Facultad Regional Venado Tuerto

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    On the Double-Arcing Phenomenon in a Cutting Arc Torch.
    (2011) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    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 (Boulos et al., 1994). 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 a high power density. To this end, the 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 with large axial and azimuthal velocity components is forced through the nozzle to provide arc stability and to protect its inner wall. In such case the hot arc is confined to the center of the nozzle, while centrifugal forces drive the colder fluid towards the nozzle walls, which are thus thermally protected. The axial component of the gas flow continuously supplies cold fluid, providing an intense convective cooling at the arc fringes. In addition, the vortex flow enhances the power dissipation per unit length of the arc column, resulting in high temperatures at the arc axis. 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 a few hundred amperes, and the gas pressure is several atmospheres. Arc axis temperatures around 15 kK are usual, but larger values, close to 25 kK or even higher, have been reached.
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    On the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon
    (2011) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    Experimental observations on the plasma dynamics inside the nozzle of a 30 A oxygen cutting torch operated at conditions close to the double arcing are reported. It is employed a technique previously developed in our laboratory consisting in using the nozzle as a large-sized Langmuir probe. Based on the behavior of the ion current signal and simple estimations, it is concluded that (1) the non-equilibrium plasma inside the nozzle is far from the steady state in time, in contrast to what is frequently assumed. The power supply ripple was identified as the main fluctuations source and (2) large-scale plasma fluctuations inside the nozzle could cause transient (total duration of the order of 100 ls) Townsend avalanches developing in the space-charge layer located between the arc plasma and the nozzle wall. Such events trigger the so called non-destructive double-arcing phenomena without appealing to the presence of insulating films deposited inside the nozzle orifice, as was previously proposed in the literature.
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    Numerical investigation of the double-arcing phenomenon in a cutting arc torch.
    (2014) Mancinelli, Beatriz; Minotti, Fernando; Prevosto, Leandro; Kelly, Héctor
    A numerical investigation of the double-arcing phenomenon in a cutting arc torch is reported. The dynamics of the double-arcing were simulated by using a two-dimensional model of the gas breakdown development in the space-charge layer contiguous to the nozzle of a cutting arc torch operated with oxygen. The kinetic scheme includes ionization of heavy particles by electron impact, electron attachment, electron detachment, electron–ion recombination, and ion–ion recombination. Complementary measurements during double-arcing phenomena were also conducted. A marked rise of the nozzle voltage was found. The numerical results showed that the dynamics of a cathode spot at the exit of the nozzle inner surface play a key role in the raising of the nozzle voltage, which in turn allows more electrons to return to the wall at the nozzle inlet. The return flow of electrons thus closes the current loop of the double-arcing. The increase in the (floating) nozzle voltage is due to the fact that the increased electron emission at the spot is mainly compensated by the displacement current (the ions do not play a relevant role due to its low-mobility) until that the stationary state is achieved and the electron return flow fully-compensates the electron emission at the spot. A fairly good agreement was found between the model and the experiment for a spot emission current growth rate of the order of 7 x 104 A/s.
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    Experimental Characterization of a Low-Current Cutting Torch.
    (2004) Kelly, Héctor; Mancinelli, Beatriz; Prevosto, Leandro; Minotti, Fernando; Márquez, Andrés
    An experimental characterization of a low-current (30-40 A) cutting torch is presented. To avoid contamination of the plasma arc by removed anode material, a rotating steel cylinder was used as the anode and the arc was anchored onto the cylinder lateral surface. The cathode-anode and cathode-nozzle voltage drops, together with the gas pressure in the plenum chamber were registered for different values of the mass flow rate injected into the plenum chamber. By employing an optical system with a large magnification (≈ 15 X), the arc radius at the nozzle exit was also determined with a digital optical camera. The obtained experimental quantities were used to evaluate several flow properties at the nozzle exit (hot arc plasma and cold gas temperatures, arc and gas velocities, etc.) by employing a simplified theoretical model for the plasma flow in the nozzle. The obtained results are in reasonable agreement with the data reported in the literature by other authors. Explanations of the origin of the clogging effect and the nozzle voltage are also presented.
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    Determination of plasma velocity from light fluctuations in a cutting torch.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
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    Análisis y diseño de un convertidor resonante para aplicaciones de arcos de plasma.
    (UTN FRVT, 2012-10) Amigo, Jorge; Oggier, Germán; García, Guillermo
    En el presente trabajo se presenta un análisis y diseño de un convertidor resonante para alimentar una Antorcha de Arco Transferido (AAT), para corte de metales en procesos metalúrgicos. La posibilidad de utilizar un convertidor en modo resonante permite incrementar la frecuencia de conmutación, lo que permite reducir el volumen y el costo del convertidor. En este trabajo se analizan las pérdidas de potencia que se producen en los semiconductores de potencia de una topología resonante serie para una aplicación de AAT. Se establecen condiciones que permiten optimizan el diseño del convertidor y se incluyen resultados de simulación para validar el análisis.