High-current Free-burning Arcs Research Articles

Possibilities of application of plasma technologies to recycle organic-containing substances: Particularities of high current free burning arcs

Studies of the free burning high current arcs fanned by weak nitrogen or argon flow, with parameters typical for plasmochemical reactors and currents of about 10 kA, are performed. The electrodes were made of carbon and metals. In this case we show that, under the arc stabilization by nitrogen or argon, a significant number of charge carriers enter the arc due to ionization of carbon and metal atoms. When the high current arc is switched by a melt the main part of the arc energy is imparted into the melt and not more than 35% of the energy is transferred into the bulk of the reactor.

A Non‐equilibrium Treatment for Free Burning High Current Arcs.

ABSTRACT: A two‐temperature variable‐density treatment has been developed for description of high current free burning arcs, including departures from thermodynamic and chemical equilibrium in the plasma. The treatment includes the arc and the electrodes, and considers the separate energy balance of the electrons and the heavy particles, together with the continuity equations for these species throughout the plasma. For a 200 A arc in pure argon at 1 atm, we calculate large differences between the temperatures of the electrons and the heavy particles in the plasma region near the cathode tip. In the main body of the arc, at high plasma temperatures, we predict minor differences between the temperatures of the electrons and the heavy particles. However, we predict large departures from local chemical equilibrium throughout the plasma, due to overpopulation of the ground state level of the neutral atoms caused by the injection into the arc core of a large mass flow of cold gas, resulting from the arc constriction at the tip of the cathode.

Non-equilibrium modelling of transferred arcs

A two-temperature, variable-density, arc model has been developed for description of high-current free-burning arcs, including departures from thermodynamic and chemical equilibrium in the plasma. The treatment includes the arc, the anode and the cathode and considers the separate energy balance of the electrons and the heavy particles, together with the continuity equations for these species throughout the plasma. The output includes a two-dimensional distribution for the temperatures and densities both of the electrons and of the heavy particles, plasma velocity, current density and electrical potential throughout the arc. For a 200 A arc in pure argon at 1 atm, we calculate large differences between the temperatures of the electrons and the heavy particles in the plasma region near the cathode tip, together with large departures from local chemical plasma equilibrium. In the main body of the arc at high plasma temperatures, we predict minor differences between the temperatures of the electrons and the heavy particles, which are inconsistent with recent measurements using laser-scattering techniques showing differences of up to several thousand degrees. However, we find that, for the region in front of the cathode tip, the ground-state level of the neutral atoms is overpopulated relative to the corresponding populations under conditions of LTE, in agreement with experimental observations. These departures from LTE are caused by the injection of a large mass flow of cold gas into the arc core due to arc constriction at the tip of the cathode.

Departures from local thermodynamic equilibrium in high-current free burning arcs in argon

A magnetohydrodynamic arc model has been combined with a kinetic plasma model to investigate departures from equilibrium in the plasma of high-current arcs. Calculations made for arcs operating in argon with various arc parameters show that the ground state levels of the neutral atoms in the region of the arc close to the cathode tip are almost always overpopulated relative to their populations under conditions of local thermodynamic equilibrium. The results suggest that this is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients generated by the magnetic pinch force in the cathode region of the arc. Calculated plasma emission coefficients for the 696.55 nm Ar I line are in agreement with experimental measurements showing that the values of these coefficients decrease in the cathode region of the arc.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

Large effect of cathode shape on plasma temperature in high-current free-burning arcs

The temperatures of the cathode surface and of the plasma for an atmospheric-pressure high-current free-burning argon arc have been measured for a range of cone angles for cathodes of thoriated tungsten. These measurements have shown that both the surface temperature of the cathode and the temperature of the plasma depend strongly on the cathode shape. For cathodes with conical shape, plasma temperatures were found to be a maximum for a cathode cone angle of 60 degrees .

Temperature measurements for high-current free-burning arcs in nitrogen

A Fowler-Milne technique using the relative intensity of the 746.8 nm N I line has been used to measure the plasma temperature of free-burning arcs in nitrogen at 1 atm pressure. Temperature profiles are presented for different arc currents. The maximum plasma temperature depends on the arc current and is 27000 K near the tip of a 3.2 mm diameter thoriated tungsten cathode with a 60 degrees cone angle for a 200 A, 5 mm long arc. Comparisons are made between high-current arcs burning in argon and in nitrogen.

An approximate model for high-current free-burning arcs

A single-dimensional model, based on an integral formulation of the conservation equations, has been developed which makes it possible to predict the behaviour of these arcs. The radial distribution of axial plasma velocity v2(r,z) is represented by a function vz(0,z)(1-(r/ra)2)n where ra is the arc radius and r and z are the radial and axial coordinates. The solution for n by by the use of radial integral of the axial momentum equation makes it possible to develop a simple but detailed picture of the axial velocity distribution in the arc column. A comparison of the axial distribution of plasma velocities with published experimental results shows that turbulent exchange processes are dominant for currents greater than about 500A.

Axial velocity variations in high-current free-burning arcs

A simple method for estimating mean arc velocities in high-current free-burning arcs is presented. Typical results are presented for arcs burning to copper, graphite and steel cathodes employing experimental values of arc current, electric field, radius and radiation loss. These results are compared with laser-Doppler velocity measurements for the case of copper. The method has also been applied to the studies of Bowman (1972) where a direct comparison with measured velocities has also proved possible and successful. The velocity estimates reveal a number of interesting experimental phenomena. Of particular interest is the implied presence of large velocity gradients in both space and time. The spatial gradients indicate that great care will be necessary in plasma velocimetry techniques based on particle tracking.

Radiation losses from high-current free-burning arcs between copper electrodes

Measurements are reported of the total radiation loss from high-current pulsed arcs which burn freely and stably between copper electrodes in atmospheric air. The diagnostic system developed for these measurements is described, and typical results are presented. It is shown that the radiation loss, which has important temporal and spatial variations, is a major component in the arc power balance. The radiation data, together with a knowledge of the electrical input power to the arc column, have enabled the percentage of the input power lost as radiation to be determined.