Simulation of Arc Root Fluctuation in a DC Non-Transferred Plasma Torch with Three Dimensional Modeling

It is well known that the coating quality of plasma spraying is strongly influenced by the instability of jets in the plasma spray, which is due to arc root fluctuation. Three dimensional (3D) unsteady-state modeling was employed in this research to analyze the arc root fluctuation in a DC non-transferred plasma torch. Numerical calculations on the distributions of gas temperature and velocity in the plasma torch were carried out using argon as the plasma gas. The electrical current density and potential were also discussed. The results indicate that the fluctuation of arc inside the plasma torch is mainly induced by the movement of the arc root on the anode surface. The arc root moves downstream with the flow of gas, and simultaneously the arc is bent by electromagnetic force. When the arc bends close enough to the anode boundary, a new arc root is formed somewhere upstream of the current attachment. In this paper the nature of the arc root fluctuation is presented, and also it is demonstrated that the voltage-drop calculated is larger than that measured experimentally because the plasma inside the torch has some deviation from the local thermodynamic equilibrium state hypothesis used in the current study.