Numerical Study Of The Influence Of Dielectric Tube On Atmosphere Pressure Plasma Jet Based On Coplanar Dielectric Barrier Discharge

Atmospheric pressure plasma jets(APPJs)can be produced outside the discharge electrodes in ambient air.Due to the advantages of high electron temperature,low gas temperature,highly active species and easy controllability,APPJs have been widely used in health care and biomedicine,materials processing,environmental remediation and pollution control and other fields.Better understanding of the characteristics of APPJ is essential for extending and utilizing APPJ technology as well as optimizing the designs.In this thesis,fluid model was employed to numerically investigate the formation and propagation of APPJ based on coplanar dielectric barrier discharge(DBD)in helium.Firstly,the characteristics of APPJ are introduced based on experiments.The formations and developments of DBD and APPJ are obtained through time-resolved ICCD camera.The velocities of plasma jet in dielectric tube and air are calculated.The jet velocity changes dramatically when traveling at the interface of two dielectrics.It is considered that the diffusion of the air and the change of dielectric is responsible for this change.Besides,the ring-shaped structure is observed in experiments.The diffusion of the air into the helium flow and the property of dielectric tube might play an important role on the formation of ring-shaped structure.Secondly,a two-dimensional fluid model in a symmetric configuration is developed.The evolutions of DBD and APPJ in pure helium are simulated.The spatiotemporal distributions of electron/ion density,spatial charge,electric field and the APPJ structure are obtained under different conditions.The results show that the APPJ is formed after the DBD bridges the two electrodes.The high-density ions above the cathode induce a strong axial electric field pointed outwards.Which makes the discharge develop forwards near the tube surface that limits the helium flow and sustains the APPJ outside the cathode.In the meantime,there exists a radial field in the plasma jet,causing the APPJ tend to shrink to the central axis.As a consequence,the APPJ will shrink gradually and change from a hollow structure to a solid structure.Thirdly,the influence of the relative permittivity,diameter,thickness of dielectric tube on DBD-based APPJ are studied.The results show that the change of the relative permittivity has great impact on the jet propagation but not on the DBD itself.The jet velocity changes drastically when passing from a tube of higher permittivity to lower one,resulting in an increase in electric field and jet length.This change around the interface of dielectrics is caused by the change of the capacitor,enhancing the axial field and promoting the discharge.The radius of dielectric tube has great influence on the jet structure.A smaller tube makes the jet to be solid structure and the length of plasma jet increasing.The thickness of tube has no effect on the jet propagation,but has a little influence on the DBD.The present numerical results can well explain the results obtained by experimental measurements,such as unique behavior of plasma jet from the dielectric tube into the ambient air and the hollow structure of plasma jet(or ring-shaped bullet)in experiments.It is expected that the present work is helpful for controlling the APPJ in applications.