DC Non-transferred Arc Plasma Torch Research Articles

Numerical analysis of direct-current (DC) plasma processing for high-efficient steel surface modification

Nowadays, direct-current (dc) non-transferred arc plasma torch has drawn significant interest from both academia and industry due to the capability to process products in an efficient and convenient way. The core of this technology is to clarify and manipulate the arc behavior at the interior of the dc plasma torch to produce ideal plasma jets for processing. To solve this problem, a quasi-steady axisymmetric model is built to simulate and compare the arc characteristics in different operating conditions and different nozzle structures of the plasma torch. The results uncover distinct aspects of the study on arc characteristics, including the detection of the primary arc attachment region and its spatial features caused by the choking effect of torch structure. The thermal efficiency focused on processing substrate is also calculated in this paper for estimating the performance of plasma processing. The calculated results show that increasing the mass flow rate brings better thermal efficiency and the greatest promotion is at least 6% in the same current value, whereas improving the arc current value causes the opposite result. Meanwhile, two types of nozzle are compared to the original design in thermal efficiency, where the wide nozzle is chosen for torch optimization due to its best power efficiency. The secondary arc attachment on the metal substrate is discovered though its impact scope is only within a radius of 10 mm from the torch axis, its effect on the processing could be ignored for the extremely low electric current value.

Comparative Study of Plasma Torch Characteristics Using Air and Carbon Dioxide

The plasma torch is the key component of the thermal plasma-assisted material processing system. Particularly, direct current (dc) nontransferred arc plasma torch is widely used in high-temperature material processing applications. The characteristics of the thermal plasma jet emanating from the plasma torch depend on many factors, including arc current, type of plasma forming gas, gas flow rates, gas injection configuration, torch geometry, and so on. This experimental study focuses on the comparison between the effects of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and air on the arc voltage, electrothermal efficiency, and specific enthalpy of the plasma torch operated at the atmospheric pressure. For this purpose, a low-power dc plasma torch consisting of a rod-type thoriated tungsten cathode and the copper anode was customized. CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and air were used as the plasma forming gases along with a feeble amount of argon (15 lpm). The plasma torch was operated over a wide range of current (100–175 A) with fixed gas flow rates. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and the electrothermal efficiency of the plasma torch were measured, and results were compared. The maximum electrothermal efficiency and specific enthalpy of the plasma torch when operated with CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were reported as 70% and 1900 KJ/Kg, respectively, which is higher than the same torch operated with air as the plasma forming gas. The electron number density and electron temperature of the air and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma jet were measured by optical emission spectroscopy (OES). The calculated electron number density and electron temperature of the plasma jet with CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.89\times 10\,\,^{\mathrm{ 16}}$ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> and 8598 K, respectively, which is higher than air plasma jet. It was found that CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has a stronger influence than air on the arc voltage fluctuation, electrothermal efficiency, and operating regime of the plasma torch.

Theoretical and experimental investigation of Tialloy powder production using low-power plasma torches

This study was carried out to investigate the possibility of titanium alloy metal powder production using low-power plasma torches. An argon DC non-transferred arc plasma torch was designed, and numerical analysis was conducted to determine the plasma jet properties and wire temperature. The highest velocities inside the nozzle attachment were between 838 and 1178 m/s. The velocities of the jets at the apex were between 494 and 645 m/s for different gas flow rates. The studied plasma gas flow rates had no significant effect on the effective plasma jet length. It was shown that the plasma jet length can be estimated by numerical analysis using the temperature and velocity changes of the plasma jet over distance. It was observed that the powders produced were spherical without any satellites. As a result of this study, a plasma torch was developed and powder production was performed successfully by using relatively low torch power.

A novel experimental method of investigating anode-arc-root behaviors in a DC non-transferred arc plasma torch

A novel experimental method of investigating anode arc root was presented, and a plasma torch with multiple anodes was designed. With this apparatus, the number of anode arc roots, their position, and movement with changes in plasma current and gas-flow rate were studied. Results showed that multiple anode arc roots can coexist and their number was more related to plasma current than gas-flow rate. The number of anode arc roots increased with increased plasma current. The anode arc root can move forward and backward in the arc plasma jet, and its position was related to current and gas-flow rate. The anode arc root moved forward with increased current, whereas the anode arc root moved forward first and backward with increased gas-flow rate. Based on the finding, a new non-transferred arc model was established. Finally, the anode arc root motion was classified as steady, slight vibration, alternating motion, jumping motion, and multiple arc root modes. These results can serve as a reference for revealing the mechanism of plasma discharge, optimization design of arc plasma jet, and high-quality plasma generation.

Role of return currents in the dynamics of a magnetically rastered plasma torch

The role of return currents in the plasma dynamics of a dc non-transferred arc plasma torch has been investigated. The anode arc root connects the main plasma column and return current in the anode and its motion governs the return current formation in the anode. Thus by investigating the return currents, we can explore complex spatio-temporal evolution of the plasma in detail. In this work, we have used electrical and magnetic diagnostics. The electrical diagnostic comprises of a high voltage probe, and the magnetic diagnostic consists of a garland of several B-dot probes incorporated inside the anode water cooling channels. The magnetic (B-dot) probes are designed to pick up the dominant component of the time-varying magnetic fields. Results show that, beyond a threshold, volume return currents transform to constricted line currents in the anode return path. This is attributed to eddy currents arising due to space varying rather than time varying return current components in the anode. The probes capture fast rotation of the arc root that is not captured by fast imaging. Also, arc root shunting (restriking) phenomena and its location on the anode periphery are clearly captured by the garland of magnetic probes. A physical model explaining the processes is also presented.

Influence of the Shroud Gas Injection Configuration on the Characteristics of a DC Non-transferred Arc Plasma Torch

The characteristics of the plasma jet emanating from a dc non-transferred plasma torch is affected by many factors including arc current, type of gas, gas flow rate, gas injection configuration and torch geometry. The present work focuses on experimental investigation of the influence of shroud gas injection configuration on the I–V characteristics and electro-thermal efficiency of a dc non-transferred plasma torch operated in nitrogen at atmospheric pressure. The plasma gas is injected into the torch axially and shroud gas is injected through three different nozzles such as normal, sheath and twisted nozzles. The effects of flow rates of plasma/axial gas and arc current on I–V characteristics and electro-thermal efficiency of the torch holding different nozzles are investigated. The I–V characteristics and electro-thermal efficiency of the torch are found to be strongly influenced by the shroud gas injection configuration. The effect of arc current on arc voltage decreases with increasing the axial gas flow rate. At higher axial gas flow rate (> 45 lpm), the I–V characteristics of the plasma torch are similar irrespective of the nozzle used. The variation of electro-thermal efficiency with arc current is almost similar to that of arc voltage with arc current. As expected, the electro-thermal efficiency is increased when the axial gas flow rate is increased and at higher axial gas flow rate, it is not influenced by the arc current and shroud gas configuration. The plasma torch with normal nozzle may be better in the range of operating conditions used in this study.

Prediction of operational characteristics of a dc non-transferred arc plasma torch using similarity criteria

The operational characteristics of a dc non-transferred arc plasma torch have been predicted using the techniques of dynamic similarity. The torch used has a distinct design incorporating both axial plasma generating gas and a shroud gas that initially forms a cold boundary layer but eventually takes part in the plasma formation. The current–voltage characteristics are correlated with electrode heat losses and plasma power for different gas flow conditions using the techniques of dynamic similarity. It is shown that this torch can operate in various power levels at high electro-thermal efficiencies. Unique dimensionless numbers are created for its wide operating regimes. The voltage and operational characteristics are predicted using these numbers for a number of experimental conditions, which are in good agreement with actual experimental results. Predictions are made on the limits of operational characteristics of this torch and class of torches at higher powers.

Direct probing of anode arc root dynamics and voltage instability in a dc non-transferred arc plasma jet

The transient dynamics of the anode arc root in a dc non-transferred arc plasma torch is captured through fast photography and directly correlated with the associated voltage instability for the first time. The coexistence of multiple arc roots, the transition to a single arc root, root formation and extinction are investigated for the steady, takeover and re-strike modes of the arc. Contrary to the usual concept, the emerging plasma jet of a dc non-transferred arc plasma torch is found to carry current. An unusually long self-propelled arc plasma jet, a consequence of the phenomenon, is demonstrated.

Three-dimensional simulation of an argon–hydrogen DC non-transferred arc plasma torch

Simulations of a DC non-transferred arc plasma torch operating with argon–hydrogen have been performed by using a three-dimensional model. An artificially high electrical conductivity layer is employed to allow the current passing through the low temperature region near the anode wall. A new way by using two equations to describe the current density distribution is developed. Besides, a new method for determining the location of the arc-root attachment is proposed, in which the minimum total heat transfer rate through the anode wall is considered as the criterion for the lowest energy loss. Based on this criterion, the real arc core radius and length are predicted. Moreover, the influences of arc current and mass flow rate on the plasma arc characteristics are also investigated. The results obtained show that the location of the arc-root attachment predicted by the minimum total heat transfer rate principle is in good agreement with the previous work and the experimental data. Additionally, it is found that arc length can be reduced by increasing current or decreasing flow rate. Also, higher current and flow rate lead to higher temperature and velocity inside the plasma torch.

Axial evolution of radial heat flux profiles transmitted by atmospheric pressure nitrogen and argon arcs

Axial evolutions of radial heat flux profiles in argon and nitrogen plasma jets from an atmospheric pressure dc non-transferred arc plasma torch are determined using a double calorimetric technique. Results are presented for power levels suitable for the processing of high temperature ceramic oxides, where the heat flux data reported in the literature is rare. Variations of the profile widths and profile maxima are presented as a function of axial distance as well as power. Relatively uniform profile width over prolonged axial distance for nitrogen plasma compared to argon is an important observation which has the potential to offer a much longer dwell time of the injected particles inside the plasma, avoiding the problem of unmelts, especially for ceramics. A comparative study of the heat flux profiles for argon and nitrogen plasma is presented. The obtained results are compared with the data reported in literature.

Inertization of Heavy Metals Present in Galvanic Sludge by DC Thermal Plasma

Galvanic sludge results from the treatment of effluents generated by the industrial metal surface treatment of industrial material, which consists in the deposition of a metal on a surface or a metal surface attack, for example, electrodeposition of conductors (metals) and non conductive, phosphate, anodizing, oxidation and/or printed circuit. The treatment proposed here is exposure of the galvanic sludge to the high temperatures provided by thermal plasma, a process which aims to vitrify the galvanic sludge and render metals (iron, zinc, and chromium) inert. Two different plasma reactors were assembled: with a DC transferred arc plasma torch and with a DC nontransferred arc plasma torch. In this way it was possible to verify which reactor was more efficient in the inertization of the metals and also to investigate whether the addition of quartzite sand to the sludge influences the vitrification of the material. Quantification of water content and density of the galvanic raw sludge were performed, as well as analyzes of total organic carbon (TOC) and identify the elements that make up the raw sludge through spectroscopy X-ray fluorescence (XRF). The chemical composition and the form of the pyrolyzed and vitrified sludge were analyzed by scanning electron microscopy with energy-dispersive X-ray spectrometer (SEM-EDS) analysis, which it is a analysis that shows the chemical of the sample surface. The inertization of the sludge was verified in leaching tests, where the leachate was analyzed by flame atomic absorption spectroscopy (FAAS). The results of water content and density were 64.35% and 2.994 g.cm(-3), respectively. The TOC analysis determined 1.73% of C in the sample of galvanic raw sludge, and XRF analysis determined the most stable elements in the sample, and showed the highest peaks (higher stability) were Fe, Zn, and Cr. The efficiency of the sludge inertization was 100% for chromium, 99% for zinc, and 100% for iron. The results also showed that the most efficient reactor was that with the DC transferred arc plasma torch and quartzite sand positively influenced by the vitrification during the pyrolysis of the galvanic sludge.

Numerical Simulation of Fluid Flow and Heat Transfer in a DC Non-Transferred Arc Plasma Torch Operating Under Laminar and Turbulent Conditions

In this work, a magnetic fluid dynamics (MHD) model is used to simulate the electromagnetic field, heat transfer and fluid flow in a DC non-transferred arc plasma torch under laminar and turbulent conditions. The electric current density, temperature and velocity distributions in the torch are obtained through the coupled iterative calculation about the electromagnetic equations described in a magnetic vector potential format and the modified fluid dynamics equations. The fluid-solid coupled calculation method is applied to guarantee the continuity of the electric current and heat transfer at the interface between the electrodes and fluid. The predicted location of the anodic arc root attachment and the arc voltage of the torch are consistent with corresponding experimental results. Through a specific analysis of the influence of mass flow rates and electric current on the torch outlet parameters, the total thermal efficiency, thermal loss of each part, and the laws of the variation of outlet parameters with the variation of mass flow rates and electric current was obtained. It is found that operation under a laminar condition with a limited area of the anode could increase the total thermal efficiency of the torch.

Comparative Observation of Ar, Ar-H2 and Ar-N2 DC Arc Plasma Jets and Their Arc Root Behaviour at Reduced Pressure

Results observed experimentally are presented, about the DC arc plasma jets and their arc-root behaviour generated at reduced gas pressure without or with an applied magnetic field. Pure argon, argon-hydrogen or argon-nitrogen mixture was used as the plasma-forming gas. A specially designed copper mirror was used for a better observation of the arc-root behaviour on the anode surface of the DC non-transferred arc plasma torch. It was found that in the cases without an applied magnetic field, the laminar plasma jets were stable and approximately axisymmetrical. The arc-root attachment on the anode surface was completely diffusive when argon was used as the plasma-forming gas, while the arc-root attachment often became constrictive when hydrogen or nitrogen was added into the argon. As an external magnetic field was applied, the arc root tended to rotate along the anode surface of the non-transferred arc plasma torch.

Arc Root Attachment on the Anode Surface of Arc Plasma TorchObserved with a Novel Method

The arc-root attachment on the anode surface of a dcnon-transferred arc plasma torch has been successfully observed using anovel approach. A specially designed copper mirror with a boron nitridefilm coated on its surface central-region is employed to avoid theeffect of intensive light emitted from the arc column upon theobservation of weakly luminous arc root. It is found that the arc-rootattachment is diffusive on the anode surface of the argon plasma torch,while constricted arc roots often occur when hydrogen or nitrogen isadded into argon as the plasma-forming gas.

Application of Steenbeck's minimum principle for three-dimensional modelling of DC arc plasma torches

In this paper, physical/mathematical models for the three-dimensional, quasi-steady modelling of the plasma flow and heat transfer inside a non-transferred DC arc plasma torch are described in detail. The Steenbeck's minimum principle (Finkelnburg W and Maecker H 1956 Electric arcs and thermal plasmas Encyclopedia of Physics vol XXII (Berlin: Springer)) is employed to determine the axial position of the anode arc-root at the anode surface. This principle postulates a minimum arc voltage for a given arc current, working gas flow rate, and torch configuration. The modelling results show that the temperature and flow fields inside the DC non-transferred arc plasma torch show significant three-dimensional features. The predicted anode arc-root attachment position and the arc shape by employing Steenbeck's minimum principle are reasonably consistent with experimental observations. The thermal efficiency and the torch power distribution are also calculated in this paper. The results show that the thermal efficiency of the torch always ranges from 30% to 45%, i.e. more than half of the total power input is taken away by the cathode and anode cooling water. The special heat transfer mechanisms at the plasma–anode interface, such as electron condensation, electron enthalpy and radiative heat transfer from the bulk plasma to the anode inner surface, are taken into account in this paper. The calculated results show that besides convective heat transfer, the contributions of electron condensation, electron enthalpy and radiation to the anode heat transfer are also important (∼30% for parameter range of interest in this paper). Additional effects, such as the non-local thermodynamic equilibrium plasma state near the electrodes, the transient phenomena, etc, need to be considered in future physical/mathematical models, including corresponding measurements.