Numerical study on the production and transport of O and OH in a helium–humid air atmospheric pressure plasma jet interacting with a substrate

Abstract

In this paper, the spatiotemporal developments of OH and O density in an atmospheric pressure helium–humid air plasma jet, both along the effluent and in particular close to the substrate surface, are evaluated under different governing parameters by performing two-dimensional numerical simulation. The results show that reducing the gas flow velocity can increase its density and shorten delivery time to the substrate surface. However, in different jet propagation stages, the dependences of the density distributions of two species on the gas flow speed are different. Before the plasma jet impinges the substrate surface, the O atom has a relatively higher density throughout the jet channel at low gas flow velocity; the increase in the flow velocity makes the peak O density shift to the jet head, whereas the OH radical is mainly distributed near the tube nozzle area and is almost unaffected by gas flow velocity. When the plasma jet moves along the substrate surface, increasing flow speed has little influence on the O density distribution but results in a longer spread length of OH along the substrate surface. By changing the relative permittivity of the substrate, it is found that both the densities and the axial extension lengths of O and OH increase with the relative permittivity, while the radial spread length over the substrate surface decreases. An earlier delivery of reactive species to the surface can be observed for the larger permittivity. The causes for these behaviors are also analyzed and discussed.