Ph.D. Thesis Defense by Stephen D. Hill
Monday, August 23, 1999

(Dr. Prateen Desai, advisor)

"Plasma Torch Interaction With a Melting Interface"

Abstract

A model of a partially ionized, high pressure plasma in stagnation flow as it melts a nonhomogeneous solid is investigated by coupling two separate processes.  The first involves melt processing across a moving boundary, and the second, an impinging plasma arc jet in stagnation flow against an electrode.

The study of a plasma arc torch that impinges on a solid surface to melt it encompasses both the analysis of the multi-fluid plasma to ascertain its bulk temperature and the heat flux profile, as well as its interaction with a receding melt interface in and around the stagnation domain.  The model examined in this study couples the plasma motion, bulk energy, electron and ion densities and temperatures, with impinging jet theory to determine the amount of heat transfer into the particular substrate.  The so-called “multi-fluid” equations are derived for an axially symmetric plasma from the Boltzmann equations for Maxwellian velocity distributions. The equations are scaled by examining the dominant effects, and the role of the driving dimensionless parameters is established.  Two separate models are coupled during the computations, the first one to ascertain the moving boundary phase change heat transfer characteristics, and the second one to characterize the plasma jet behavior.

A parametric study evaluates important torch characteristics including mass flow rate of the gas, temperature of the plasma bulk, and proximity to the surface, as it influences the substrate melt zone. Calculations of solid/liquid interfacial location, radius of the melt zone, and depth of the melt zone ensue which provide a qualitative view on the vitrification process.