Analysis Of Atmospheric-pressure Nanosecond Pulsed Coaxial Dielectric Barrier Discharge Characteristics

Plasma is widely used in various fields due to its unique properties,so it is important to explore how to generate uniform,soft,large and highly active plasma at atmospheric pressure.Dielectric barrier discharge(DBD)is considered to be an effective method to obtain plasma and has received extensive attention.In recent years,with the urgent demand for high-energy and low-consumption DBD generators and the development of pulse power technology,the combination of nanosecond pulse technology and DBD has become an emerging research hotspot in the field.Although some work has been carried out on atmospheric-pressure nanosecond pulsed DBD in recent years,there are still some problems.For example,most of these studies are based on parallel plate DBD,and there are few studies on coaxial electrode configuration.In addition,most of these studies focus on improving the discharge intensity by optimizing the parameters of pulse power supply or the structural parameters of DBD,and few studies focus on the discharge efficiency of DBD and explore new ways to improve the discharge intensity.Therefore,in this thesis,a one-dimensional fluid model of argon nanosecond pulsed DBD at atmospheric pressure is established by COMSOL Multiphysics software,and the accuracy of the model is verified.Based on this model,the discharge mechanism and evolution law of atmospheric pressure nanosecond pulsed coaxial DBD are studied in this thesis.The effects of pulse voltage parameters and coaxial DBD structure parameters on discharge characteristics are discussed.In addition,a method of increasing DBD discharge intensity by increasing the number of electrons is proposed.The specific research contents are as follows:(1)The evolution process of voltages,current density,electron density,ion density and accumulated charge on the dielectric layer with time in atmospheric pressure nanosecond pulse coaxial DBD are simulated and calculated.The spatial distribution of electron density,ion density and spatial electric field at different times is obtained.On this basis,the discharge mechanism and evolution law are analyzed.The results show that there are two opposite discharges in one discharge cycle,and the discharge intensity of the first discharge is significantly higher than that of the second discharge.These two discharges occur at the rising edge and falling edge of the pulse voltage respectively.The evolution process of the first discharge and the second discharge is similar,and both discharges are affected by the space residual charge.(2)The effects of pulse power parameters(pulse voltage amplitude,pulse voltage rise and fall time and pulse width)and structural parameters of coaxial DBD(relative dielectric constant,dielectric layer thickness and discharge gap width)on discharge characteristics were systematically studied.The characteristic parameters used to describe the discharge intensity are: the peak current density of the first discharge and the second discharge,the average dissipated power density,the electron density and the average electron energy.In addition,a ratio R is defined to describe the discharge efficiency.R is the ratio of total discharge current to total dissipated power density in a discharge cycle.The results show that increasing the pulse voltage amplitude can significantly improve the discharge intensity and discharge efficiency of DBD,especially when the pulse voltage amplitude increases from 1000 to 1500 V,the growth rate of discharge intensity and discharge efficiency is the fastest.When the pulse voltage rise and fall time increases,the discharge intensity increases,and the discharge efficiency increases first and then decreases.When the pulse voltage rise and fall time is 1000 ns,the discharge efficiency is the highest.When the pulse voltage width increases,the discharge intensity and discharge efficiency of DBD decrease slightly.The reason for this change is related to the space residual charge.Increasing the relative dielectric constant or reducing the thickness of the dielectric layer will increase the discharge intensity and discharge efficiency of DBD.When the discharge gap width increases,the spatial electric field intensity increases,the average electron density decreases,the peak current density increases first and then decreases under the influence of these two factors,the average dissipation power density decreases,and the discharge efficiency increases.(3)A method to improve the discharge intensity by increasing the number of electrons is proposed,that is,the hot electron emission material is loaded on the coaxial DBD reactor.The effects of pulse voltage amplitude and temperature on the discharge intensity of coaxial DBD and coaxial DBD with hot electron emission material are compared and studied.The characteristic parameters describing their discharge intensity are: two current density peaks and the density of excited argon atoms at the time of the first discharge current density peak.The results show that when the temperature is 1073 K,the discharge intensity of coaxial DBD and coaxial DBD with thermal electron emission material increases with the increase of pulse voltage amplitude.When the pulse voltage amplitude is 600 V,increasing the temperature can significantly improve the discharge intensity of coaxial DBD and coaxial DBD loaded with hot electron emission material.The temporal and spatial distribution of the excited argon atom density shows that the discharge area of the coaxial DBD is on the instantaneous anode side,while the discharge area of the DBD loaded with the hot electron emission material has two discharge areas,which are on the instantaneous anode side and the instantaneous cathode side,but the discharge mechanism causing these two discharges is different.When the pulse voltage amplitude is 1500 V,when the temperature increases,the characteristic parameters of both coaxial DBD and coaxial DBD loaded with hot electron emission materials show a trend of increasing first and then decreasing.In addition,for the parameter range studied in this thesis,under the same external conditions,the loading of hot electron emission materials on the coaxial DBD can significantly improve the discharge intensity.