Three-dimensional modeling of arc root rotation by external magnetic field in nontransferred thermal plasma torches

A three-dimensional (3-D) transient numerical model has been developed to investigate the arc root rotation driven by an external magnetic field and its influences on the thermal plasma characteristics in the nontransferred plasma torches with rod-type cathode (RTC) and well-type cathode (WTC). The 3-D distributions of electric current density are obtained from a current continuity equation along with the generalized Ohm´s law, while the magnetic fields induced by the arc, superimposed on the external field, are calculated by a magnetic vector potential equation. The coupled interactions between the arc and the plasma flow are described in the framework of time-dependent magnetohydrodynamic (MHD) equations in conjunction with a K-ε turbulence model. Numerical simulations have been focused on finding the magnetically driven rotating velocities of the anode arc root for the RTC torch and the cathode arc root for the WTC torch, respectively. The external application of magnetic field turns out to be a practical method for rotating the arc root rapidly to reduce the electrode erosion in the typical torch operation. The 3-D simulations also reveal that a large swirling motion is induced by the external magnetic field, thereby the distribution of plasma temperature is helically distorted. In addition, it is shown for the RTC torch that the rotation velocity of arc root rises in proportion to the square root of external field strength and that it increases with input current but decreases with gas flow rate.