Passive and active (plasma based) swirling jets

Jet flows characterized by the addition of an azimuthal velocity component to a conventional round jet are referred to as swirling jets. These flows enhance heat transfer, support flame stabilization, and improve turbulent mixing in combustion processes, making them widely applicable in industrial settings. Swirling motion is typically generated using passive methods, such as guided vanes (helical inserts) or rotating perforated plates. While these passive techniques are reliable, they are optimized for specific design parameters and may become less effective, or even counterproductive, if the flow conditions deviate from these parameters. Additionally, due to the internal inserts, these methods can induce flow blockage, resulting in a pressure drop and requiring additional energy consumption. Alternatively, active swirl generators can be employed, imparting a tangential velocity component to the axial jet flow, while allowing for real-time control of the swirl intensity. In this study, swirling fluid motions were induced using four Dielectric Barrier Discharge plasma actuators (DBD-PAs), positioned along the axial direction of the injector and evenly distributed around its circumference. The actuators extended approximately two nozzle diameters along the axial direction (z) and were mounted at the nozzle outlet to generate the tangential velocity component. By varying the supplied peak-to-peak voltage (V_{pp}), the impact of different control inputs on the resulting jet flow was examined. Moreover, the actuators electrical characteristics were also assesed in ad hoc experiments. The experimental campaign was conducted at the Politecnico di Torino's aerospace laboratory Modesto Panetti where Stereoscopic Particle Image Velocimetry (S-PIV) was employed to measure the velocity field at several axial locations downstream of the nozzle exit. The results revealed that the swirl numbers achieved using DBD- PAs were comparable to those obtained with a passive swirler with 4-guided vanes. However, the flow field produced by a passive swirler with 4-outer vanes qualitatively mimicked the effect of the DBD-PAs but did not match the swirl number produced by the active devices. This study confirmed the feasibility of the active control strategy, demonstrating its ability to generate and regulate swirling jets without the need for moving parts or internal inserts. Finally, the case study was extended by increasing the jet exit velocity to determine whether the effect of the active swirlers remained significant under these new conditions. It was found that, under the higher velocity scenario, the DBD-PAs were less effective compared to the passive swirlers due to the reduced azimuthal momentum they introduced, which was insufficient relative to the axial momentum of the jet.