Modelling of the arc reattachment process in plasma torches

The need to improve plasma spraying processes has motivated the development of computational models capable of describing the arc dynamics inside plasma torches. Although progress has been made in the development of such models, the realistic simulation of the arc reattachment process, a central part of the arc dynamics inside plasma torches, is still an unsolved problem. This study presents a reattachment model capable of mimicking the physical reattachment process as part of a local thermodynamic equilibrium description of the plasma flow. The fluid and electromagnetic equations describing the plasma flow are solved in a fully-coupled approach by a variational multi-scale finite element method, which implicitly accounts for the multi-scale nature of the flow. The effectiveness of our modelling approach is demonstrated by simulations of a commercial plasma spraying torch operating with Ar-He under different operating conditions. The model is able to match the experimentally measured peak frequencies of the voltage signal, arc lengths and anode spot sizes, but produces voltage drops exceeding those measured. This finding, added to the apparent lack of a well-defined cold boundary layer all around the arc, points towards the importance of non-equilibrium effects inside the torch, especially in the anode attachment region.