Two-dimensional Numerical Simulation Of Helium Atmospheric Pressure Plasma Jet At Different Flow Velocities

In recent years,atmospheric pressure plasma jet(APPJ)has attracted considerable attention due to its unique properties.APPJ is usually generated by barrier discharges in a noble gas flow driven in thin dielectric tubes and injected into ambient air.Compared with other plasma jet sources,it can propagate in the open space rather than in a confined discharge gap and the discharge region and application region are separated,which is very useful for the applications where safety is a strict requirement.These advantages make them very suitable for surface treatment of delicate materials,as well as biomedical and environmental applications.At present,although APPJ has been extensively studied by experiments and simulations,many aspects still remain the subject of debate.It is known that the gas flow velocity is an important factor that affects the length and stability of the plasma jet.In this dissertation,based on two-dimensional plasma fluid model and the neutral gas hydrodynamic model,we studied numerically the propagation characters and the stability of helium atmospheric pressure plasma jets propagating into humid air under laminar and turbulent flow regimes.The electrode configuration that we used in simulation is a single ring electrode which is forced by positive voltage pulse.The details are as follows:In Chapter 3,a two-dimensional axis-symmetrical fluid model is developed to study the generation and propagation of helium plasma jet,as well as the diffusion process of the neutral gas under laminar flow state,especially the influence of helium flow rate on plasma jet propagation as well as their underlying physical mechanisms.The results show that under laminar flow regime,the structure of APPJ is stable and axis-symmetric.With the increase of helium flow velocity,the propagation length of plasma jet increases linearly,but the radial radius of plasma jet tends to decrease.These behaviors result from the helium molar faction distribution at different gas flow rates.Under laminar flow regime,the spatial distributions of active species densities generated by plasma jet are also studied and for all the species,a ring-shaped profile can be observed.The densities of metastable species are higher than that of the charged species.For the simulation condition,increasing the applied voltage amplitude can lead to the increase of the velocity and propagation length of plasma jet,as well as the rising of active species densities.But the applied voltage cannot affect the stabilities plasma jet in laminar flow regime.To achieve a longer plasma jet,it is a very effective way to increase the velocity of gas flow.When the velocity of helium flow is high enough,the helium flow will transit from laminar to turbulent mode,which may greatly affect propagation characteristics of APPJ.In Chapter 4,based on two-dimensional plasma fluid model and the neutral gas hydrodynamic model with incorporation k-? turbulent model,the influence of turbulent-dominated gas flow on propagation characteristics of atmospheric pressure plasma jet is studied.The numerical results show that under the turbulent regime,the donut shape structure of plasma jet disappears and the plasma jet becomes unstable.After a small distance away from the exit the plasma jet starts fluctuating and propagating in a snake-like form.Compared with the plasma jet in laminar mode,the jet length becomes short and the densities of active species decrease.With the increase of gas flow rate,the plasma jet length increases slightly and the distance which plasma jet keeps stable becomes shorten.The simulation results under different applied voltages show that the plasma jet length increases significantly with the voltage amplitude,but the plasma becomes more unstable and the radial transport of plasma is more obvious.