The Numerical Simulation Of Dielectric Barrier Discharge (DBD) Plasmas At Atmospheric Pressure

Dielectric-barrier-discharges(DBDs) at atmospheric pressure have a large number of industrial applications because they can be operated at strongly non-equilibrium conditions and at reasonable high power levels, without using sophisticated pulsed power supplies and expensive and complex configuration of vacuum system. As a main feasible method to produce atmospherical pressure plasma, the DBDs have been studied extensively in theoretical and experiments in recent years. However, due to the complicated processes of charged particles, neutral atoms and molecules, and the active radicals in the DBD discharges, and also owing to the lack of the proper diagnostics in atmospherical pressure preventing the measurement and observation of the characteristics of DBD plasmas, up to now the mechanism of DBDs is still not fully explained. The numerical modeling and simulation will provide an essential tool to investigate and understand the DBD plasmas.In the thesis, two dimensional diffusion—drift fluid model is used to simulate and study the generation and development processes of the atmospherical pressure air DBD plasmas in plane-parallel configuration. In simulation, the particle flux continuity equations and the coupling Poisson's equation are solved by the flux-corrected transport technique(FCT) and the iterative symmetric successive over-relaxation (SOR) method with Chebyshev (semi-iteration) acceleration respectively.The temporal and spatial development of charged particle densities, electric field strength, discharge current and discharge propagation velocity during the progression of discharges in different initial conditions are investigated and the results indicated that:The processes of DBDs have four phases: (1)avalanche phase; (2)streamer formation phase: (3) streamer propagation; (4)formation of surface charge and discharge quenching.In the initial stage of avalanche phase, with the increasing of the electrons, the visible avalanche outline is wedge-shaped and the visible avalanche radius is growing linearly with the axial distance z. When the transverse avalanche size reaches the characteristic ionization lengthα≈0.1cm the broadening of the avalanche head slows down dramatically. Then the avalanche propagates without evident size change.When the charged particles density and the electric field adjacent to the anode barrier reach the streamer formation criterion Meek condition, the induced electric field start to shift to the cathode. The streamer is started and propagates to cathode. Accompany with this, the electric field in the front of streamer increases continuously and causes the enhancement of the ionization. So the electron density increased rapidly, while the move speed to cathode becomes larger and larger. The discharge enters the streamer propagation stage.During the propagation, the streamer causes the accumulation of the charges in the front of the streamer head and increase the electric field greatly. This enhances the ionization rate enormously. At same time the excited particles are produced, which emitting photons produce photoionization. The collision and the photon ionizations generate a large number of second electrons and initiate secondary avalanches, resulting the enormous increase of the electron density in the head of streamer and induce the ionize wave to cathode.When the streamer connects to the surface, positive space charge is dropped onto the cathode barrier and is transformed into surface charge. Substantial local field enhancements are now established at both barrier surfaces, and a relatively homogeneous distribution covers the remaining gap. As surface charge densities are formed on the barriers, the air gap electric field decreases and becomes less than the ionization threshold. Then new avalanches are prevented and the discharge extinguished.It is worth emphasized from simulation that the photoionization is important during the streamer propagation phase to cause the high velocity of the ionization front and the barrier discharge expansion along the cathode dielectric surface.