Fluid dynamic study of direct current plasma jets for plasma spraying applications

Recently, direct current (dc) plasma torches equipped with converging-diverging (Laval) nozzles, instead of standard cylindrical ones, have been shown to present several advantages for both vacuum and atmospheric plasma spraying, such as diminishing the gradients of temperature and velocity and reducing the turbulence intensity in the jet fringes. The present study was concerned with the diagnostics of the plasma jets produced by three nozzles of various contours: a standard cylindrical anode and a Mach 2.5 and Mach 3 Laval nozzle. Emission spectroscopy (absolute intensity) and enthalpy probe techniques were used to measure temperature and velocity fields. Special attention was given to the effects of spray chamber pressure on flow regime inside the nozzles and to the distribution of the temperature and velocity fields in the plasma jet. Results showed that under the chamber pressure conditions used (vacuum), for which Laval nozzles originally were designed, the generated plasma jets had greater centerline velocities and larger high temperature zones compared to standard cylindrical nozzles. The results showed significant improvement in the deposition efficiency by using nozzles with these computed contours.