Numerical Investigations of Swirling flow in a Conical Diffuser

Paper in proceeding, 2004

When a hydraulic power plant is operating at off-design, a swirling flow exits the runner and is convected through the draft tube. The swirling flow gives rise to several features that decrease the efficiency and/or may damage the construction. As a step towards making reliable numerical predictions of the swirling flow in draft tubes, steady computations in an idealised model have been carried out. The model geometry is a straight conical diffuser and O. G. Dahlhaug of NTNU, Trondheim, Norway, has provided experimental data. In this paper, numerical 3D RANS investigations are quantitatively compared to the available experimental data. Good agreement with experimental data was obtained. The discrepancies are partly reminiscent of the nature of the k-omega turbulence model that was used in this work. Given symmetric geometry and boundary conditions, a fluid flow is most often thought to behave in an equally symmetric manner. If the flow is swirling, this is not generally true. Due to the unstable properties of the symmetric solutions for the averaged Navier-Stokes equations, the flow will collapse into an asymmetric mode. In the first computational cases of this paper, the disturbance that triggers the instability is shown to be imperfections in the CAD-geometry. Also included is a discussion concerning the development of counter-rotating vortices in the boundary layer of swirling flow in a circular pipe.