Vortex Breakdown in Decelerated Swirling Flows

The decelerated swirling flows, such as the ones encountered in the discharge cone of hydraulic turbines, may develop a central stagnant region (the so-called vortex breakdown) with possible flow instabilities that hinder the turbine operation at off-design regimes. We investigate numerically such a vortex breakdown phenomenon induced in a decelerated simple swirling flow originating from a constant axial velocity profile and rigid-body rotation. The main finding of the paper is that a rather small alteration of the incoming swirl axial velocity profile can completely eliminate the vortex breakdown. This result could impact the design choices of the hydraulic turbines runner in order to extend their operating range.