Numerical Modeling Of Plasma Streamers Guided By Multi-Ring Electrodes In Atmospheric Pressure Plasma Jets

As a non-thermal equilibrium plasma discharge technology,atmospheric pressure plasma jets(APPJs)have attracted much attention.It has become one of the research hotspots in the area of atmospheric-pressure plasma discharge.Compared with the traditional non-equilibrium plasma discharge technology,the atmospheric pressure plasma jets do not require a complicated vacuum chamber,which overcomes the shortage of the narrow discharge space.The plasma temperature of APPJs is close to the room temperature,meaning that it has better performance in treating temperature-sensitive materials.Furthermore,the atmospheric pressure plasma jet can separate the discharge area from the treatment area,thereby enhancing the stability of the discharge system.Based on the above advantages,the plasma jet has extremely wide application prospects.To better understand the plasma plume behaviors,such as plume shape and propagation characteristics,experiments and numerical simulation studies on the atmospheric pressure plasma jet have been done.It is reported that in the atmospheric pressure plasma jets,the propagation characteristics of the streamer heavily depend on the electric field established by the electrodes.Placing a ring electrode on the outsider of the dielectric tube is an effective way to ignite the plasma discharge and promote the streamer/ionization wave to propagate inside and outside the dielectric tube.Although the shape and propagation of streamers have been concerned,the mechanism of the formation of various plume structures is still unknown,and further study is desirable.A two-dimensional axisymmetric model is used to simulate APPJs with a needle and multi-ring electrode in this work.In Chapter 3.1,based on the needle and multi-ring electrode model,the effect of the number of ring electrodes and their arrangement positions on the discharge characteristics is discussed.The layout of multi-ring electrode outside the dielectric tube can determine the potential gradient or electric field inside the dielectric tube,and thereby control the discharge intensity,streamer behavior,and plume formation.(1)The increasing amount of grounded ring electrode promotes the streamer propagation as well as leads to a longer plasma plume.(2)Putting more ring electrodes within a certain length around the dielectric tube can increase the propagation speed of streamer.(3)If all the ring electrodes are arranged close to the needle,the streamer length is shorter,but the instantaneous speed of streamer propagation is higher.However,when the space between the adjacent ring electrodes is over large,the electric field may be insufficient to promote the propagation of the plasma jet.In Chapter 3.2,the needle-quad-ring electrode structure is employed to further explore the influence of voltage polar on plasma propagation.The voltage polarity and proper arrangement of the ring electrodes are essential for accelerating the propagation of streamers.(1)When the ring electrode is grounded,increasing the pulse voltage on the needle electrode can make the plasma plume propagate faster and further.(2)The performance of the needle and ring electrodes with identical voltage polar but with different amplitudes is the same as that reducing the needle voltage(ring electrodes are grounded).Conversely,the needle and ring electrodes with opposite voltage polar increase the potential gradient and enhance the discharge.(3)By connecting the ring electrodes to positive or negative voltage polarity alternately,the potential gradient is changed,along with the propagation speed and plume shape.When the potential gradient is sufficiently high between adjacent ring electrodes,it may ignite discharge directly between ring electrodes which belongs to a typical dielectric barrier discharge.In Chapter 3.3,the influence of the width of the grounded ring electrode on the discharge characteristic based on the needle-quad-ring electrode model is further studied.The results are shown in the following.(1)The wider ring electrode can build a thin cathode sheath near the ring electrode,and more obvious annular structure of the jet can be observed;(2)There are four peaks of propagation speed related to the four-ring electrodes.The streamer with the wider ringelectrodes propagates faster at first and then slides more slowly than with the narrower ringelectrodes.It is shown that the last three peaks of streamer speed with the wider ring-electrodes are smaller than with the narrow ring-electrodes.