 3D, unsteady numerical simulations are performed using the CFD codes ANSYS-CFX and an in-house code. The computational domain includes the entire flow passage from the spiral casing inlet to the draft tube exit. For turbulence modeling the SST k-ω, standard k-ε, and BSL EARSM models are applied. The numerical results at two guide vane openings (20° and 6°) and different operating points at each guide vane opening are compared to the experimental results. The comparisons between the CFD predictions and experimental data shows that the CFD predictions of all turbulence models are in good agreement with the experimental data for 20° guide vane opening whereas for the 6° guide vane opening, which is critical for the synchronization, only the k-ε and BSL EARSM turbulence models are showing a reasonable agreement with the experimental data. Based on the detailed analysis of the experimental data and CFD results focusing especially on the flow features in the vaneless space and at the runner inlet, the onset and development of the flow instabilities are explored.