Modelling and diagnostics of multiple cathodes plasma torch system for plasma spraying

Abstract

Usage of a multiple-arcs system has significantly improved process stability and coating properties in air plasma spraying. However, there are still demands on understanding and controlling the physical process to determine process conditions for reproducible coating quality and homogeneity of coating microstructure. The main goal of this work is the application of numerical simulation for the prediction of the temperature profiles at the torch outlet for real process conditions. Behaviour of the gas flow and electric arcs were described in a three-dimensional numerical model. The calculated results showed the characteristic triangular temperature distribution at the torch nozzle outlet caused by three electric arcs. These results were compared with experimentally determined temperature distributions, which were obtained with specially developed computed tomography equipment for reconstructing the emissivity and temperature distribution of the plasma jet close to the torch exit. The calculated results related to temperature values and contours were verified for the most process parameters with experimental ones.

Keywords

plasma spraying / electric arc / three-cathode plasma torch / numerical simulation / computed tomography

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Acknowledgements

The authors gratefully acknowledge the financial support of the German Research Foundation (DFG) within the project “Homogenization of Coating Properties in Atmospheric Plasma Spraying” (Bo 1979/7-1 and Sche 428/6-1).

Notations
\overrightarrow{B}	magnetic flux density
cp	specific heat capacity by constant pressure
\overrightarrow{D}	electric displacement field
\overrightarrow{E}	electric field
{\overrightarrow{f}}_{\mathrm{L}}	Lorentz force density
f_{\mathrm{T}}	temperature distribution
f_{\mathrm{v}}	velocity distribution
\overrightarrow{g}	gravitation acceleration
\dot{m}	mass flow
\overrightarrow{H}	magnetic field
h	enthalpy density
\overrightarrow{j}	current density
p	pressure
r	radial coordinate
SJoule	joule heating rate density
SRad	radiation rate density
S_{\mathrm{N}\mathrm{e}\mathrm{t}}	net emission rate density
S_{\mathrm{A}\mathrm{b}\mathrm{s}}	reabsorbed radiation rate density
T	temperature
t	time
v	plasma velocity
{\dot{W}}_{\mathrm{e}\mathrm{f}\mathrm{f}}	effective power
\overrightarrow{u}	gas velocity
Greek letters
\lambda	thermal conductivity
\rho	gas density
\mathrm{\Theta }	angular coordinate
{\rho }_{\mathrm{f}}	charge density
\sigma	electric permeability
\overrightarrow{\tau }	stress tensor
{\mu }_{\mathrm{f}}	magnetic permeability

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