Vortex breakdown modes in co/counter-swirling non-reacting and reacting flows

The vortex breakdown mechanism in swirling flows is not fully understood yet despite being a classical problem seen in many applications. The complexity increases further when there are concentric swirling streams. In the present study, the vortex breakdown (VB) modes of concentric co- and counter-swirling streams are investigated at various momentum ratios (M) by employing both stereo-particle image velocimetry and two-dimensional particle image velocimetry under both non-reacting and reacting conditions. For co-swirling flows at low M, a bluff-swirl vortex breakdown mode is observed, whereas a bubble vortex breakdown mode is prominent in counter-swirling flows. The VB mode changes when M increases; however, the transition occurs at different M for co- and counter-swirl. At high M, both co- and counter-swirl show signatures of a conical breakdown mode (CVB). Similar VB modes and their transitions are also observed for reacting conditions. The strength and shape of the recirculation zone and the axial and azimuthal vorticities are compared to those in single swirling flows. Notably, the CVB modes in co- and counter-swirling flows share some common aspects with the CVB modes of single-swirling flows. In addition, the underlying physical mechanisms for different breakdown modes are discussed.