The effect of methane gas flow rate on the streamer propagation in an atmospheric-pressure methane-air plasma jet

Abstract

In this paper, a two-dimensional axisymmetric fluid model is applied to investigate the streamer discharge characteristics in an atmospheric pressure methane-air plasma jet as a function of methane flow velocities (2.5 m/s and 20 m/s, respectively). Although the streamer ignition and propagation in the dielectric tube are not sensitive to the methane gas flow velocity, the concentration field of methane and air in the mixing layer established by a balance between convective methane flow and back-diffusion of air ambient is crucial for streamer propagation in the gap. As the methane flow velocity is 2.5 m/s, the structure of the streamer head transits from ring-shape into solid disk-shape, while the streamer head always maintains a donut-shaped pattern at high flow velocity of 20 m/s until it impinges on the substrate. At lower gas velocity, the back-diffusion of ambient air into the methane jet is even more pronounced, which causes a larger space charge density at the streamer head, and thus the local electric field near streamer head is greater. Therefore, the overall trend in streamer propagation speed versus methane flow velocity is that the larger the flow velocity, the lower plasma bullet speed. Besides, as the gas flow velocity increases from 2.5 to 20 m/s, less oxygen/nitrogen radical species and charged ions are produced in the streamer discharge, while the produced methane-related particles increase slightly. As for different methane flow velocities, the streamer advances within the methane core.