Comparative study of two- and three-dimensional modeling on arc discharge phenomena inside a thermal plasma torch with hollow electrodes

Abstract

A comparative study between two-and three-dimensional ͑2D and 3D͒ modeling is carried out on arc discharge phenomena inside a thermal plasma torch with hollow electrodes, in order to evaluate the effects of arc root configuration characterized by either 2D annular or 3D highly localized attachment on the electrode surface. For this purpose, a more precise 3D transient model has been developed by taking account of 3D arc current distribution and arc root rotation. The 3D simulation results apparently reveal that the 3D arc root attachment brings about the inherent 3D and turbulence nature of plasma fields inside the torch. It is also found that the constricted arc column near the vortex chamber plays an important role in heating and acceleration of injected arc gases by concentrating arc currents on the axis of the hollow electrodes. The inherent 3D nature of arc discharge is well preserved inside the cathode region, while these 3D features slowly diminish behind the vortex chamber where the turbulent flow begins to be developed in the anode region. Based on the present simulation results, it is noted that the mixing effects of the strong turbulent flow on the heat and mass transfer are mainly responsible for the gradual relaxation of the 3D structures of plasma fields into the 2D axisymmetric ones that eventually appear in the anode region near the torch exit. From a detailed comparison of the 3D results with the 2D ones, the arc root configuration seems to have a significant effect on the heat transfer to the electrode surfaces interacting with the turbulent plasma flow. That is, in the 2D simulation based on an axisymmetric stationary model, the turbulence phenomena are fairly underestimated and the amount of heat transferred to the cold anode wall is calculated to be smaller than that obtained in the 3D simulation. For the validation of the numerical simulations, calculated plasma temperatures and axial velocities are compared with experimentally measured ones, and the 3D simulation turns out to be more accurate than the 2D simulation as a result of a relatively precise description of the turbulent phenomena inside the torch using a more realistic model of arc root attachment. Finally, it is suggested that the 3D transient formulation is indeed required for describing the real arc discharge phenomena inside the torch, while the 2D stationary approach is sometimes useful for getting practical information about the time-averaged plasma characteristics outside the torch because of its simplicity and rapidness in computation.

Thermal Plasma Decomposition of Fluorinated Greenhouse Gases

“…In the numerical simulation, the governing fluid equations consisting of mass, momentum, and energy conservation are solved under the steady-state, twodimensional, and axis-symmetric conditions. Although the turbulent thermal plasma is an inherently threedimensional time transient phenomena, a time averaged axis-symmetric modeling on thermal plasma jets have been in good agreement with experimentally measured results [50][51][52]. Since the k-ε turbulence model is widely used for turbulent thermal plasma flow modeling, it is also incorporated to include turbulence effects [53].…”

“…(1) plasma characteristics in the arc discharge region of the torch interior are numerically analyzed by using a MHD (magnetohydrodynamics) code which has been developed to simulated thermal plasmas [46][47][48][49][50]. Second, the thermal plasma characteristics inside the decomposition reactor region where a complex thermal flow mixture between plasmas jet and waste gas is formed, can be precisely and stably simulated by using a commercial CFD (computational fluid dynamics) code, FLUENT (ANSYS), which has been widely used for solving complex hydrodynamic flows.…”

Effects of Constrictor Geometry, Arc Current, and Gas Flow Rate on Thermal Plasma Characteristics in a Segmented Arc Heater

“…For the numerical simulation, the governing fluid equations consisting of mass, momentum and energy conservation, were solved under the steady-state, two-dimensional, and axis-symmetric conditions. Although the arc discharge is an inherently three-dimensional time transient phenomena, an axis-symmetric steady-state modeling on thermal plasma jet have been in good agreement with experimentally measured results (7)(8)(9) . Because both the constricted arc column and the enhanced mixing effects of the turbulent flow are responsible for the gradual relaxation of the time transient three-dimensional structures of plasma fields into the steady-state two-dimensional axis-symmetric fields (9) .…”

“…In order to include radiation effects, optically thin plasma with the net emission coefficient in a local thermodynamic equilibrium (LTE) state was assumed (10) . Since the k-ε turbulence model has been widely used for a numerical modeling for the high velocity arc plasma jet, it was also incorporated to include turbulence effects inside the arc heater (4)(5)(6)(7)(8)(9)(10)(11) .…”

“…The arc discharge and the thermal plasma jet generated in the segmented arc heater was simulated by using a magnetohydrodynamic (MHD) code, DCPTUN, which has been developed in the author's laboratory (4)(5)(6)(7)(8)(9) . For the numerical simulation, the governing fluid equations consisting of mass, momentum and energy conservation, were solved under the steady-state, two-dimensional, and axis-symmetric conditions.…”