Effects of anode nozzle geometry on ambient air entrainment into thermal plasma jets generated by a non-transferred plasma torch

In plasma spraying, higher temperature and velocity of thermal plasma jets are preferable for producing high qualities of protective coatings for their industrial uses, because the sprayed coating powders are melted and accelerated in the plasma flame ejected from a non-transferred DC arc torch through an ambient air toward a substrate. In the typical plasma spray process operated under an atmospheric-pressure condition, the entrainment of ambient air into the thermal plasma jet is inevitable. An air inflow to the flame alters chemical compositions of the plasma species, cools the thermal plasma flame, and decreases the jet velocity. Furthermore, the dissociation of entrained air enhances the specific heat of plasma, and then the plasma temperatures decrease even when operation power level of the plasma torch is not changed. Consequently, the gradients of temperature and velocity in the plasma jet increase with the degree of air entrainment. Therefore, the ambient air entrainment should be controlled to the lower degree for getting the better quality of coating products with higher purity, density and bond strength. In this experimental work, the geometrical effects of anode nozzle of the non-transferred plasma torch on the air entrainment are examined by measurements using a quadruple mass spectrometer (QMS). Two different types of anode nozzle, i.e., tubular and stepped nozzles are employed for the torch. For each nozzle, air contents in the thermal plasma are measured to find the effects of nozzle geometry on the ambient air mixing with the plasma species. The radial and axial distributions of plasma temperature and velocity are also measured by using an enthalpy probe method based on the assumption of local thermodynamic equilibrium (LTE). By analyzing the measured results of the thermal plasma characteristics and the geometrical effects of nozzle shape on the air entrainment, the suitable design requirements of the nozzle are determined for optimal processes of plasma spraying.