A comparison of measurements and calculations of demixing in free-burning arcs

Spectroscopic measurements of the composition and temperature of free-burning arcs in mixtures of argon with helium, nitrogen and hydrogen are compared with the predictions of a two-dimensional numerical model. The measurements have been performed by two different techniques, each requiring the measurement of the emissions from two lines, and the measurement of emission from a pure argon arc. Both the measurements and the calculations show that significant demixing occurs in arcs in all gas mixtures, with the lighter gas being concentrated near the arc axis. Generally good agreement is found between the measurements and the calculations. Discrepancies are tentatively explained as resulting from departures from local thermodynamic equilibrium.