Particle Velocity and Temperature Histories in a Plasma Plume: A Comparison of Measurements and Predictions

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PARTICLE VELOCITY AND TEMPERATURE HISTORIES IN A PLASMA PLUME: A COMPARISON OF MEASUREMENTS AND PREDICTIONS G. Trapaga, R. Westhoff, J. Szekely Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, MA. 02139 J. Fincke and W. D. Swank Idaho National Engineering Laboratory, Idaho Falls, ID. 83415 ABSTRACT A mathematical formulation is presented to describe the interaction between a plasma and injected particles. In the formulation, a realistic allowance is made for intraparticle conduction, vaporization and non-continuum effects. The theoretical predictions are compared with experimental measurements and quite reasonable agreement has been obtained. A discussion is presented assessing the relative importance of the various factors that govern plasma-particle interactions. 1. INTRODUCTION The work described in this paper is part of a continuing effort aimed at the development of a faithful representation of plasma-particle interactions. This is a joint program between researchers at MIT and the Idaho National Engineering Laboratory. Plasma-particle interactions are of obvious practical interest, since the principal uses of plasma systems include the production of fine particles, reactions between plasmas and particles, and the melting of particles as part of a coating or spray forming operation. In order to develop a realistic representation of plasma-particle interactions one must understand:

"•the velocity, temperature, and concentration fields in the plasma in the absence of particles "•the trajectory of particles as they are injected into the plasma "•the actual heat, mass and momentum transfer process that takes place between the plasma gases and the injected particles. Focussing on the last mentioned point, while fluid-particle interactions have been studied for many years, notably by chemical and mechanical engineers, these phenomena take on added complexities in a plasma environment. The principal contributing effects are the very high velocities, acceleration and deceleration rates, the very high absolute temperatures and temperature gradients, particle vaporization, and possible departures from continuum behavior. A selection of references highlighting some of these effects follows.

"•Calculation of plasma plume velocities and temperatures as a basis for particle models(l-43).6 "•Presentation of the particle equations of motion and analysis of the Basset history term( - ). "•The effect of variable plasma properties in the particle boundary layer on drag coefficient and 4 57 8 heat transfer coefficient( - , - ).

9 11 "*Evaporation and vaporization effects on particle momentum and heat transfer( - ). "•The effect of deviations from continuum behavior (Knudsen effects)(1 2- 13). "*The effects of internal heat conduction in the particle(I 0 , 14 "15). "•Coupling between plasma and particles in dense loading conditions( 16 ).

Despite these and many other efforts over the past years, a fully unified description of the interaction between injected particles and plasma