Numerical Study Of Propagation Of Ar Plasma Jet At Atmospheric Pressure And Generation Of Reactive Species

Atmospheric-pressure plasma jets(APPJs)have attracted much attention due to their good performances in applications ranging from material processing,environment,to biomedicine,which has already become one of the most important research fields.APPJs are multi-parameter systems,which means that any parameter condition such as electrode structure,driving voltage,working gas,ambient environment,could significantly influence the performance of APPJs.Therefore,deep insight into the dynamic characteristic of APPJs under different discharge parameters is necessary for optimizing and controlling APPJs' behaviors to meet the detailed demands of applications.In this paper,a two-dimensional fluid model coupling with a neutral gas dynamic model is used to study the properties of atmospheric-pressure argon plasma jet propagating into ambient nitrogen driven by pulsed voltage,with emphasis on the influence of the voltage amplitude and gas flow rate on dynamic characteristics of the plasma jet and the generation of reactive species.The results are concluded in the following:The simulation results under certain parameters show that the argon APPJs propagating into ambient nitrogen exhibits a cylindrical shape channel.With the increase of propagation length,the jet channel gradually shrinks and the front gradually becomes conical.The propagation speed of the plasma jet varies with time.Inside the dielectric tube,the plasma jet accelerates propagation,and reaches its maximum at the nozzle.Then after exiting from the tube,the propagation speed of plasma jet quickly decreases,and finally changes slowly.The argon species are mainly generated inside the dielectric tube and delivered through the argon flow channel.These species have a similar density distribution as electron,which is uniform along the radial direction.The species related to nitrogen are mainly generated at the mixture layer of Ar-N2 interface outside the tube and transported along the mixture layer.Their densities are extremely low at the central axis and thus form a ring-shaped hollow structure.The flow rate of working gas is a key factor that affects the APPJ characters.The simulation results of different argon flow rates show that under the condition of laminar flow,as the increase of flow rate,the argon molar fraction inside the jet channel increases,and resulting in the increase of jet length.However,when the flow rate exceeds a critical value,the length of plasma jet fluctuates with the flow rates,and after that,it increases slowly.Inside the dielectric tube,the jet propagation speed is unaffected by the flow rate,but after leaving the tube,the propagation speed of plasma jet increases with the flow rate.When the flow rate exceeds a critical value,the propagation speed becomes irregular.The effect of flow rate on the averaged density of active species is insignificant,but the effect of flow rate on the spatiotemporal distribution of active species density is apparent.When the flow rate is low,more nitrogen can mix into the argon channel,and thus the densities of argon species decrease along with the axial and radial directions.Simultaneously,the ring-shaped diameter of nitrogen species reduces,and the maximum density moves towards the central axis,leading to the hollow structure disappearing.Another key factor that determines the propagation behaviors of plasma jet is the electric field in the ionization front.The more intensive the electric field,the more energy that charges can obtain in the same free path,the stronger ionization can occur in the ambient gas,and the plasma jet can propagate longer.The simulation results of APPJs with different driving voltage shows that when the voltage amplitude is relatively high,the electric field in the plasma head is intensive,the plasma could propagate further and the jet channel is wider.When the driven voltage is increased from 5 k V to 12 k V,the length of plasma jet almost increases linearly with driven voltage.Commonly,argon APPJ performs a cylindrical shape with a conical plasma front.However,when the driven voltage is lower and the jet is shorter,the plasma jet exhibits a ring-shaped structure.It is because the ionization wave slides along the inner surface of the dielectric tube.With the driven voltage increases,the ring-shaped structure disappears.The density of active species increases with the applied voltage,especially the active species generated from direct impact ionization,which has a significant increase in density.