Plasma-particle interactions in plasma spraying systems
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I. INTRODUCTION: M A T H E M A T I C A L M O D E L I N G OF PLASMA-PARTICLE INTERACTIONS
A great deal of work has been done on the modeling of plasma-particle interactions in recent years, and many significant milestones have been reached. Some examples include the following: (1) calculation of the plasma velocities and temperatures in a plasma plume as a basis for particle models; 0-5] (2) presentation of the particle equations of motion and analysis of the Basset history term; ]6-9] (3) the effect of variable plasma properties in the particle boundary layer on drag coefficient and heat-transfer coefficient;t6,7,10-131
(4) evaporation and vaporization effects on particle momentum and heat t r a n s f e r ; 03-]6] (5) the effects of deviations from continuum behavior (Knudsen effects); [17,181 (6) the effects of internal heat conduction in the particle; [15A9'2~ and (7) coupling between plasma and particles in dense loading conditions. E21] Despite these and many other efforts over the past years, a fully unified description of the interaction between injected particles and plasmas has not yet been presented. Experimental information concerning velocities, temperatures, trajectories, and particle sizes is still needed. Modeling efforts in plasma systems make use of parameters derived from empirical correlations which may not be applicable to plasma systems. In addition, vaporization and noncontinuum phenomena require more study. To test the description provided by plasma-particle R. WESTHOFF, Research Assistant, G. TRAPAGA, Postdoctoral Associate, and J. SZEKELY, Professor, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. This article is based on a presentation made in the symposium "Spray Processing Fundamentals: Coating and Deposition" presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee. METALLURGICAL TRANSACTIONS B
interaction models, a comparison with careful experimental measurements is needed. The work presented here includes a single particle model for the calculation of velocities, trajectories, and thermal histories of the particles. This model has incorporated the results from another computer code (developed in this laboratory), which calculates the fluid flow and thermal characteristics of the plasma jet. In this formulation, allowance has been made for noncontinuum effects, for particle vaporization, and for temperature gradients within the particle. The theoretical predictions are compared with experimental measurements obtained from two different sources, namely, experimental data reported by Lesinski et a1.[22] for the injection of alumina particles into a roomtemperature turbulent air jet and simultaneous measurements of the temperature, size, and velocity of alumina particles in a nontransferred plasma jet (with the injection being from inside the torch). This second source consists of work described in earli
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