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

Spectroscopic measurements of the composition and temperature of free-burni ng arcs in mixtures of argon with helium, nitrogen and hydrogen are compare d with the predictions of a two-dimensional numerical model. The measuremen ts have been performed by two different techniques, each requiring the meas urement of the emissions from two lines, and the measurement of emission fr om a pure argon are. Both the measurements and the calculations show that s ignificant demixing occurs in arcs in all gas mixtures, with the lighter ga s being concentrated near the are axis. Generally good agreement is found b etween the measurements and the calculations. Discrepancies are tentatively explained as resulting from departures from local thermodynamic equilibriu m.