FRVT - Artículos en Revistas
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Item Investigation of the relevant kinetic processes in the initial stage of a double-arcing instability in oxygen plasmas.(2018) Mancinelli, Beatriz; Prevosto, Leandro; Chamorro, Juan Camilo; Minotti, Fernando; Kelly, HéctorA numerical investigation of the kinetic processes in the initial (nanosecond range) stage of the double-arcing instability was developed. The plasma-sheath boundary region of an oxygen operated cutting torch was considered. The energy balance and chemistry processes in the dis charge were described. It is shown that the double-arcing instability is a sudden transition from a diffuse (glow-like) discharge to a constricted (arc-like) discharge in the plasma-sheath boundary region arising from a field-emission instability. A critical electric field value of 107 V/m was found at the cathodic part of the nozzle wall under the conditions considered. The field-emission instability drives in turn a fast electronic-to-translational energy relaxation mechanism, giving rise to a very fast gas heating rate of at least 109 K/s, mainly due to reactions of preliminary dissocia tion of oxygen molecules via the highly excited electronic state populated by electron impact. It is expected that this fast oxygen heating rate further stimulates the discharge contraction through the thermal instability mechanism.Item Quantitative Schlieren Diagnostic Applied to a Nitrogen Thermal Plasma Jet.(2018) Chamorro, Juan Camilo; Prevosto, Leandro; Cejas, Ezequiel; Kelly, Héctor— A quantitative interpretation of the schlieren technique applied to an atmospheric pressure, vortex-stabilized nitrogen thermal plasma jet generated in a direct-current nontransferred arc plasma torch (nitrogen gas flow rate of 25 NL/min, power level of 15 kW), discharging into ambient air is reported. A Z-type, two-mirror schlieren system was used in the research. The technique allowed inferring the temporally averaged values of the temperatures and densities of different species present in the plasma jet in a wide range of radial and axial distances. Deviations from kinetic equilibrium in the calculation of the plasma refractive index were accounted for, but maintaining the assumption of the local chemistry equilibrium. The influence of several assumptions on the accuracy of the measurements was considered. The results have shown that for a distance of 3.5-mm downstream from the nozzle exit, the kinetic equilibrium is realized (being both electron and gas temperatures values around 11 000 K), but noticeable deviation from kinetic equilibrium appears toward the jet border. On the other hand, a marked deviation from the kinetic equilibrium was found in the whole far field of the plasma jet, where the electron temperature remains still quite high (about 10 000 K at 30-mm downstream of the nozzle exit), well decoupled from the gas temperature (about 7000 K at the same distance). The obtained results are in reasonable good agreement with those previously reported by some of the authors by using a double floating probe method in the same plasma torch.Item Plasma Cutting of Concrete: Heat Propagation and Molten Material Removal From the Kerf.(2019) Chamorro, Juan Camilo; Prevosto, Leandro; Cejas, Ezequiel; Milardovich, Natalio; Mancinelli, Beatriz; Fischfeld, GerardoAn experimental investigation of heat propagation in the case of plasma cutting of concrete is reported. The experiments were carried out by using a high-enthalpy nitrogen plasma jet generated in a dc vortex-stabilized nontransferred arc torch. Concrete plates of different thicknesses up to 52 mm and with and without steel reinforcement were used. The plates were placed horizontally while cutting. The heat conduction losses inside the material were estimated by comparing thermocouple measurements and theoretical temperatures obtained with an analytical model of the heat propagation in the material. The influence of the molten concrete layer that separates the plasma to the solid material due to the high viscosity of the liquid concrete was accounted for. The power losses below the material in the extinguishing plasma have also been determined from calorimet ric measurements. For different plate thicknesses and cutting velocities, a complete power balance of the process is performed with the calculation of the cutting efficiency on the basis of various relevant power terms. In addition, the hydrodynamics of the molten concrete layer in the kerf is analyzed. For a mean power level of 11.2 kW and a nitrogen gas flow rate of 25 Nl/min, the torch is able to cut a concrete plate of 52 mm in thickness with a speed of 20 mm/min and a whole efficiency of about 30%. The viscosity force is the main limiting factor on the cutting velocity in thick plates.Item On the Gas Heating Mechanism for the Fast Anode Arc Reattachment in a Non-transferred Arc Plasma Torch Operating with Nitrogen Gas in the Restrike Mode.(2015) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz; Chamorro, Juan CamiloThe present work provides a detailed kinetic analysis of the time-resolved dynamics of the gas heating during the arc reattachment in nitrogen gas in order to understand the main processes leading to such a fast reattachment. The model includes gas heating due to the relaxation of the energy stored in the vibrational as well as the electronic modes of the molecules. The results show that the anode arc reattachment is essentiality a threshold process, corresponding to a reduced electric field value of E/N * 40 Td for the plasma discharge conditions considered in this work. The arc reattachment is triggered by a vibrational instability whose development requires a time of the order of 100 ls. For E/N\80–100 Td, most of the electron energy is transferred to gas heating through the mechanism of vibrational–translational relaxation. For larger values of E/N the electronic– translational energy relaxation mechanism produces a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few ls of the vibrational instability give rises to a sudden transition from a diffuse (glow-like) discharge to a constricted arc with a high current density (*107 A/m2 ). This sudden increase in the current density gives rise to a new anode attachment closer to the cathode (where the voltage drop between the original arc and the anode is the largest) thus causing the decay of the old arc spot.Item Ambient Species Density and Gas Temperature Radial Profiles Derived from a Schlieren Technique in a Low Frequency Non-thermal Oxygen Plasma Jet.(2017) Chamorro, Juan Camilo; Prevosto, Leandro; Cejas, Ezequiel; Kelly, Héctor; Mancinelli, Beatriz; Fischfeld, GerardoA quantitative interpretation of the schlieren technique applied to a non-thermal atmospheric-pressure oxygen plasma jet driven at low-frequency (50 Hz) is reported. The jet was operated in the turbulent regime with a hole-diameter based Reynolds number of 13,800. The technique coupled to a simplified kinetic model of the jet effluent region allowed deriving the temporally-averaged values of the gas temperature of the jet by processing the gray-level contrast values of digital schlieren images. The penetration of the ambient air into the jet due to turbulent diffusion was taken into account. The calibration of the optical system was obtained by fitting the sensitivity parameter so that the oxygen fraction at the nozzle exit was unity. The radial profiles of the contrast in the discharge off case were quite symmetric on the whole outflow, but with the discharge on, relatively strong departures from the symmetry were evident in the near field. The time-averaged gas temperature of the jet was relatively high, with a maximum departure of about 55 K from the room temperature; as can be expected owing to the operating molecular gas. The uncertainty in the temperature measurements was within 6 K, primarily derived from errors associated to the Abel inversion procedure. The results showed an increase in the gas temperature of about 8 K close to the nozzle exit; thus suggesting that some fast-gas heating (with a heating rate *0.3 K/ls) still occurs in the near field of the outflow.Item Modelling of the Plasma–Sheath Boundary Region in Wall-Stabilized Arc Plasmas: Unipolar Discharge Properties.(2017) Mancinelli, Beatriz; Prevosto, Leandro; Chamorro, Juan Camilo; Minotti, Fernando; Kelly, HéctorA two-dimensional model of the non-equilibrium unipolar discharge occurring in the plasma–sheath boundary region of a transferred-arc was developed. This model was used to study the current transfer to the nozzle (1 mm diameter) of a 30 A arc cutting torch operated with oxygen. The energy balance and chemistry processes in the discharge were described by using a kinetic block of 45 elementary reactions and processes with the participation of 13 species including electronically excited particles. The nonlocal transport of electrons was accounted for into the fluid model. The dependence of the ion mobility with the electric field was also considered. Basic discharge properties were described. It has been found that a large part (* 80%) of the total electric power (1700 mW) delivered in the bulk of the sheath region is spent in heating the positive ions and further dissipated through collisions with the neutral particles. The results also showed that the electron energy loss in inelastic collisions represents only * 25% of the electron power and that about 63% of the power spent on gas heating is produced by the ion– molecule reaction, the electron–ion and ion–ion recombination reactions, and by the electron attachment. The rest of the power converted into heat is contributed by dissociation by electron-impact, dissociative ionization and quenching of O(1 D). Some fast gas heating channels which are expected to play a key role in the double-arcing phenomena in oxygen gas were also identified.Item On the physical processes ruling an atmospheric pressure air glow discharge operating in an intermediate current regime(2015-02-06) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz; Chamorro, Juan Camilo; Cejas, EzequielLow-frequency (100 Hz), intermediate-current (50 to 200 mA) glow discharges were experimentally investigated in atmospheric pressure air between blunt copper electrodes. Voltage–current characteristics and images of the discharge for different inter-electrode distances are reported. A cathode-fall voltage close to 360V and a current density at the cathode surface of about 11 A/cm2, both independent of the discharge current, were found. The visible emissive structure of the discharge resembles to that of a typical low-pressure glow, thus suggesting a glow-like electric field distribution in the discharge. A kinetic model for the discharge ionization processes is also presented with the aim of identifying the main physical processes ruling the discharge behavior. The numerical results indicate the presence of a non-equilibrium plasma with rather high gas temperature (above 4000 K) leading to the production of components such as NO, O, and N which are usually absent in low-current glows. Hence, the ionization by electron-impact is replaced by associative ionization, which is independent of the reduced electric field. This leads to a negative current-voltage characteristic curve, in spite of the glow-like features of the discharge. On the other hand, several estimations show that the discharge seems to be stabilized by heat conduction; being thermally stable due to its reduced size. All the quoted results indicate that although this discharge regime might be considered to be close to an arc, it is still a glow discharge as demonstrated by its overall properties, supported also by the presence of thermal non-equilibrium. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907661]