An Assessment of the Heat and Fluid Flow Phenomena Inside Plasma Torches in Non-transferred Arc Systems

A mathematical representation has been proposed to represent the behavior of non-transferred arc plasma plumes without the need to specify the temperature and velocity of the plasma gas upon exiting the torch. The need to make this assumption has been the principal drawback of plasma modeling efforts to date. In the present work we postulate an idealized shape for the plasma column within the torch, with a constant heat generation rate per unit volume and then solve the coupled heat flow and fluid flow equations both within and outside the torch. The computed results, in the generation of which allowance has been made for heat exchange between the plasma and the torch walls, were very interesting, in that they showed that the temperatures and velocities found for the plasma plume were relatively insensitive to the dimensions postulated for the plasma column. Indeed, for the particular conditions examined a nearly 17-fold change in the volume of the arc column produced only a 13.5% difference in the maximum exit temperature from the torch. It follows that by making "reasonable assumptions" for the dimensions of the arc column one may provide fairly good predictions for the temperature and velocity profiles in the plasma plume; in fact the predictions based on this model were found to be in very good agreement with experimental measurements. Another potentially interesting finding was that any initially imposed swirl on the plasma gas was overwhelmed by the thermal expansion of the plasma within the torch; in fact very special arrangements would be needed in order to produce a strong swirl in the exit gas.