The Simulation Of Magnetically Dispersed Arc Plasma Coupled With Electrode

The arc plasma is widely used in industry. Being highly concentrated energy and great parameters gradient, arc plasma is difficult in large scale industry application.Compared with contractive arc, the magnetically dispersed arc plasma, which is called "Large Area Dispersed Arc Plasma Source-LADAPS", present diffusive arc column and diffusive anode arc root, as well as various forms of cathode arc root, such as shrink, split and diffusive, and these forms are dynamic and transformable.In this paper, Lowke’s one-dimensional model for the arc plasma coupled with the cathode is developed for computation program compiled with FLUENT. With this program, the arc plasma with axial magnetism in different boundary conditions is simulated. Various plasma configurations, including cathode arc root, arc column and anode arc root, are obtained and discussed. The mechanism of the interaction among cathode arc root, arc column and anode arc root are analyzed.The fully dispersed arc plasma in a small scale (Φ10) is simulated with two-dimensional model. The plasma parameter distribution and flow in different conditions such as cathode shape, magnetic field intensity, gas velocity, arc currents, work function of the cathode material and so on, is discussed. The results show that:(1) with axial magnetic field, the cathode arc root, arc column and anode arc root appear to be dispersed (diffusion), and the current density are much smaller than those of free burning arc respectively.(2) As the cathode apex angle increasing, the cathode arc root moves to the side from the center of the front end. The arc root diffusive area increases and the current density decreases, so as that the average and maximum temperature of plasma, the maximum velocity and the arc voltage decrease, but the plasma configuration (location and shape) is essentially same.(3)The cathode arc root moves to the side from the center of the front end as the magnetic field increasing. The arc root diffuses much and the current density decreases. The plasma temperature decrease and plasma region moves upstream. The anode arc root is compress in the axial direction, the current density of the anode arc root increases.(4) As the input gas velocity increasing, the cathode arc root is blown to the tip of the cathode, and the current distribution on the cathode tip has no obvious change, the thickness of the plasma increases, and the arc voltage increases, the gas flow intensifies.(5) The arc current has no effect on the location or the shape of the plasma, but only increases the values of the parameter. The cathode arc root diffuses much easily as arc currents increasing.(6) The work function of the cathode affects current distribution on the arc root. A smaller work function leads cathode arc root diffusing greater.A free burning arc configurated of needle-plate electrodes with axial magnetic field is simulated. The results show that:with the axial magnetic field intensity increasing, the anode current density decreases at the axial, and the peak value decreases and transfers in radial direction, the arc column near the anode expends, and the arc column near the cathode shrinks. As axial magnetic field increasing, the cathode arc root gets much contractively, which is contrary to that of the dispersed arc in coaxial electrodes plasma generator. That is resulted from the pinch of gas flow affected by the cathode jet, which is enhanced by Lorentz force as axial magnetic field being increasing. With the axial magnetic field increasing, a vortex appears in the heartland near anode. Those results are proved by experiments.The dispersed arc plasma in large scale generator (Φ70) is simulated. The temperature distribution is similar with the experiment result. But the difference of value indicates that the plasma is NLTE.Lastly, a three-dimensional model is employed to simulate the dispersed arc plasma in a small scale generator, the result shows that the arc column and the anode arc root appear excellent symmetry, but the cathode arc root is non-axisymmetric.According to the numerical simulations and the experimental results, Main conclusions are drawn as follow:(1) The configuration of the cathode arc root and of the arc column near cathode region simulated with coupled cathode model are different with un-coupled model. The parameters distribution of the cathode root and of the arc near cathode region affects each other, and they affect the characteristics of the arc column and the anode arc root.(2) The arc plasma at atmospheric pressure can be dispersed uniformly; the arc plasma configuration can be affect by the boundary condition by means of flow.(3) It is easy to maintain the arc plasma column and the anode arc root uniformly-dispersed in coaxial electrodes magnetically dispersed arc plasma generator, but the cathode arc root may be non-axisymmetric.