Numerical Study Of Atmospheric-pressure Plasma Helium Jet Excited By A Sinusoidal Voltage

Atmospheric-pressure plasma jets(APPJs)have a variety of promising applications in biomedicine,material processing,environment engineering due to their unique advantages that are not limited by the size of discharge gap and can produce rich reactive species.APPJs are multi-parameter systems,which means that any parameter condition such as electrode structure,driving voltage,working gas,propagation environment,could significantly influence the behaviors of APPJs.Although APPJs have been studied widely,deep insight into the dynamic characteristic of APPJs under different discharge parameters is still a hot research topic to optimize and control APPJs’behaviors to meet the detailed demands of applications.In practical applications,the plasma jet length and the generation of reactive species are two key factors.The existing researches show that,compared with other control conditions,the excitation voltage has an important effect on the plasma jet length and active particle density.In this thesis,a two-dimensional fluid model coupling with a neutral gas dynamic model is developed to study atmospheric-pressure helium plasma jet propagating into ambient air driven by sinusoidal voltage,with emphasis on dynamic characteristics of the plasma jet and the generation of reactive species under different biased sinusoidal voltages.The results are concluded in the following:In Chapter 3,the generation and propagation of helium plasma jet with a ring-plane electrode structure driven by sinusoidal voltage are studied in detail,and the influence of frequency on the dynamic characteristics of plasma jet is also discussed.The simulation results show that the plasma jet can be ignited in both positive and negative half periods of sinusoidal voltage and positive and negative jets exhibit different propagation characteristics.In positive half period,a column-shaped plasma jet with a spherical head is formed and the ionization rate of the jet head is the highest.In contrast,in negative half period of sinusoidal voltage,a conical head is observed in the column-shaped plasma jet.The ionization rate of negative jet is significantly lower than that of the positive jet,and the discharge is more diffuse.There is an obviously interaction between positive and negative jets.The generation of negative jet inhibits the propagation of positive jet.In the process of negative jet propagation,positive jets almost stop propagating forward.Meanwhile,the positive jet also hinders the propagation of the negative jet.When the negative jet approaches the positive jet head,its velocity decreases rapidly,and once it exceeds the positive jet head,the velocity increases again.In the process of jet propagation,the formation and transport of different active particles are different.Under the current simulation conditions,the helium-related particles are mainly generated in the dielectric tube,and their distribution is different.He~+concentrates near the jet axis,while He~*particles almost fill the whole dielectric tube.After exiting the tube,He~+density presents an annular distribution,while He~*density presents a columnar distribution.Nitrogen and oxygen related particles are mainly generated in the ambient gas,and their densities are distributed in ring-like hollow shape.With the propagation of the jet,the density distributions of various particles all have a tendency to contract towards the central axis,but the contraction speed is different.Under different driving frequencies,the dynamic characteristics of plasma jets are different.When the frequency is low,the jet propagates closer to the axis in dielectric tube,while at high driving frequency,a relatively uniform jet channel is formed within the whole dielectric tube.Moreover,as the increase of driving frequency,the ionization rate inside the jet channel increases,thus resulting in the increase of active particle density.The experimental results show when APPJ is excited by a biased sinusoidal voltage,the plasma jet will behave differently.In chapter 4,the influence of different polarity bias voltage on plasma jet behavior is studied.The results show,when the sinusoidal voltage with amplitude of 5 k V is superimposed on the negative bias voltage of 1 k V and 2 k V respectively,compared with the jet without bias voltage,the jet length and propagation velocity decrease in positive half period.But after entering negative half period,the jet propagation velocity increases rapidly,and the higher the negative bias voltage is,the faster the propagation velocity increases.The final jet length is obviously larger than that without bias voltage,and the produced active particle density is also increased greatly.When a sinusoidal voltage with a voltage amplitude of 5 k V is superimposed on a positive bias voltage of 2 k V,compared with the jet driven by a sinusoidal voltage with a voltage amplitude of 7 k V,the jet driven by a positive bias voltage has a longer propagation distance and higher particle density.Therefore,the introduction of bias voltage can not only increase the length of plasma jet,but also improve the generation efficiency of active particles.