Modelling of plasma–particle two-phase flow using statistical techniques

Abstract

A Lagrangian two-phase flow approach has been used to simulate the gas flow and simultaneously the motion and heating of particles in a plasma jet. This enables the interaction between the gas flow and feedstock powder particles to be investigated during the plasma spraying of engineering coatings. An essential element of the Lagrangian two-phase flow model applied in the present study is that, instead of tracing the motion and heating process of each individual particle, the particles are grouped into ‘parcels’. The particles in the same parcel will have the same location and physical properties. In addition, instead of using an average particle size, a statistical method has been used to simulate the more realistic case of injected particles with a normal size distribution. Numerical computations are carried out to correlate the influence of inter-phase interaction on gas flow and particle motion/heating with the powder injection conditions. The model is applied to study the influence of turbulence on the dispersed particles, gas–particle interactions and the motion and heating of the in-flight particles. The errors introduced by neglecting the existence of the dispersed phase are calculated and used as a measure of the practical importance of the model. The results indicate that the gas–particle interaction produces irregularities in the pattern of the plasma jet owing to the asymmetry of the powder trajectory together with the random components in the injection velocity and the size of the feedstock particles. The magnitude of the interaction increases monotonically with the powder feed rate. The critical feed rate above which gas–particle interactions become significant depends upon the operating conditions but would normally be approximately 2–3 kg/h. The effect of phase interaction above this critical rate on the temperature and velocity of in-flight particles would be expected to be greater than 5% and could be as high as 15%. The results indicate that a two-phase model taking into account particle–gas interactions should be used to simulate plasma spraying, particularly at high feed rates.