Investigating Arc Instability with Binary Gas Mixture

Summary form only given. A mixture of hydrogen with primary plasma gas argon has been used to investigate some fundamental aspects of arc instability in an SG-100 spray torch. In such arcs, owing to inherent demixing processes, the boundary layer and the core of the plasma column are dominated by hydrogen and argon respectively. Therefore, associated variations in instability features due to variation in the argon flow may reflect phenomena happening at the core and similar effects for variations in the hydrogen flow may reflect the phenomena happening in the boundary layer. It has been observed that for a given hydrogen flow and arc current, the basic instability features remain mostly unaffected over a wide range of argon flow rates and only the average arc voltage varies. On the other hand, a small variation in hydrogen flow by as little as 1 slm, significantly changes the instability features under similar conditions. Identical behaviors have been observed for all values of arc currents studied. The observations suggest that the arc instability originates from the boundary layer over the anode surface, the region, strongly influenced by the hydrogen flow. This, together with the observation of strong dependence of the instability behavior on the magnitude of arc current dictates that the instability-originating zone over the anode boundary layer must also carry current. In other words, the study identifies the region over the anode arc root as the instability-originating zone similar to that postulated in earlier studies. Overall behavior of the instability features and the average arc voltage are presented for the operating range of arc current 300 A to 700 A, hydrogen flow rate 1 slm to 5 slm and argon flow rate 42 slm to 70 slm.