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

Abstract

Abstract-The geometrical effects of an anode nozzle in a nontransferred plasma torch on air entrainment are examined by measurements of plasma composition using a quadruple mass spectrometry. In addition, the radial and axial distributions of plasma enthalpy, temperature, and velocity are measured by using an enthalpy probe method. Two types of anode nozzle geometry, i.e., cylindrical and stepped nozzles, are employed for the torch in this experiment. As a result of gas composition measurements, the new stepped nozzle turns out to produce a thermal plasma jet having lower air content in it compared with the conventional cylindrical nozzle. The plasma jet produced by the stepped nozzle exhibits higher enthalpy and temperature, especially around the core of the plasma flame, due to less intrusion of ambient air. Furthermore, the axial velocity distribution with a slowly changing variation is observed in the stepped nozzle case because of the plasma flow less disturbed by air entrainment. From these experimental results of thermal plasma characteristics and nozzle geometry effects on air entrainment, a high quality of coating products are expected in plasma spraying by using the stepped nozzle due to higher plasma enthalpy and temperature and lower velocity drop along the plasma jet.Index Terms-Air entrainment, enthalpy probe, nozzle geometry effects, plasma spraying, plasma torch, quadruple mass spectrometry.

Effect of Shroud in Plasma Spraying on Chemical Composition and Thickness of Titanium Coatings

“…There were such extremely high temperatures in the plasma flame that the in-flight titanium particle reactions would take place. If the powder feedstocks were sprayed in an open and static surrounding atmosphere without a shrouding system, a turbulent shear at the outer edge of the plasma flame would produce a few of sizeable vortices, which would then create some eddies of cold ambient air swept within the plasma flame [26,[29][30][31][32]. Molecules of nitrogen and oxygen entrained into the plasma jet from the surrounding air were susceptible to thermal dissociation into highly reactive monotonic atoms when heated at the high temperatures by plasma.…”

“…Thus, titanium particles in flight could stay at higher temperatures, and more particles would be melted, which consequently would increase the deposition efficiency when particles impact on the substrate. On the other hand, air entrainment and friction with the surrounding air would also remove kinetic energy from the plasma when spraying without the shroud attachment, leading to a sharp drop in velocity [32]. Therefore, higher velocity of particles in flight could be obtained due to less air entrainment when it was sprayed with the shroud.…”