On the double-arcing phenomenon in a cutting arc torch

LEANDRO PREVOSTO; HÉCTOR KELLY; BEATRIZ MANCINELLI

Numerical Simulations of Physical and Engineering Processes

In Tech Open Access Publisher

Año: 2011; p. 1 - 22

Transferred arc plasma torches are widely used in industrial cutting process of metallic materials because of their ability to cut almost any metal and the very high productivity that can be achieved with this technology. The plasma cutting process is characterized by a transferred electric arc that is established between a cathode, which is a part of the cutting torch, and a work-piece (the metal to be cut) acting as the anode. In order to obtain a high-quality cut, the plasma jet must be as collimated as possible and also must have high power density. To this end, a transferred arc is constricted by a metallic tube (a nozzle) with a small inner diameter (of the order of one millimeter). Usually, a vortex-type flow is forced through the nozzle to provide arc stability and to protect its inner wall. In such case the arc is confined to the center of the nozzle, while centrifugal forces drive the cold fluid towards the walls of the nozzle, which is thus thermally protected. In addition to the circumferential component of the vortex flow, there is also a superimposed axial component that continuously supplies cold fluid. The intense convective cooling at the arc fringes due to the vortex flow enhances the power dissipation per unit length of the arc column, which in turn, results in high arc axis temperatures. Since the nozzle is subjected to a very high heat flux, it is made of a metal with a high thermal conductivity (copper is broadly used). The arc current is of the order of ten up to few hundred amperes, and the gas pressure is a few atmospheres. Arc axis temperatures close to 25 kK or even higher have been reached. A typical configuration of a cutting torch is presented in Fig. 1(a). In the normal mode of operation ?Fig. 1(a)?, the nozzle is a floating conductor (i.e., it is not electrically connected to any part of the torch circuit). However, such operating mode is somewhat unstable. The much higher electrical conductivity of the nozzle as compared with that of the confined arc column would cause instability. Under certain operating conditions (too large arc current, too small gas mass flow, or a nozzle with a too small bore diameter or a too large length) the level of arc stabilization provided by the vortex flow can be insufficient and the arc, which normally connects the cathode and the work-piece, is broken into two. One of them connects the cathode with the nozzle and the other connects the nozzle with the anode ?see Fig. 1(b)?; following the path of smallest electrical resistance. Such type of arc instability is called double-arcing. From a practical point of view, the double-arcing is very undesirable, since the arcs root on the nozzle (specially the arc originated from the cathode) wall usually destroy the nozzle almost instantaneously.   Figure 1. Normal (a) and double-arcing (b) modes of nozzle operation of transferred arc cutting torch.   As the double-arcing event takes place in relatively small, bounded regions, inaccessible to most experimental probing, experimental data on this phenomenon are hard to obtain. However, the metallic nozzle that bounds the arc can be considered itself as a large-sized Langmuir probe, and it can be used to collect charges from the contiguous plasma (i.e., to build the current-voltage characteristic curve of the nozzle) and therefore to obtain information about the plasma state inside the nozzle. The necessary condition for a comprehensible use of the probe (that is: the plasma should not be perturbed sufficiently far away from the probe surface) is accomplished since in this case the nozzle-probe behaves as a boundary condition to the arc. This chapter is devoted to a comprehensive study of the double-arcing phenomenon (common not only to arc cutting processes but to other industrial processes as well) which is one of the main drawbacks that put limits to increasing capabilities of the plasma arc cutting process. The starting point for such a study is the analysis and interpretation of the nozzle current-voltage characteristic curve. A complementary numerical study of the space-charge sheath formed between the plasma and the nozzle wall of a cutting torch is also reported. A physical interpretation on the origin of the double-arcing phenomenon is presented, that explains why the double-arcing (that it is established when the space-charge sheath adjacent to the nozzle wall breaks-down) appears for example at low values of the gas mass flow.