Analytical and Numerical Studies of Internal Swirling Flows

Doktorsavhandling, 2008

Swirling flows are common in technical applications, e.g. turbines, pumps, fans, compressors and combustors. The objectives of the present work are to acquire an understanding of the physics of swirling flow in general and unsteady swirling flow in draft tubes of hydro turbines in particular, and to find a simulation method suitable for industrial purposes. An analysis was made of the quasi-cylindrical approximation of the Navier-Stokes equations. The analysis shows that there are no quasi-cylindrical solutions at certain critical levels of swirl. Furthermore, it is shown that this property of the equations is connected to the vortex breakdown phenomenon, i.e. the sudden change of flow structure often observed in swirling flow. In draft tubes in hydraulic power plants, a vortex breakdown gives rise to a precessing vortex core that induces a temporally periodic load on the machine, which in turn causes bearing wear and increases the risk of fatigue failure. An industrially applicable hybrid LES/RANS method was generalized and employed. The hybrid method is based on a dynamic filtering procedure of the turbulent length and time scales obtained from an eddy-viscosity RANS turbulence model. The method has been used for detailed investigations of the dynamic behaviour of swirling flows through a sudden expansion and a hydro turbine draft tube. It is shown that the filtering procedure yields solutions that contain accurate unsteady information. In addition, the time-averaged results obtained using the filtered model are significantly better than those obtained using other hybrid methods and unsteady RANS simulations.