Experimental Observation And Numerical Analysis Of D.C. Arc Plasmas Dispersed By Magnetism

Direct-current arc plasmas at atmospheric pressure have many diverse industrial uses nowadays as their special properties of high temperature, enthalpy and chemical activity. The majority of arc plasmas presently in industrial use are usually of small volume, concentrated energy, and inhomogeneous parameters as a direct result of their inherent shrinkage. However, in some application areas, especially for the chemical synthesis, material processing/preparation and etc, a large-volume with uniformity in plasma parameters is one of the most important requirements that the arc plasma source should satisfy to advance the product quality and production efficiency. With this motivation, a large-scale magnetically rotating arc plasma generator is designed and utilized to obtain diffuse plasma in this study. The generator consists of an 80mm diameter graphite anode chamber and a concentric graphite cathode. A solenoid coil wrapping the anode is used to produce an AMF (axial magnetic field).Rotation induced by the AMF results in improved stability and drastic changes in plasma structure. With the increase in arc current and AMF strength, the arc plasma in the chamber is observed in various configurations, it temporally and spatially evolves from a contractive column to fully diffuse plasma cloud which fills the entire chamber cross section. The fully diffuse plasma is distributed throughout the electrode gap and no anode attachment can be seen in the chamber cross-section, the diffuse anode root is symmetrically distributed over the electrode. Furthermore, this diffuse plasma runs more steadily and its voltage fluctuation is significantly reduced. The diffuse plasma consumes more power but its emission is weaker, which implies that the volume of the fully diffuse plasma should have been greatly enlarged.An axisymmetric-swirl model based on the commercial CFD code FLUENT is also used to qualitatively discuss the AMF effects on the flow and heat transfer of the fully diffuse plasma inside an assumed magnetron arc torch. The AMF induces a low pressure region around the torch axis and a strong backflow at its spout. So as that the AMF plasma is significantly retracted axially and expanded radially and as a result, the arc shape seems to be a flag dancing windward. The plasma is of high temperature at the flagpole but cooling a little at the banner and this is responsible for the plasma intensity distribution on the chamber cross-section is more uniform.When the ambient gas outside the chamber is air, the air backflow induced by the AMF mixes with the argon inflow and significantly heightens the arc voltage. The air quantity depends on both the AMF strength and the inlet gas velocity, i.e., it increases with the former but decreases with the latter. An increase in the amount of the argon inflow restrains the quantity of the air backflow, and this should be responsible for a lower arc voltage in such an AMF torch when a larger gas inflow is used.Finally, we simulate the plasma flow inside an AMF welding argon arc and the heat transfer from the arc to its anode at the atmospheric pressure. Numerical results indicate that the imposed AMF induces most of the plasmas flowing by the arc mantel which leads to the appearance of a low-pressure region at the arc core. The AMF arc significantly retracts axially and presents a hollow bell shape. The characteristics of temperature, overpressure and current density distributions on the arc-anode interface are also discussed. Using a small AMF is contributive to expand the arc attachment on the anode, however, strengthening the AMF can't excessively expand the arc radially but pinch it finally.