High Speed Video Camera and Electrical Signal Analyses of Arcs Behavior in a 3-Phase AC Arc Plasma Torch

A 3-phase AC plasma torch has been developed and aims at overcoming some limits of the classical DC torches in terms of efficiency, cost and reliability. However, the arc behavior in 3-phase plasma torch remains poorly explored. This paper is dedicated to the high speed video camera at 100,000 frames per second and electrical signal analyses of arcs behavior in a 3-phase AC arc plasma torch. First, a reference case at 150 A, in nitrogen as working gas, has been deeply analyzed. Afterwards, a parametric study based on current and inter-electrode gap has been carried out. Results show that only one arc can exist at a given time and arcs rotate by switching from a pair of electrodes to another one, following the maximal electrical gap potential. However, a particular “abnormal” arc behavior was sometimes observed. Indeed, the arc motion within the inter-electrode gap increases the heat exchange and stabilizes the 3-phase discharge whereas the system is unbalanced when the arc is in the periphery. The analysis highlights that the arc motion is strongly influenced by the electrode jet velocity and repulsive Lorentz forces. The parametric study shows that the current increases both jet velocity and arc discharge stability. Elsewhere, the increase of the inter-electrode gap can also stabilizes the electrical 3-phase arc discharge. Furthermore, the correlation between arc motion and current waveform is highlighted. This work is likely to open the way toward a better understanding of 3-phase discharges in the perspective of their further optimization.     

Influence of the Electromagnetic Forces on Momentum and Heat Transfer in a 3-Phase ac Plasma Reactor

A new 3-phase ac plasma reactor has been developed within the framework of research on hydrocarbon cracking for the production of carbon black and hydrogen. ^(1,2) One of the main characteristics of the system is related to the 3-phase, 50 Hz ac current plasma generator which induces a very particular arc motion affecting the heat and mass transfer inside the reactor. In a first step, the general flow inside the reactor in the absence of hydrocarbon injection has been studied. A simplified approach to characterize the heat and mass transfer inside the reactor is presented in this paper. The arc zone analysis is carried out simultaneously by a theoretical analysis of the electromagnetic forces and by an ultrahigh-speed cine-camera analysis. The flow in the reactor is modeled with a CFD commercial code. Results are compared with experimental temperature measurements.     

Comparison of Simple Particle-Radiation Coupling Models Applied on a Plasma Black Process

The effect of soot formation on the radiative heat transfer inside a plasma reactor for carbon black synthesis has been modelled. For this purpose, three methods to quantify the soot volume fraction have been tested and evaluated: (1) Assuming local thermodynamic equilibrium, (2) assuming instantaneous conversion of the hydrocarbon and (3) using a single-step soot model. These approaches have been studied using a two-dimensional axis-symmetric and a three-dimensional steady computational fluid dynamics (CFD) model based on the commercial software FLUENT (v.5.6). The CFD model includes turbulence effects (by standard k– model), an electric arc sub-model to describe the time-average and spatial-average Lorentz forces and ohmic heating generated by the three-phase power-supply, methane transport in a nitrogen plasma and radiation calculations. The calculations show that for the simulated operating conditions the resulting temperature distributions obtained are very similar with the three methods in spite of major differences in the treatment of particle formation in the different methods.     

A Comparison Between MHD Modeling and Experimental Results in a 3-Phase AC Arc Plasma Torch: Influence of the Electrode Tip Geometry

Arc behavior in 3-Phase AC plasma technology remains poorly explored. This system noticeably differs from the classical DC plasma torches and aims to overcome certain limitations, such as efficiency, equipment cost and reliability. A MHD model of a 3-Phase AC plasma torch was recently developed at Mines-ParisTech. The model does not include the electrodes in the computational domain. In parallel, experiments were conducted using a high-speed video camera shooting 100,000 frames per second. In this paper, the comparison between MHD modeling and experimental results shows that the arc behavior is in line with the results from the MHD model. Particularly, the strong influences of both the electrode jets and Lorentz forces on the arc motion are confirmed. However, some differences between experimental and numerical electrical waveforms are observed and particularly in the current–voltage phase shift. A new model was then developed by integrating the electrodes into the computational domain and adjusting the electrode tip geometry. With this simulation, we were able to reproduce the phase shift, power and voltage values with a good accuracy showing the strong influence of electrode tip geometry on the 3-Phase arc plasma discharge.     

MHD Modeling of the Tip-to-Plane Plasma Arc Behaviour at Very High Pressure in CF4

Plasma Chemistry and Plasma Processing - In this paper, the behaviour of a tetrafluoromethane (CF4) plasma arc under conditions of low current (0.25–0.4 A) and very high pressure (10...