| Suspension plasma spray is an important and complex surface processing technology. This technology can be utilized to manufacture finely structured coatings. In this process, liquid suspended with micrometer- or nanometer-sized solid particles is injected into a plasma jet. It involves plasma jet generation, droplet injection, solvent evaporation, and discharge, acceleration, heating and melting of the solid particles. Mass, momentum and heat transfer between particles and the flow field. The theoretical study on plasma spray is lagged far behind the technological applications due to the complex phenomena involved. This thesis aims at understanding the micro-and nanoparitcles’ dynamic and thermal behaviors. For this purpose, we develop a numerical model for the transport phenomena between particles and turbulent plasma jet, dealing the problem that is difficult to be measured in experiment, interpreting the mechanisms of complex phenomena, and optimizing the process parameters.A comprehensive three dimensional numerical program, integrating Eulerian method and Lagrangian approach, was developed to solve the governing equations of particle and flow field. Firstly, the flow field was solved using Eulerian method. The plasma jet was treated as multi-component, compressible, and chemical reacting gas. Explicit algorithm was used to solve the temporally difference equations of the plasma jet. The calculated centerline gas temperature profile was compared with experimental results. The projected gas temperature profile shows reasonable agreement with experimental data, and the deviation is within 10%. Providing the information of plasma gas field around the particle, all particles with different diameters were tracked as Lagrangian entities. A one-dimensional model was adopted to calculate the heating on droplet surface, solvent evaporation and heat conduction inside the micron particle. After the solvent evaporated out, momentum, heat and mass transport between discharged nanoparticles and the plasma jet were taken into account. Micro- and nanoparticles multiphase model was used to simulate the particles accelerating, collision, atomization, heating, melting and evaporation.Using the above three dimensional numerical model, dynamic behaviors of single and multiple partiles were studied. Firstly, the forces exerting on micro- and nano-sized particle in plasma flow field were analyzed. The three most important forces, including the drag force, Saffman lift force and Brownian force were considered in present model. The critical size of particle that can penetrate the boundary layer in front of the substrate was deduced under present operating conditions. Particles’s moving, atomization, collision, heating and evaporation processes were also simulated. Multiple particles effect on the velocity and temperature of the flow field was considered. Turbulent dispersion on multiple particles spatial distribution was also studied. The velocity and temperature of particles were predicted. Micro- and nano-sized particles’ accelerating, heating and spatial distributions were analyzed. The deviation of flow field temperature and velocity using two-way coupling and one-way coupling calculation was studied.Parametric study on plasma spray process was conducted. The effects of gas composition and power input on plasma flow field were analyzed. The deposition rate with different spray distances and substrate sizes were simulated. The effect of powder injection mode on the heating of particles was studied. The effects of injection parameters, such as, injection position, angle, velocity and the sizes of droplets and agglomerates on nanoparticles’discharge and heating processes were also studied. Parameters that are favorable for suspension plasma spray were presented.Using the present computational model, motion and thermal histories of single and multiple particles were discussed. Comprehensive parametric study was helpful to improve our understanding of suspension plasma spray process. |
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