Effect of humidity on streamer propagation process in dielectric barrier discharge plasma actuator*

Dielectric barrier discharge plasma actuators (DBD-PAs) have been developed as active control devices to improve the aerodynamic properties of fluid machinery. Since air, the working fluid in practical fluid machinery, contains moisture, it is necessary to investigate the humidity effect on the performance of DBD-PAs. Although several previous studies experimentally investigated the effect of humidity on discharge current and body force of DBD-PAs, the humidity effect on streamer propagation process is unclear. To clarify this effect, we conducted two-dimensional plasma simulations based on the drift-diffusion approximation for DBD-PAs under different humidity conditions. In this study, a simple three-fluid model is employed, which considers electrons, one generic type of positive and negative ions. To reveal the fundamental characteristics of a single positive streamer, a positive step voltage of 3 kV is applied to a DBD-PA. A notable difference is that the addition of water vapor increases the propagation speed of the positive streamer and the electron number density at the streamer head. This result is attributed to increase in the ionization coefficient in high reduced electric field region above 200 Td due to the addition of water vapor. Consequently, humidity activates the chemical reactions of the electrons generated during the streamer head formation and lead to increase the electron number density at the streamer head.