Droplet impact on a sliding dielectric barrier discharge plasma: dynamic and thermal response

Three-electrode dielectric barrier discharge actuators comprising two exposed electrodes and an encapsulated one generate a nonthermal plasma sheet known as sliding plasma (SP-DBD). While the most common type of DBD actuator is a two-electrode configuration, the streamwise length of the plasma region is limited to about 1 cm. SP-DBD actuators exhibit a uniform plasma density across a larger surface delimited by two exposed electrodes. The discharge is accompanied by an induced air current and significant heat generation which can be exploited for flow control, de-icing of aircraft wings, or disinfection of surfaces. Under said operating conditions, interactions of water droplets with active actuators may occur.

In this study, high-speed and infrared thermal imaging were used to characterize the electrodynamic and thermal effects of water droplets impacting the plasma region of an SP-DBD actuator. Filamentary plasma structures conforming to the sliding plasma sheet were observed. Upon impact, the droplet spreads due to inertial effects and Coulomb forces. Plasma discharges took place radially outward at the droplet’s edge, effectively disrupting the otherwise one-directional filamentary structures. Furthermore, the droplet obstructed the path between electrodes, preventing the plasma filaments from reaching the downstream electrode. This creates a “wake” in the discharge. Infrared images revealed that the wake has a significantly lower temperature than its surrounding areas, creating a large temperature gradient in the spanwise direction of the actuator.

A secondary effect of the water droplet on the discharge surface was an intermittent electric arc formation across the electrodes. The occurrence of arc discharges, as well as the thermal response of the plasma-droplet system, are presented with respect to droplet location relative to the exposed electrodes.