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C.H. Chang
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Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 115-124, May 8–11, 2000,
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Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition (ZrO2) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity and direction, etc.) is introduced "one phenomenon at a time" to distinguish and characterize its effect and enable comparisons of separate effects. A final calculation then considers all phenomena simultaneously, to enable further comparisons. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is most significantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction, and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.
Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 141-148, May 8–11, 2000,
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Thermal spray processing of functionally graded materials requires that the spray patterns of different particle types coincide at impact and that each particle type arrives with the appropriate temperature and degree of melting. Measurements of particle velocity, temperature, and size along with spray pattern characteristics have been obtained for co-injected NiCrAlY and zirconia powder. The plasma and particle flow fields were also simulated with a pseudo 3-D model using the LAVA computer code. The model assumes that the gas flow is axisymmetric while the particles are treated in a fully 3-D manner. A stochastic discrete-particle model that includes turbulent dispersion dictates particle behavior. The simulation produced reasonably accurate velocities and particle trajectories, although, particle temperature is consistently over predicted. Comparisons between the calculated and measured velocity and temperature statistical distributions and calculated molten fractions are discussed.
Proceedings Papers
ITSC1998, Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 315-327, May 25–29, 1998,
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The first part of this overview on plasma spraying covers the general behavior of plasma jets generated by d.c. and r.f. plasma torches, including the fluid dynamics of such jets. In the second part, interactions of injected powders with the plasma are considered with emphasis on those processes which dominate heat and momentum transfer from the plasma to the powder particles. Both experimental studies as well as modeling efforts are included in this overview.
Proceedings Papers
ITSC1996, Thermal Spray 1996: Proceedings from the National Thermal Spray Conference, 541-546, October 7–11, 1996,
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High power supersonic plasma guns operating in excess of 200 kW can produce molten particles with 3 to 4 times the impact velocity of conventional plasma sprays. With this increased range of particle velocity it is important to understand the relationship between the torch input parameters and the sprayed particle velocity, temperature, pattern and size. Stainless steel particle velocity, temperature, size and relative number are measured for a high power plasma spray system operating at 110 kW. At the same torch operating conditions the plasma and particle flow fields are simulated with a newly developed computational model. It was found that the injection geometry plays an important role in the particle entrainment, heating and acceleration. In spite of the complexity of the system, i.e. supersonic plasma velocity with a high swirl component, the simulation produced reasonalble particle trajectories resulting in good agreement between the calculated and measured particle velocity, temperature and size distributions.