Influence of injection mode on properties of DC plasma jets for thermal plasma spraying

Abstract

In plasma spraying powder particles are transported to the plasma jet with the help of a carrier gas. The influence of this gas was investigated by means of an enthalpy probe system with a mass spectrometer for measuring plasma temperature, velocity and plasma gas composition, and visualized by means of Schlieren optics. The enthalpy probe system does not allow measurements of the plasma flow containing solid particles. Therefore, to establish changes of the jet characteristics the carrier gas was supplied through different ports without addition of any powders. Nitrogen and helium were used as carrier gases. They were supplied into the jet with flow rates from 5 to 20 slpm either directly into the plasma beam through a hole in the nozzle or with help of an external injector positioned at a distance of several millimeters from the exit nozzle. Injection of the carrier gas led to high jet perturbations. Values of the centerline temperature and the velocity were reduced. The higher the carrier gas flow rate the stronger the changes of the jet properties. Nitrogen carrier gas perturbed plasma jet flow more than helium.