Technique of Formation of an Axisymmetric Heterogeneous Flow During Thermal Spraying of Powder Materials
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V.I. KuzÕmin, A.A. MikhalÕchenko, O.B. Kovalev, E.V. Kartaev, and N.A. Rudenskaya (Submitted November 30, 2010; in revised form October 11, 2011) The paper presents an investigation of a unit of annular injection of powder materials into a thermal plasma flow. The unit is designed for the electric-arc direct-current plasma torch with a sectioned interelectrode insert up to 100 kW, which was developed earlier. Energy characteristics (thermal efficiency and thermal power of the plasma jet) and spectra of plasma torch current and voltage fluctuations are described. The characteristics of the radial temperature distribution in the plasma jet in the annular and point powder injection cases are compared. A multi-channel spectrometer with a photo-diode array was implemented for the measurements. It is shown that, in contrast to point injection of powder particles, which is carried out across the jet on the nozzle exit, distributed annular injection with gas-dynamic focusing provides a dense axisymmetric heterogeneous flow, in which almost all particles pass through a high-temperature and high-speed area near the plasma jet axis.
Keywords
axisymmetric heterogeneous flow, plasma torch with an inter-electrode insert (IEI), refractory material spraying, unit of annular injection of particles
The efforts to develop a plasma torch with axial injection of particles through a cathode hole failed because the powder material itself and its vapors affect the
Nomenclature
1. Introduction By now, the conventional technique of injection of a processed powder material into a thermal plasma jet generated by a commercial plasma torch is the point transverse injection (usually at an angle of 90 to the plasma jet axis) through a tube flux duct at the nozzle exit or through an orifice in the nozzle (Ref 1-4). This kind of powder injection significantly disturbs the carrier flow, which enhances the interaction between the plasma jet and the ambient medium, thus, resulting in fast dissipation of energy in the high-temperature area of the jet and in distortion of the radial temperature profile. Because of the resultant nonuniformity of temperature and velocity fields in the plasma jet, the particles in the jet cross sections may have appreciably different thermal and kinetic energy and even may be in different aggregate states. For this reason, the quality of coatings obtained by the plasma spraying method is appreciably deteriorated. Moreover, local point injection has a very low efficiency of plasma jet energy utilization. Normally, it does not exceed 6% (Ref 5).
V.I. KuzÕmin, A.A. MikhalÕchenko, O.B. Kovalev, and E.V. Kartaev, Khristianovich Institute of Theoretical and Applied Mechanics SB RAS, Novosibirsk, Russia; and N.A. Rudenskaya, Scientific and Technological Park of the BNTU ‘‘Metolit’’, Minsk, Belarus. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
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T h k c x, y
Temperature Planck constant Boltzmann constant Light velocity in vacuum Coordinates in the Cartesian system of coordinates (mm)
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