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

摘要

A numerical model has been developed to calculate the spatial distributions of plasma gas temperature, enthalpy, velocity and fractions of dissociated and ionized species in a vacuum plasma spraying (VPS) plasma jet under a range of plasma current, Ar flow rate, H-2 flow rate and chamber pressure, and the trajectories, temperatures and velocities of Ti particles under typical processing conditions. The model uses FLUENT V4.2 commercial software, incorporating approximations to describe dissociation, ionization and recombination reactions in the plasma jet. The calculations show that the spatial distributions of plasma gas temperature, enthalpy, velocity, and degrees of dissociation and ionization in the plasma jet are mainly controlled by the initial boundary values at the plasma gun exit, which are functions of the VPS processing conditions. The model predicts that the plasma jet length increases with increasing plasma current and decreasing Ar flow rate and chamber pressure, and shows a maximum with varying H-2 flow rate, agreeing well with measurements. Particle trajectory is largely determined by the initial particle position at the plasma gun exit. Particle temperature and velocity increase rapidly in the first 100 mm of the plasma jet and then become nearly constant at axial distances >150 mm. Particle temperature and velocity in the plasma jet decrease with increasing particle size and initial radial position at the plasma gun exit. [References: 64]