Modeling of Micro- and Nanoparticle Characteristics in DC Suspension Plasma Spray
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Xue-ming Shao, Kai Zhang, and Hong-bing Xiong (Submitted October 30, 2013; in revised form October 17, 2014) Suspension plasma spray is a promising technology for surface coatings. In this work, a comprehensive numerical model was developed to investigate the multiphase flow of suspension droplets and nanoparticles in direct-current (DC) plasma spraying. A three-dimensional computational model was developed to describe the plasma jet flow fields coupled with the axial injection of suspension droplets in which the zirconia micro- and nanoparticles were dispersed. The suspension droplets were tracked using Lagrangian coordinates, considering particle heating, melting, and evaporation. After evaporation of the solvent surrounding the particle, the nanoparticles were discharged into the plasma flow. In addition to the viscous force exerted by the flow on the micrometer-sized particles, the Brownian force and the Saffman lift force were taken into account. The effects of the noncontinuum on particle momentum transfer and evaporation on heat transfer were also considered. The numerical predictions of gas flow temperature were compared with experimental data and numerical data obtained with a different computational fluid dynamics code. The agreement was reasonable. The trajectories, velocity, and temperature of nanoparticles were calculated, and compared with those of microparticles. The results showed that the Brownian force plays a major role in acceleration and heating of nanoparticles. Compared with the conventional plasma spray process with micrometer-sized feedstock, the nanoparticles in suspension plasma spraying were found to have a wider spatial distribution and higher temperature. The effects of operating parameters, such as the power input to the plasma gas and plasma gas composition, on the gas velocity and temperature were investigated. The parameters that have a significant effect on the heat and momentum transfer to the particles injected in the plasma jet were identified.
Keywords
Nomenclature
Lagrangian method, particle force, plasma spray, suspension plasma spraying, two-phase flow
1. Introduction Suspension plasma spray (SPS) is a newly developed technology for surface processing that has attracted increasing attention in recent decades (Ref 1-4). It uses much finer particles, to achieve finely structured coatings. As a result, coatings with improved characteristics, such as high adhesion and low porosity, can be obtained (Ref 5). Compared with coatings made by conventional thermal spraying, nanostructured coatings have superior resistance to wear, erosion, and cracking (Ref 6). The SPS process involves liquid feedstock preparation and injection into a plasma jet, liquid atomization, plasma jet generation, and interaction of the plasma jet with the droplets and particles. In this process, submicrometer particles are usually dispersed in a solvent, and injected into the plasma jet by an atomization device. After the
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