Towards plasma-based swirling jets

Swirling jets, generated by superimposing an azimuthal velocity component onto a conventional round jet, find widespread utility in industrial applications for their enhanced heat transfer and flame stabilization capabilities. Traditional methods for inducing swirling fluid motions rely on passive techniques, such as guided vanes (e.g. helical inserts) or rotating perforated plates, while emerging approaches, like dielectric barrier discharge plasma actuators (DBD-PAs), offer active control without moving parts. This work emphasizes the performance of passive swirl generators designed to impose predetermined swirl numbers (S = 0.1, 0.2, 0.3) and presents a representative active case - employing an array of DBD-PAs - to highlight differences in swirl intensity and flow behaviour between the passive and the active approach. The experimental campaign, conducted using stereoscopic particle image velocimetry, focused on evaluating the induced flow field characteristics. The PAs-induced flow swirl number was compared against the passive cases with known design swirl numbers, using a novel proposed approach. The results hereby discussed indicate that passive methods produce higher swirl numbers (S = 0.11-0.29) compared to the active case (S = 0.08), due to limitations imposed by the dielectric material used for the PAs embodiment. Radial velocity profiles, vorticity fields, and three-dimensional reconstructions highlight the superior entrainment and uniformity of passive systems, whereas the active configuration exhibits localized tangential velocity generation near the injector walls with minimal impact on the jet core.