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The effects of the plasma's gas-flow rate on phenomena such as the negative anode fall and the heat flux to the anode in high-intensity arcs are discussed on the basis of a numerical model of the anode's boundary layer. The modelling system consists of a rotationally symmetrical argon plasma formed between the outlet of a constrictor tube and a water-cooled flat copper anode perpendicular to the axis of the plasma flow which is directed towards the anode. The arc is operated at atmospheric pressure and at a current level of 200 A. The boundary conditions for the electron temperature and the electron number density at the anode's surface are obtained by solving the electron-energy equation and the diffusion equation at the anode's surface simultaneously with all conservation equations for the calculation domain. A diffuse anode attachment is obtained for a mass flow rate exceeding 0.2 g and a constricted anode attachment is found for rates below 0.02 g . There is a (probably unstable) transition region between these flow rates. The anode boundary layer of the diffuse attachment is strongly affected by the mass-flow rate. Increasing the mass-flow rate shifts the location of the peak of the electrical potential towards the anode while its magnitude decreases and the total heat flux to the anode increases. This is a consequence of the effects of the mass-flow rate on the axial profiles of electron and heavy-particle temperatures and of the electron number density. In contrast, the anode boundary layer of the constricted attachment seems not to be affected by the mass-flow rate.

A numerical model for an electric arc stabilized by a water vortex has been proposed. The two-dimensional axisymmetric model includes the discharge area between the cathode and the orifice of the arc chamber. The production of water plasma, i.e. the rate of evaporation of a water wall, is taken either from experiments or is determined numerically by fitting of the outlet plasma parameters to the experimental ones. The computer results concern thermal, fluid dynamic and electrical characteristics of such arcs for the currents 300, 400, 500 and 600 A. It is found, for example, that the role of thermal diffusion within the discharge increases with current. The power losses from the arc due to radial conduction and radiation represent around 50% of the input power. Rotation of the plasma column due to the induced tangential velocity component has negligible effect on the overall arc performance. The calculated velocities, pressure drops and electrical potentials are in good agreement with experiments carried out on the water plasma torch PAL-160 operating at our Institute.

This paper presents a numerical investigation of characteristics and processes in the worldwide unique type of thermal plasma generator with combined stabilization of arc by argon flow and water vortex, the so-called hybrid-stabilized arc. The arc has been used for spraying of ceramic or metallic particles and for pyrolysis of biomass. The net emission coefficients as well as the partial characteristics methods for radiation losses from the argon–water arc are employed. Calculations for 300–600 A with 22.5–40 standard litres per minute (slm) of argon reveal transition from a transonic plasma flow for 400 A to a supersonic one for 600 A with a maximum Mach number of 1.6 near the exit nozzle of the plasma torch. A comparison with available experimental data near the exit nozzle shows very good agreement for the radial temperature profiles. Radial velocity profiles calculated 2 mm downstream of the nozzle exit show good agreement with the profiles determined from the combination of calculation and experiment (the so-called integrated approach). A recent evaluation of the Mach number from the experimental data for 500 and 600 A confirmed the existence of the supersonic flow regime.

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