Enthalpy probe measurements and three-dimensional modelling on air plasma jets generated by a non-transferred plasma torch with hollow electrodes

Abstract

Thermal flow characteristics of air plasma jets generated by a non-transferred plasma torch with hollow electrodes are experimentally and numerically investigated in order to provide more reliable scientific and technical information, which has been insufficient for their practical applications to material and environmental industries. In this work, a thermal plasma torch of hollow electrode type is first designed and fabricated, and similarity criteria for predicting operational conditions for the scale-up to high-power torches are derived from the arc voltage characteristics measured with various operating and geometry conditions of the torch. The thermal flow characteristics of air plasma jets ejected from the torch are measured by enthalpy probe diagnostics and turn out to have relatively low temperatures of around 3000–7000 K, but show features of other unique properties, such as high energy flux, broad high temperature region and long plasma jet with moderate axial velocity, which are promising for their applications to material syntheses and hazardous waste treatments. Such high enthalpy at a relatively low temperature of air thermal plasma compared with the argon one is due to the high thermal energy residing in the vibrational and rotational states and oxygen dissociation, besides the translational states in monatomic gases such as argon. It is expected that this high specific enthalpy of the air plasma will enable material and environmental industries to treat a large amount of precursors and waste materials effectively at a lower temperature for a longer residence time by the low plasma velocity. It is also found from the measurements that the turbulence intensity influenced by the size of the electrode diameter has a significant effect on the axial and radial profiles of plasma jet properties and that a longer plasma jet is more readily achievable with a larger electrode diameter reducing the turbulence intensity in the external region of the torch. In the numerical studies based on magnetohydrodynamics (MHD) theory, a precise three-dimensional transient numerical model for the internal arc discharge plasma of the torch has been developed along with a practical two-dimensional stationary one for the external thermal plasma jet by considering highly localized distributions of arc roots with circumferential non-uniformity on the electrode wall surfaces, so that more reliable and realistic descriptions on the arc thermal plasma properties become feasible both inside and outside the torch. The numerical calculation results are compared with the experimental data obtained from the probe measurements and found to be in good agreement with them.

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].…”

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) .…”

“…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) . Furthermore, both arc spots on anode and cathode surfaces are almost fixed in axial direction due to segments structure, and they rotate azimuthally because of the magnetic body force induced by the arc discharge.…”

“…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.…”

“…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) .…”