The distribution of OH radicals in a pulsed DC He + H2O plasma jet with the presence of a dielectric material

2021 
The distribution and density of OH radicals in a pulsed DC He + H2O plasma jet are studied by a two-dimensional (2D) plasma jet model. This article shows the effect of adding H2O to the pure He plasma jet on the OH density distribution and the changes of the OH density under different H2O contents. The process of streamer propagation toward the dielectric surface and development of the streamer on the dielectric surface are discussed in this article. The total OH radicals are generated from two parts in a He + H2O atmospheric pressure plasma jet (APPJ). The first part is produced with the water in ambient air outside the tube, and the other part is generated with the water in the helium flow. At the time when the streamer approaches the dielectric surface, it is found that electron-impact dissociation of H2O and electron neutralization of H2O+ are the main reactions to generate OH species and three-body combination of OH and OH with He is the main reaction to consume OH species. The OH density shows a donut shape in a pure He APPJ, while the density distribution of OH radicals under the nozzle gradually becomes uniform with the increase in the H2O content added to the helium flow, but the maximum value is at the both ends of the streamer outside the tube. The electron density initially increases with the increase in the proportion of H2O added to the He plasma jet and then gradually decreases with the H2O ratio greater than 0.5%. The enhancement of Penning ionization with the increasing H2O content is the main reason for the increase in electron density, while the increasing consumption of electrons by the excitation energy loss causes the decrease in the electron density with the H2O ratio more than 0.5%. The increase in the H2O content will bring both positive and negative effects. On the one hand, the increase in the H2O content added to the He gas flow will promote the generation of OH species. On the other hand, the decrease in the electron density caused by the increasing excitation energy loss slows the rapid growth of the reaction rates of the two main reactions related to the production of OH species. Eventually, OH density reaches saturation with the H2O ratio at 1.5% under the combined effect mentioned above.
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