Modelling and experimental analysis of vacuum plasma spraying. Part II:prediction of temperatures and velocities of plasma gases and Ti particles ina plasma jet

Abstract

A numerical model has been developed to calculate the spatial distributions ofplasma gas temperature, enthalpy, velocity and fractions of dissociated andionized species in a vacuum plasma spraying (VPS) plasma jet under a range of plasma current, Ar flowrate, H2 flow rate and chamber pressure, and the trajectories, temperaturesand velocities of Ti particles under typical processing conditions. The modeluses FLUENT V4.2 commercial software, incorporating approximations to describedissociation, ionization and recombination reactions in the plasma jet. Thecalculations show that the spatial distributions of plasma gas temperature,enthalpy, velocity, and degrees of dissociation and ionization in the plasmajet are mainly controlled by the initial boundary values at the plasma gunexit, which are functions of the VPS processing conditions. The model predictsthat the plasma jet length increases with increasing plasma current anddecreasing Ar flow rate and chamber pressure, and shows a maximum with varyingH2 flow rate, agreeing well with measurements. Particle trajectory is largelydetermined by the initial particle position at the plasma gun exit. Particletemperature and velocity increase rapidly in the first 100 mm of the plasmajet and then become nearly constant at axial distances >150 mm. Particletemperature and velocity in the plasma jet decrease with increasing particlesize and initial radial position at the plasma gun exit.