Simulation Study On Cold Plasma Jets At Atmospheric Pressure Using Two Dimensional Fluid Model

The atmospheric plasma jet has attracted extensive attention as a new non-thermal equilibrium plasma source. It is one of the hot topics in the fields of low temperature plasma research and practical application. Under the influences from gas flow and electric field, the various active components such as radicals, charged species, ultraviolet, excited species, and metastable species produced during the discharge can be delivered to relatively remote locations for localized treatment. This will not only improve the stability of the discharge system, but also satisfy the needs of high reactive chemical species. In addition, the plasma produced comes with no electrical or thermal shocks. This makes it possible for plasma jet to contact with living organisms directly, bringing breakthrough to both traditional industrial applications and the biomedical field. Although lots of research (both experimental and numerical) has been carried out on plasma jet, further investigation is still needed to study some unclear discharge behaviors as well as potential physical mechanism. In this dissertation, based on a two-dimensional axisymmetric fluid model, the research on helium plasma jets at atmospheric pressure is organized as follows.A deep understanding of the plasma jet-surface interaction is important and necessary to precisely control the treatment processes. In chapter 3, a two-dimensional plasma fluid model is developed to study the interaction between atmospheric pressure plasma jet and surfaces (conductor vs. insulator) in needle-plane electrode discharge. Meanwhile the evolution of discharges in the vicinity of the cathode plane is analyzed. Numerical result shows rapid deceleration of the plasma jet when it arrives at the boundary of the cathode sheath. The subsequent evolution of the discharge strongly depend on the surface electrical properties. Upon encountering a conductive surface, the plasma jet root formed on the surface turns into a spot. While upon encountering a dielectric surface, the plasma jet spreads out radially along the surface. The secondary electron emission process and the accumulated surface charges on dielectric layer have a significant influence on the discharge dynamic. The sheath thickness decreases with the secondary electron emission coefficient. And the ion flux to the surface increases with the relative permittivity.Interactions between plasma jets, which directly affect the stability and homogeneity of the discharge system, are vital problems to be fixed. In chapter 4, based on a two-dimensional plasma fluid model, a numerical simulation of two counter-propagating helium plasma jets in ambient air with a constant mixing layer is developed to study the effect of driving voltages on the interactions between the two jets. Results show that the plasma jets interaction inhibits the propagation. Besides, the physical processes that occur during the interaction depend strongly on the driving voltages. When the plasma jets are driven by equal voltages, two identical plasma jets are generated. The plasma jets will stop the propagation without merging, forming a minimum approach distance, which decreases with the increase of driving voltages and the initial electron density and increases with the relative permittivity. When the driving voltages are unequal, the plasma jet driven by lower voltage will stop propagation, followed by a merge of the two plasma jets. The merging location depends strongly on the difference of the applied voltages.In the fields of atmospheric pressure plasma jet researches, achieving longer plume length, higher plasma density and higher plasma stability by optimizing the atmospheric pressure plasma sources has always been a hot topic. In chapter 5, a two dimensional coupled fluid model is developed to simulate the helium plasma jet at atmospheric pressure driven by dual power electrodes. The advantages of the dual power electrodes plasma jet, namely, the effect of the needle electrode is studied and analyzed. Results show that the needle electrode has an evident effect on the discharges. Compared to single ring electrode, the existence of the needle electrode leads to a plasma jet with higher propagation velocity, higher plasma density, and larger discharge width. Then we investigate the characteristics of the plasma jet to discover the influences from the needle parameters. It is found that the needle-to-ring discharge gap has much more impacts on the discharge than needle radius. The length, density, and discharge width of the plasma jet can be increased by decreasing the distance between needle and ring electrodes.In chapter 6, a two-dimensional computational study of helium plasma jet at atmospheric pressure emerging in oxygen ambient is performed. Physical processes including discharge ignition and propagation inside the tube, streamer (plasma bullet) propagation in the open gap, and interaction with a substrate are studied. Upon ignition, the discharge first propagates along the inner wall of the containing tube, and then detaches from the wall forming an ionization wave (streamer or plasma bullet). The streamer evolves from a hollow (donut-shaped) feature to one where the maximum of ionization is on axis. The streamer propagates in the gap between the tube exit and the substrate with a speed of-3><105 m/s. Upon encountering a conductive substrate, the streamer turns into a spot on the substrate, then the intensity of the discharge decreases as time. Upon encountering a dielectric substrate, the streamer spread-out radially along the surface, and the intensity of the discharge reduces as the spreading distance increases. For a conductive substrate, the radial species flux to the surface peaks on the symmetry axis. For a dielectric substrate, a ring-shaped flux distribution is observed. The "footprint" of plasma-surface interaction will increase when either the gap between tube exit and substrate or the dielectric constant of the insulating substrate decreases.