Inviscid Vortex Breakdown in Decelerated Swirling Flows and Flow Control Methods
The paper is focused on decelerated swirling flow behavior as the upstream swirl intensity is increased. We define simplified flow model and boundary conditions in order to analyze the basic flow features. Numerical experiments are carried out for incompressible, inviscid, axisymmetric flow, with incoming Burgers vortex in a sinusoidal shaped diffuser. As the swirl intensity increases we find first a steady flow without breakdown, then a steady axisymmetric vortex breakdown with a central stagnation region, and finally an unsteady vortex breakdown with vortex rings developed and convected downstream. The main result of the paper is that the unsteady vortex breakdown can be removed by injecting a weak axial jet on the inlet section, with no more than 0.5% of the overall discharge. This flow control technique is particularly relevant for swirling flow in the draft tube cone of Francis turbines operated at part load.