Stability of a DC SF/sub 6/ arc in an axially accelerating flow

The stability of a DC arc burning in an axially accelerating flow of SF/sub 6/ is investigated. The method of normal modes is applied to the equations governing an axisymmetric, infinitesimal perturbation of a self-similar arc, and solutions are obtained numerically. Results are presented for a range of discharge conditions. The physical mechanisms causing instability are deduced from equations for the time-averaged, squared enthalpy, and velocity of a perturbation. The steep radial gradient of the density at the edge of the arc is shown to cause instability. The effect of density fluctuations in comparison with velocity fluctuations is not negligible, and the possible implication of this result for turbulent arc modeling is discussed.