Nonequilibrium Modeling of Steam Plasma in a Nontransferred Direct-Current Torch

Abstract

This paper proposes a magnetohydrodynamic model that does not assume local thermal equilibrium (LTE) to predict the flow field and electron temperature and species density distributions inside a direct-current nontransferred-arc steam torch with a well-type cathode at atmospheric pressure. A steady, axisymmetric flow is assumed, and the azimuthal velocity component is not neglected. A finite volume method is adopted to solve the continuity equation, the current continuity equation, the momentum equation, and the energy equation according to a k − ε turbulence model. Ohm's law is used to calculate the induced magnetic field, and Ampere's circuital law is adopted to compute the current distribution in space. Fifty-two chemical and plasma kinetic equations are employed to describe the interactions among the 12 main species of water plasma, H electrons, considered in this paper. The electron temperature is predicted from the transport equation based on the electron energy balance, whereas the transport coefficients of the plasma are obtained from the Chapman-Enskog solution of the Boltzmann equation. The thermal plasma flow of steam inside the plasma torch, which measures 503 mm in length and 9 mm in radius, is predicted to have I = 180 A and Q = 5 g/s. The non-LTE simulation suggests that a plasma temperature of 11 600 K and flow velocity of 3.8 km/s can be achieved at the center of the torch outlet. Compared with the LTE model, the non-LTE calculation yields lower plasma temperature and higher axial velocity at the torch outlet. The calculated electron temperature principally varies between 1 and 3 eV, and energetic electrons occur not only near the electrode surfaces but also within the axial electric arc. The model predictions suggest that the thermal plasma is well approximated as LTE in the hot plasma core, which is characterized by an isothermal contour of 10 kK, and in the vicinity of the electrodes. The deviation from the LTE condition predominantly grows with the radial coordinate, and the temperature ratio approaches unity at the electrode surfaces. The dominant species components of the water plasma in the inner region (r < 6 mm) at the torch outlet are monoatomic and ionic species of H and O, whereas the diatomic species and water molecule become relatively important in the outer region (r > 6 mm) at the torch outlet.

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

“…in the torch by evaporation of a liquid with a strong tangential velocity component. There are only a few research studies using plasma torches with a configuration creating a swirl plasma velocity component, for example [33][34][35], for the purpose of future industrial application. Ando [33] created a vortex plasma flow in argon with air as a carrier gas for photo-catalytic titanium oxide film deposition by thermal plasma chemical vapor deposition at atmospheric pressure.…”

“…However, no numerical modeling of the mixing of the plasma species (argon and air) in this torch was carried out. Chau [34,35] presented numerical modeling of a direct-current nontransferred-arc steam torch at atmospheric pressure with the non-trivial strong tangential velocity comp onent and turbulent effects. The problem can be treated as axisymmetric since the plasma torch has the geometry of a hollow pipe.…”

Modeling of argon–steam thermal plasma flow for abatement of fluorinated compounds

“…According to the author's knowledge there are only a few researches using plasma torches with a configuration creating a swirl plasma velocity component, for example [32][33][34], for the purpose of future industrial application. Experimental research [32] presents a vortex plasma flow in argon with air as a carrier gas for photo-catalytic titanium oxide film deposition by TP chemical vapor deposition at atmospheric pressure.…”

“…The torch operates at 100 A with air flow rate 3 l min −1 and argon flow rate 1-20 l min −1 . Chau [33,34] presented numerical modeling of direct-current nontransferred-arc steam torch at atmospheric pressure with the non-trivial strong tangential velocity component and turbulent effects. The problem is treated as an axisymmetric since the plasma torch has the geometry of a hollow pipe.…”

“…The problem is treated as an axisymmetric since the plasma torch has the geometry of a hollow pipe. The arcs operate at high currents (180-900 A), the steam plasma can be accelerated to a mean velocity of 1500 m s −1 at torch outlet with the mass flow rate of 5 g s −1 at 180 A. Nonequlibrium model with mole fractions of different steam plasma species is also presented [34].…”