Plasma-based swirling jets

Jet flows characterized by the superposition of an azimuthal velocity component on a conventional round jet are defined as swirling jets. The induction of rotational motion to a free flow generated with a round nozzle enhances heat transfer and flame stabilization capability, making these jets widely used in industrial applications. The swirling motion is usually achieved by passive methods such as guided vanes (helical inserts) or rotating perforated plates. The aim of this study was to validate the feasibility of generating and controlling swirling jets without the use of moving parts or internal inserts. Four pairs of Dielectric Barrier Discharge plasma actuators (DBD-PAs) were instead positioned along the axial direction of the injector and evenly distributed around its circumference, so that the body force exerted by them would impart a circumferential velocity component to the axisymmetric jet flow, allowing the swirl number to be adjusted without any drag penalty and providing the capability of active control. The jet facility at the Politecnico di Torino aerospace department's laboratory Modesto Panetti was reconfigured with a new nozzle (specifically designed and 3D-printed) and a seeding system. The former made it possible to easily change the setup configuration in order to evaluate different cases, while the latter allowed the measurement campaign to be carried out using the Stereoscopic Particle Image Velocimetry technique. The effects induced by the active swirl generators were compared with those of the 3D-printed passive swirl generator by acquiring the flow field at several x/D distances from the nozzle exit section. The swirl numbers obtained using DBD-PAs were found to be lower than those obtained using guided vanes, this being attributed to the dielectric material which did not allow the employment of higher voltages. The study successfully validated the potential of this active control design, identifying areas for future enhancement. A novel actuation approach, consisting in the sequential triggering of the DBD-PAs, was also tested and validated.