High-speed schlieren and particle image velocimetry of a highly-efficient atmospheric plasma jet*

Cold plasma jets present challenges in characterization due to their multifaceted, non-equilibrium, and transient nature, necessitating high spatiotemporal resolution. Building on our work from last year, we systematically perform flow visualization of an atmospheric plasma jet designed with a novel mechanism. This energy-efficient jet is a helium plasma plume exiting from a glass capillary tube with an inner diameter of 0.9 mm, which passes through a gap region of an evanescent-mode cavity resonator. The specific design of the cavity concentrates the electric field mainly over the 100s of µm critical gap, and thus gas breakdowns even at very low values of input microwave power. The plasma jet is generated at 2.45 GHz with an input power in the range of 0.5 to 10 W. A Z-type high-speed schlieren imaging system is used to study the hydrodynamic characteristics of the jet within a steady-state flow rate range of 1 to 7 SLPM. We perform imaging of the jet both with and without plasma to clearly observe the effects of plasma actuation on parameters such as jet core size, spread angle, shear layer unsteadiness, and the mixing of the helium jet with the surrounding air. Additionally, we introduce sodium chloride crystals into the plasma jet to create a so-called dusty plasma. These particles are leveraged as seeding to perform high-speed planar PIV, allowing to quantify the jet’s velocity statistics. This benchmarking study offers detailed information for potential applications in flow control and propulsion systems.