The Arc Rotational Characteristics Inside a Plasma Torch with an External Magnetic Field

Abstract

This work investigates the dynamic behavior of an arc column in a DC thermal plasma torch using a three-dimensional transient model. The model assumes an axial magnetic field of constant magnitude inside the plasma torch domain which mimics a magnetic field that is generated either by a solenoid or a permanent magnet encircling the anode. Under the influence of a strong external magnetic field, the plasma arc swirls inside the torch with one end of the arc sweeping the inner surface of the anode while the other end pivoted at the cathode tip. The dynamic variation of the electric arc and its structure is analyzed by simulating a special case with a step-change in magnetic field strength. The transient simulation traces the formation of a helical arc structure as soon as the magnetic field is imposed. The arc column is seen to be axially constricted and radially diffused in the presence of a strong magnetic field. The impact of the magnetic field is also reflected in the arc attachment gap, which is the axial distance between cathode and anode attachments. The influence of magnetic field strength, arc current and gas flow rate on arc rotational frequency, voltage drop, and temperature distribution are analyzed.