Abstract
In DC plasma spray torches, anode erosion is a common concern. It mainly depends on the heat flux brought by the arc and on the dimensions and residence time of the arc attachment to a given location on the anode wall. The latter depend, to a great extent, on the attachment mode of the arc on the anode wall. This paper compares the anode arc attachment modes predicted by an LTE (Local Thermodynamic Equilibrium) and 2-T (two-temperature) arc models that include the electrodes in the computational domain. It deals with a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring, while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage showed that the 2-T prediction is closer to the actual voltage. Also, the post-mortem observation of a new anode ring of the actual plasma torch operated under the same conditions for a short time confirmed a diffuse arc attachment on a new anode.
Highlights
Anode erosion is a common concern in plasma spraying
This paper compares the anode arc attachment modes predicted by an LTE (Local Thermodynamic Equilibrium) and 2-T arc models that include the electrodes in the computational domain
The actual arc voltage and pictures of a new anode ring operated with exactly the same conditions as that of the model (500 A; 60 NLPM of argon) are used as a first attempt to validate the predictions
Summary
Anode erosion is a common concern in plasma spraying. It brings about variation in arc dynamics, voltage and attachment mode on the anode wall. It may modify the development of the arc column inside the plasma torch and the plasma jet issuing from the torch (Ref 1-4); it limits the lifetime of the anode and causes production shutdowns and increased operating cost. An azimuthal displacement of the anode arc attachment is achieved by a swirling injection of the gas. A high swirling component at the gas injection is required in order to have a significant effect on the anode arc attachment further downstream. If the axial movement of the anode arc attachment occurs over a large portion of the anode, it affects the stability of the plasma jet and so the injection and processing of the powder or suspension in the plasma jet
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