Modeling of Air Plasma in Direct-Current Torch at Nonatmospheric Pressure

Abstract

This paper employs a magnetohydrodynamic model to study the atmospheric and nonatmospheric characteristics of air plasma jets inside a nontransferred plasma torch operating with direct-current arc. The continuity, momentum, and energy equations, with the inclusion of a revised turbulence model, are selected to depict the flow and temperature behaviors of the investigated thermal plasma flow, whereas Ohm’s and Ampere’s laws are employed to account for the applied electric field along the induced magnetic effect arising in the plasma torch. The governing equations are solved via a finite volume method under the steady and axisymmetric assumption, where an azimuthal component is additionally considered in the axisymmetric simulation to capture the strong swirling motion of the thermal plasma due to an azimuthal injection of incoming gas. The air plasma jet inside a well-type-cathode plasma torch, which measures 690 mm in length and 11 mm in radius, is predicted at a fixed current of $I = 100$ A and a constant flow rate of $Q = 100$ L/min. The calculated interelectrode length varies from 178 to 210 mm as the working pressure $P$ decreases from 3 to 1 bar. The numerical simulation also suggests that a gas temperature of 7900 K accompanied by an axial velocity of 346 m/s is achieved at the center of the torch outlet under the atmospheric pressure, whereas the plasma jet at the outlet center suffers from a 26% loss in the gas temperature as well as a 77% decrease in the axial velocity with a threefold working pressure. The mean gas temperature at the torch outlet declines from 5850 to 4070 K with a rise of working pressure from 1 to 3 bar. Similar to the mean temperature at the torch outlet, the axial and azimuthal counterparts show a negative dependence on the working pressure. The mean axial component at the torch outlet considerably drops from 153 to 30 m/s as the pressure is trebled from the atmospheric condition, while its mean azimuthal counterpart at $P = 3$ bar significantly amplifies from 1.7 to 16 m/s provided the working pressure reduces to an atmospheric ambience. The flow calculation suggests that the interelectrode length as well as the mean gas temperature at the torch outlet delivers a negative dependence of power-law on the working pressure, but the mean axial and azimuthal components at the torch outlet exhibit an exponential decay with the increase of the working pressure.