Numerical simulation of draft tube flow in off-design conditions

Abstract

Draft tube flow fields are simulated with 3D unsteady Reynolds averaged Navier-Stokes equations.The purpose of this study is the modeling, simulation and characterization of a complex threedimensional unsteady flow inside a Francis turbine draft tube for two specific off-design conditions: A) Part load (0.88Q), frequently characterized by the occurrence of an unsteady rotating vortex rope linked to strong pressure fluctuations and, B) High load (1.21Q), where softer pressure surges taking place because the cavity volume at this condition has an axisymmetric shape. This work takes place after overhaul works on the actual turbine, which include new runner and wicket gates and modifications on stay vanes and other passageways; where power output, efficiency and stable operating range were increased. The computational domain consists of the draft tube alone. A relative poor mesh (430k nodes) and the k-ε turbulence model are implemented in order to get a quick, but clear explanation to understand how the flow in the existing draft tube responds in front of a new velocity distribution at runner outlet for off-design conditions. Numerical results are qualitative and quantitatively analyzed and compared with experimental data from model and prototype. The unsteady and complex nature of the flow field distribution inside the draft tube for both conditions is visualized