TIME-DEPENDENT GAS DYNAMICS AND GAS-PARTICLE HEAT AND MASS TRANSFER DURING PLASMA SPRAY DEPOSITION

Abstract

This paper describes a time-dependent and 3-D computer model of the atmosphere pressure plasma spray process. The model uses a two-step procedure. The first deals with the formation of the primary plasma jet inside the plasma torch and solves simultaneously the time-dependent NavierStokes equations, conservation equation of electric current and electromagnetism equations assuming that the electromagnetism phenomena are quasi-steady. The second involves the discharge of the plasma jet in air and the heating and acceleration of the powder particles. It is based on the momentum and thermal energy equations for a three components (plasma forming gas, powder carrier gas and air) gas mixture, continuity equations for each component of the mixture and state relation. The particle model uses a stochastic and time-dependent injection description with a distribution of particle size, injection velocity and injection direction in three dimensions. Once the particles exit the injector; their acceleration, heating and vaporization are calculated with a Lagrangian scheme. Their trajectory and velocity are determined from a balance of the gravity, Archimedean, pressure gradient and drag forces. Particles are subjected to turbulent dispersion with the assumption that the turbulent eddies have random lifetimes. The temperature evolution of particles along their trajectory is calculated from a heat flux balance in the boundary layer surrounding the particles. Such a model provides physical insight on phenomena that cannot be easily measured directly as the flow fields inside the plasma torch and the time-variation of the flow characteristics and particle behaviour.