| In recent years,atmospheric pressure low-temperature plasma has attracted much attention because of its applicability in many fields,and many gas discharge methods for atmospheric pressure low-temperature plasma generation have also emerged.Based on the attractive prospect of atmospheric pressure low-temperature plasma in various industrial applications,this paper will mainly focus on the numerical simulation of two gas discharge methods for atmospheric pressure low-temperature plasma generation,to further understand its discharge mechanism and provide some guiding significance for the parameter selection of gas discharge practical application or further theoretical research.Atmospheric pressure gas discharge includes many and complex systems in natural disciplines,such as electricity,magnetism,heat,mechanics,chemistry and so on.The one-dimensional fluid model used in this paper has the advantages of high efficiency and accuracy in the study of atmospheric pressure gas discharge.In this paper,the pulse modulated discharge with pure helium as the discharge background gas and the microwave discharge with the mixture of helium and oxygen as the discharge background gas in the microwave frequency band are numerically simulated.The time scale effect of various discharge parameters in the gas discharge in the microwave frequency band and the influence of some discharge parameters on the formation of active particles in the helium oxygen discharge are analyzed.Firstly,for microwave pulse-modulated discharge,the starting point of introducing pulse modulation into gas discharge is to limit the discharge mode to mode α,which can produce large volume and uniform plasma,and inhibit the discharge from turning into mode γ,which is characterized by radial plasma aggregation and elevated temperature.However,with the introduction of pulse modulation,some new discharge phenomena,such as the first current pulse phenomenon,also appear.The appearance of the first current pulse phenomenon reflects the degree of interaction between adjacent modulation cycles in the discharge process.The more prominent the phenomenon is,the more serious the mutual interference between the two modulation cycles is.In addition,the first current pulse phenomenon can also be used as a sign to further analyze the time scale effects of discharge parameters such as voltage rise time,voltage application time and voltage off time,as well as the discharge mechanism contained in the establishment of electric field,electron migration and charged particle diffusion at the micro level of discharge,and deeply understand the influence of these different time scales on pulse modulated discharge.The simulation results show that the high voltage rise rate is the necessary condition for the emergence of the first current pulse.Under a single variable,the greater the voltage rise rate in the first cycle of each modulation cycle,the greater the value of the first current pulse;With the extension of the duration of the power on phase of the modulation cycle,that is,the voltage application phase,the value of the first current pulse also increases,but the growth rate slows down gradually;With the extension of the duration of the power-off phase of the modulation cycle,that is,the voltage off phase,the value of the first current pulse first increases and then decreases,and there is a maximum value.Secondly,for microwave helium-oxygen discharge,the main application value of microwave helium oxygen discharge doped with a small amount of oxygen in the discharge background gas is reflected in the generation of reactive oxygen species(ROS).The power excitation frequency of helium oxygen discharge simulated in this paper is raised to the microwave frequency band of gigahertz.Through the results of numerical simulation,the effects of power input power,power excitation frequency and oxygen doping on the generation of active particles are analyzed and studied one by one.The simulation results show that when the input power of the power supply increases gradually,the density of ground state atomic oxygen and excited state oxygen molecule SDO(single delta metastable oxygen)increase,while the density of ozone molecule decreases;While keeping the input power of the power supply unchanged,only changing the excitation frequency of the power supply will hardly change the density of each particle;When the doping amount of oxygen increases,the ground state atomic oxygen density first increases and then decreases,and there is a maximum value.At this time,the oxygen doping is recorded as the best oxygen doping for the ground state atomic oxygen,and this value will increase with the increase of the excitation frequency of the power supply,while the excited state oxygen molecules SDO and ozone molecules change monotonically with the increase of oxygen doping,decreasing monotonically and increasing monotonically respectively.To sum up,the gas discharge plasma with the power source excitation frequency raised to the microwave frequency band is accompanied by new discharge phenomena and subsequent further application prospects.In this paper,the numerical simulation of microwave pulse modulation discharge and microwave helium oxygen discharge is mainly carried out.On the one hand,the mechanism of time scale effect of discharge parameters in pulse modulation discharge with the power source excitation frequency in the microwave frequency band is analyzed,on the other hand,the evolution of active particle density with discharge parameters in microwave helium oxygen discharge is analyzed.According to the numerical simulation and theoretical analysis in this thesis,the generation of reactive oxygen species can be effectively optimized in atmospheric microwave discharges. |
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