Fluid Dynamic Effects on Plasma Torch Anode Erosion

Abstract

Anode erosion in plasma spray torches results in coating deterioration. The usable life of a torch anode is strongly dependent on the fluid dynamic behavior of the plasma inside the torch, which in turn depends on the geometric design of the anode and the operating parameters. To study the relative importance of these effects, cold flow investigations have been performed with a torch having a glass anode with the same geometric dimensions as a commercial plasma torch. The density differences between the arc and the cold gas were simulated by injecting heated helium from the tip of the cathode into the cold argon gas flow from the regular gas injector. Flow visualization was achieved by seeding the flow with micron-sized particles. A finite-element computational fluid dynamics code was used to simulate the cold flow structure. The results were compared with erosion patterns observed with an actual plasma torch. The results indicate that recirculation eddies inside the torch will force a preferred anode attachment, which is different for different gas injectors. The minimization of such recirculation regions by appropriate fluid dynamic design will result in more random attachment of the arc and prolonged anode life.