Investigation of the Flow Instabilities of a Low Specific Speed Pump Turbine Part 1: Experimental and Numerical Analysis

This is the first part of a two-part paper focusing on the flow instabilities of pump turbines. In this first part, results of the experiments and computational fluid dynamics (CFD) simulations of the research carried out on a low specific speed model pump turbine at HSLU (Lucerne University of Applied Sciences) Switzerland are presented. The requirements of a stable and reliable pump turbine operation, under continuously expanding operating ranges, challenge the hydraulic design and demand new developments. This paper presents the results of experimental (model pump turbine at the test rig) and numerical (CFD) investigations of the pump turbine instabilities of a low specific speed (n(q) = 25) pump turbine in the turbine operating mode. Four-quadrant characteristics of a low specific speed pump turbine with two similar runners differentiating in the size (diameter) are measured. Testing of both runners with the same guide vane system provided information about the effects of the increased vaneless space on the pump turbine performance and stability. A CFD methodology is developed by applying several turbulence models to accurately predict the characteristics of the reversible pump turbines in the S-shaped region (speed no load conditions) as well as to analyze the flow features especially at off-design conditions. This CFD model is validated against the experimental data at 6 deg and 18 deg guide vane openings in turbine operating mode. Using the data of the unsteady pressure measurements and detailed investigation of unstable ranges on the pump turbine characteristics, flow instabilities in the low-specific speed model pump turbine are analyzed. Relevant frequencies such as rotating stall, steady and unsteady vortex formations are determined.