Numerical Study On Atmospheric Pressure Glow Discharge And Plasma Jets

In recent years, atmospheric pressure low temperature plasmas have numerous potential applications on biomedicine, materials surface modification, nanotechnology and pollution control, among which pulsed radio-frequency (RF) glow discharges and pulsed dc plasma jets have received broad attention due to their advantages of active chemistry and low gas temperature. Compared with traditional low pressure RF glow discharges, Atmospheric pressure pulsed RF glow discharges can be generated between two electrodes directly without the costly vacuum chamber. Besides, glow discharges are very active in chemistry. Atmospheric pressure pulsed dc plasma jets are generated in open space rather than in a confined discharge gap. Therefore, they can be used to process3d objects with complex structure. Plasma jets also have efficient reaction chemistry. Plasma plume temperature is very low and can be touched without any feeling of scorching hot or electrical shock. However, these two kinds of discharges comprise a series of complicated physical and chemical processes which are needed to be further investigated. In this work, two self-consistent hydrodynamic models, including1D and2D, solving continuity equation coupled with Possion equation, are used to simulate atmospheric pressure pulsed RF glow discharges and pulsed dc plasma jets aimed at understand the underlying mechanisms. The main contents are as follows:1. The discharge mode transition and O(5p1) production mechanism of pulsed radio frequency glow discharge. As the discharge initiates, the uniform plasma bulk is across the gas gap. When RF voltage increases, most emissions tend to move towards the cathode sheath. The transition from uniform discharge to a mode occurs. The fast increasing stochastic heating at the boundary of enlarging sheath is the key reason. He*and He2dominate the production of O(5p1) through dissociation and excitation of O2by comparing the experimental and numerical spatial-temporal777nm emission profile. The sheath does not have effect on He*and He2*density distribution, which result in uniform777nm emission. RF glow discharge can produce larger areas of reactive species than plasma jets, which lead to higher decontamination efficiency.2. The mechanism of plasma jets propagation. Because of the higher mobility of electrons than of ions, plasma sheath is formed close to inner wall of the dielectric tube. The sheath causes the plasma channel inside the tube to shrink in the radial direction. When the plasma jet exits the tube, the strong electric field and the air mixing layer lead to the streamer head becoming ring-shaped. The streamer head can be regarded as a charged sphere, in which majority of the ionization take place. The ionization domain is a site of avalanche-to-streamer transition. The charge produced in this domain pushed out the electric field toward the ground electrode, so the plasma jet moves forward. Penning ionization dominates the ionization reactions outside the tube. Also, the electrons produced by the Penning ionization increase the electric conductivity of the plasma channel and facilitate the ring-shaped streamer propagation.3. The effect of working gas impurities on plasma jets. Helium of different purity level (helium purity99.99999%,99.99%,99.9%,99%) is chosen as working gas. The impurity is air. The density of reactive species inside the tube increases significantly with working gas impurities, while the density of species outside the tube increases slightly. The photoionization between air species play a crucial role in the transition of the localized discharge to streamer inside the tube. For the high purity helium, photoionization can’t provide enough seed electrons and the transition of discharge to streamer is not observed in this case, which is attributed to the low photoionization rate and long distance between the photoionization region and ionization region. Increasing the applied voltage and decreasing the distance between the electrode and nozzle can overcome the shortcoming. Besides, photoionization can increase the propagation speed of plasma jet outside the tube.4. The production mechanisms of OH radicals in plasma jets. The simulation result of OH density is consistent with laser induced fluorescence (LIF) measurement. Comparing the contribution of reactions related to OH, electron-impact dissociation of H2O, electron neutralization of H2O+and dissociation of H2O by O(1D) are found to be the main reactions to generate the OH species in the open air. OH and H three-body recombination reaction is the main OH loss mechanism. The addition of N2, O2and air to the working gas increases the OH density outside the tube, which can be attributed to the increased electron density through Penning ionization. Although the addition of H2O increases the OH density inside the tube dramatically, the OH radical density increases only slightly outside the tube, because of the effective diffusive gas mixing. The LIF measurement shows that OH density outside the tube increases with H2O concentration increasement. This is due to the gas flow, which blows the OH generated inside the discharge tube to the detected region.1D simulation result shows that OH density keeps increasing at first11pulses and then comes to steady state afterwards. It suggests that controlling the pulse numbers is also an effective way to control the dynamics and reactivity of the plasma jet.