Active Control of Swirling Coaxial Jet Mixing with Manipulation of Large-Scale Vortical Structures

Large-scale vortical structures and associated mixing in methane/air swirling coaxial jets are actively controlled by manipulating the outer shear layer of the outer swirling coaxial jet with miniature flap actuators. In order to investigate the control mechanisms, stereoscopic particle image verocimetry (stereo-PIV) and plannar laser-induced fluorescence (PLIF) techniques are employed. It is found that intense vortex rings are produced in the outer shear layer in phase with the periodic flap motion regardless of the swirl number examined. The vortical structures in the inner shear layer, however, are strongly dependent on the swirl rate. This is because the central methane jet is accelerated by the negative axial pressure gradient, of which strength is determined by the swirl. As a result, the inner vortex formation is significantly suppressed at a higher swirl rate. On the other hand, at a relatively low swirl rate, the inner vortices are shed continuously and the methane jet is pinched off. This particular mode promotes the mixing of methane and air, so that the flammable mixture can be formed at an earlier stage of the jet flow development. In addition, the evolution of secondary streamwise vortices is prompted by the combination of the periodic vortex ring shedding and the swirl. They also contribute to the mixing enhancement in the downstream region.