Numerical simulation of hydrodynamics in a pump-turbine at off-design operating conditions in turbine mode

• Published in: IOP conference series. Earth and environmental science Vol. 15; no. 3; p. 32041
• Main Authors: Yan, J P, Seidel, U, Koutnik, J
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 26.11.2012
• Subjects: Amplitudes; CAD; Computational fluid dynamics; Computer aided design; Computer applications; Design; Design optimization; Discharge; Draft tubes; Drafting software; Fluid flow; Fluid mechanics; Guide vanes; Hydrodynamics; Impellers; Mathematical models; Rotating stalls; S curves; Simulation; Turbines
• ISSN: 1755-1307; 1755-1315
• DOI: 10.1088/1755-1315/15/3/032041

The hydrodynamics of a reduced-scaled model of a radial pump-turbine is investigated under off-design operating conditions, involving runaway and "S-shape" turbine brake curve at low positive discharge. It is a low specific speed pump-turbine machine of Francis type with 9 impeller blades and 20 stay vanes as well as 20 guide vanes. The computational domain includes the entire water passage from the spiral casing inlet to the draft tube outlet. Completely structured hexahedral meshes generated by the commercial software ANSYS-ICEM are employed. The unsteady incompressible simulations are performed using the commercial code ANSYS-CFX13. For turbulence modeling the standard k-ϵ model is applied. The numerical results at different operating points are compared to the experimental results. The predicted pressure amplitude is in good agreement with the experimental data and the amplitude of normal force on impeller is in reasonable range. The detailed analysis reveals the onset of the flow instabilities when the machine is brought from a regular operating condition to runaway and turbine break mode. Furthermore, the rotating stall phenomena are well captured at runaway condition as well as low discharge operating condition with one stall cell rotating inside and around the impeller with about 70% of its frequency. Moreover, the rotating stall is found to be the effect of rotating flow separations developed in several consecutive impeller channels which lead to their blockage. The reliable simulation of S-curve characteristics in pump-turbines is a basic requirement for design and optimization at off-design operating conditions.