Modeling of inhomogeneous mixing of plasma species in argon–steam arc discharge

This paper presents numerical simulation of mixing of argon- and water-plasma species in
an argon–steam arc discharge generated in a thermal plasma generator with the combined
stabilization of arc by axial gas flow (argon) and water vortex. The diffusion of plasma species
itself is described by the combined diffusion coefficients method in which the coefficients
describe the diffusion of argon ‘gas,’ with respect to water vapor ‘gas.’ Diffusion processes
due to the gradients of mass density, temperature, pressure, and an electric field have been
considered in the model. Calculations for currents 150–400 A with 15–22.5 standard liters per
minute (slm) of argon reveal inhomogeneous mixing of argon and oxygen–hydrogen species
with the argon species prevailing near the arc axis. All the combined diffusion coefficients
exhibit highly nonlinear distribution of their values within the discharge, depending on the
temperature, pressure, and argon mass fraction of the plasma. The argon diffusion mass flux
is driven mainly by the concentration and temperature space gradients. Diffusions due to
pressure gradients and due to the electric field are of about 1 order lower. Comparison with
our former calculations based on the homogeneous mixing assumption shows differences in
temperature, enthalpy, radiation losses, arc efficiency, and velocity at 400 A. Comparison with
available experiments exhibits very good qualitative and quantitative agreement for the radial
temperature and velocity profiles 2 mm downstream of the exit nozzle.