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.     

Direct Measurement of the Gas Entrainment Into a Turbulent Thermal Plasma Jet

An experimental study is conducted to investigate the entrainment characteristics of a turbulent thermal plasma jet issuing from a DC arc plasma torch operating at atmospheric pressure. The mass flow rate of the ambient gas entrained into the turbulent plasma jet is directly measured by use of the so-called “porous-wall chamber” technique. It is shown that a large amount of ambient gas is entrained into the turbulent plasma jet. With the increase of the gas mass flow rate at the plasma jet inlet or the plasma torch exit, the mass flow rate of entrained ambient gas almost linearly increases but its ratio to the jet-inlet mass flow rate decreases. The mass flow rate of the entrained gas increases with the increase of the arc current or jet length. It is also found that using different ways to inject the plasma-forming gas into the plasma torch affects the entrainment rate of the turbulent plasma jet. The entrainment rate expression established previously by Ricou and Spalding (J. Fluid Mech. 11: 21, 1961) for the turbulent isothermal jets has been used to correlate the experimental data of the entrainment rates of the turbulent thermal plasma jet, and the entrainment coefficient in the entrainment rate expression is found to be in range from 0.40 to 0.47 for the turbulent thermal plasma jet under study.     

Effect of Anode Arc Root Position on the Behavior of the DC Non-transferred Plasma Jet at Field Free Region

A special bi-anode plasma torch that can change the anode arc root position without changing working gas flow rate has been developed to investigate the effect of anode arc root position on the behavior of the plasma jet. It has two nozzle-shaped anodes at different axial distances from the cathode tip. The arc root can be formed at anodes either close to the cathode tip (anode I) or far away from it (anode II) to obtain different attachment positions and arc voltages. The characteristics of pure argon plasma jets operated in different anode modes were measured in the field free region by using an emalpy probe, and the thermal efficiency of the torch was determined by measuring the temperature differences between cooling water flowing in and out of the torch. The results show that compared with the normal arc operated in anode I mode, the elongated arc operated in anode II mode significantly reduced the plasma energy loss inside the torch, resulting in a higher temperature and a higher velocity of the plasma jet in the field free region.     

Generation of Long, Laminar Plasma Jets at Atmospheric Pressure and Effects of Flow Turbulence

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 times its diameter could be generated with a DC arc torch by restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including arc channel geometry, gas flow rate, and feeding methods, etc. Influence of flow turbulence at the torch nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent arc jet.     

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.     

Experimental Comparison of Methane Pyrolysis in Thermal Plasma

Methane pyrolysis via thermal plasma was investigated experimentally on a 2 kW DC arc plasma setup in argon atmosphere. Two widely applied methane pyrolysis profiles, i.e., pre-mixing methane and argon before fed into plasma torch, and injecting methane into pure argon plasma jet at torch outlet, were compared. Performances of methane pyrolysis regarding to methane conversion, acetylene selectivity, acetylene specific energy requirement (SER), and plasma stability were concluded. Results showed that pre-mixing methane and argon before fed into plasma torch would be efficient in converting methane and acetylene production, with higher conversion of methane and lower SER to acetylene at a given specific energy. Also, methane in arc zone would cause periodic fluctuations of plasma voltage and power, which could be reduced by controlling methane fraction in feed. On the other hand, when methane was injected into argon plasma jet at torch outlet, the energy efficiency in converting methane and producing acetylene would be lower. And the plasma would barely participate in the reaction other than providing heat, but the erosion of electrode was much slower and slighter. It was also validated that the SER of acetylene was limited by the thermal loss of the setup due to size-effect of reactor.     

Characteristics of Argon Laminar DC Plasma Jet at Atmospheric Pressure

Argon DC plasma jets in stable laminar flow were generated at atmospheric pressure with a specially designed torch under carefully balanced generating conditions. Compared with turbulent jets of short length with expanded radial appearance and high working noise, the laminar jet could be 550 mm in length with almost unchanged diameter along the whole length and very low noise. At gas feeding rate of 120 cm³/s, the jet length increases with increasing arc current in the range of 70–200 A, and thermal efficiency decreases slightly at first and then leveled off. With increasing gas flow rate, thermal efficiency of the laminar jets increases and could reach about 40%, when the arc current is kept at 200 A. Gauge pressure distributions of the jets impinging on a flat plate were measured. The maximum gauge pressure value of a laminar jet at low gas feeding rate is much lower than that of a turbulent jet. The low pressure acting on the material surface is favorable for surface cladding of metals, whereas the high pressure associated with turbulent jets will break down the melt pool.     

Multiscale Finite Element Modeling of Arc Dynamics in a DC Plasma Torch

The dynamics of the electric arc inside a direct current non-transferred arc plasma torch are simulated using a three-dimensional, transient, equilibrium model. The fluid and electromagnetic equations are solved numerically in a fully coupled approach by a multiscale finite element method. Simulations of a torch operating with argon and argon–hydrogen under different operating conditions are presented. The model is able to predict the operation of the torch in steady and takeover modes without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes, needed because of the equilibrium assumption. The results obtained indicate that the reattachment process in these operating modes may be driven by the movement of the arc rather than by a breakdown-like process. It is also found that, for a torch operating in these modes and using straight gas injection, the arc will tend to re-attach to the opposite side of its original attachment. This phenomenon seems to be produced by a net angular momentum on the arc due to the imbalance between magnetic and fluid drag forces.     

Design and Characteristics of a Laminar Plasma Torch for Materials Processing

Thermal plasma jets have been widely used in various materials processing techniques. However, the conventional thermal plasma torches usually generate turbulent plasma jets with the disadvantages of high axial temperature gradient, a short jet length and difficulties in the process control relatively, limiting its applications to materials processing. Therefore, this paper proposes a new laminar plasma torch (LPT) working with pure nitrogen to generate laminar plasma jet (LPJ). Its design and structural characteristics, e.g. segmented anode, axial gas injection, parallel water cooling structure, etc., are detailed to ensure the stability, the favorable temperature and velocity distribution of the generated LPJ. Experiments on the characteristics of the LPT showed that the generated LPJ possessed high specific enthalpy (ranging between 10 and 90 kJ/g), long jet length (maximum length: 480 mm) and low axial temperature gradient, and its output power a current and the gas flow rate. In addition, the thermal efficiency of the LPT was experimentally determined to be ranging between 25 and 45 %. Furthermore, experiment and simulation on the application of the LPJ for surface quenching verified the even radial temperature distribution of the plasma jet and high heat flux density brought to the surface.     

Improvement of plasma spraying efficiency and coating quality

A commercial torch has been modified to introduce an additional anti-vortex and shroud gas flow to counter the detrimental effects brought about by the vortex plasma gas flow which is used to stabilize the cathode arc attachment and to increase the anode life. Deposition efficiency and coating quality are used as criteria to judge the modified versus the nonmodified torch. High-speed videography and computerized image analysis systems are used to determine the particle trajectories, velocities, and the plasma jet geometry. The results show that the additional anti-vortex and shroud gas flow to the torch can keep the particles closer to the torch axis and reduce the amount of entrainment of cold air into the plasma jet. The consequence is that deposition efficiency and coating quality are substantially improved.     

Nanocarbon structures formed in a universal plasma reactor

The results of an instrumental study of a deposit formed on the electrodes of an arc plasma torch with propane-butane mixture feeding into the interelectrode gap are presented. The optical, electron, and Raman microscopy techniques have been used in the study. According to Raman spectra, the cathode deposit contains various forms of nanocarbon. It has been found that maleic anhydride is synthesized and covalently grafted to nanographite in the absence of a specialty catalyst during plasma torch operation. Having a large specific surface area, the nanocarbon itself acts as a heterogeneous catalyst in this case. It has been shown that an arc plasma torch of this design with a hydrocarbon feed gas can be considered a mini-reactor for synthesis of different forms of nanocarbon, its surface modification, and alteration of physicochemical properties.     

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.     

Modeling of the Subsonic–Supersonic Flow and Heat Transfer in a DC Arc Plasma Torch

Two-dimensional modeling results are presented concerning the subsonic–supersonic flow and heat transfer within a DC plasma torch used for low-pressure (or soft vacuum) plasma spraying. The so-called fictitious anode method is used in the modeling in order to avoid inclusion of the complex three-dimensional effects near the anode arc root and also to avoid the forced heating of all the incoming cold gas stream by the arc. A nonorthogonal boundary-conforming grid, nonstaggered variable arrangement and the all-speed SIMPLE algorithm are employed for the solution of the governing equations, including gas viscous effects, temperature-dependent properties, and compressible effects. Good agreement of the predictions with available results for a few benchmarked compressible flow problems shows that the new version of the FAST-2D program can be employed for the present plasma flow modeling. Temperature, axial velocity, Mach number, and static pressure contours, and streamlines within the DC arc plasma torch are presented to show the flow and heat transfer characteristics. The flow transits gradually from upstream subsonic regime into downstream supersonic regime with the subsonic–supersonic transition within the cylindrical segment of the torch nozzle. Additional numerical tests show that gas viscosity and Lorentz force have only a slight effect on the plasma flow.     

Biomass conversion to hydrogen-rich synthesis fuels using water steam plasma

In this study, an experimental plasma-chemical reactor equipped with an arc discharge water steam plasma torch was used for biomass conversion to hydrogen-rich synthesis fuels. Glycerol and crushed wood were used as biomass sources. The effects of different conversion parameters including the water steam flow rate, treated material flow rate, and plasma torch power were studied. The experimentally obtained results were compared with the model based on the thermodynamic equilibrium. Additionally, the quantification of the plasma conversion system in terms of energy efficiency and specific energy requirement was performed. It has been found that the synthesis gas can be effectively produced from the biomass using water steam plasma.     

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10⁻⁴ kg/s.     

Study of the dynamic and static behavior of de vortex plasma torches: Part II: Well-tye cathode

This work was devoted to the study of the dynamic and static behavior of de vortex plasma torch with a well-type cathode (power level below 100 kW). The dynamic behavior of the torch was characterized by the fulctuations of arc voltage and current, plasma jet radiation, and acoustic pressure. Characteristic frequencies of the arc root movement inside the torch were observed. By numerical simulation (with the numerical codeMelodie, it was shown that the position of the erosion diameter) of the axial velocity along the cathode channel near the wall. The static behavior of the torch was inverstigated for different cathode designs. The variations of voltage U with arc current I, gas flow rate G nature of the gas and cathode design were represented by semiempirical relationships established between dimensionless numbers. By dimensional analysis, the behavior of this torch was compared with that of two powerful torches: the Aerospatiale and the Plasma Energy Corporation torches.     

Probe measurements in thermal plasma jets

Measurements of composition, temperature, and velocity in atmospheric argon plasma jets are reported, using enthalpy probes. The plasma jets are generated by a commercial type plasma gun and the measurements are expected to be of particular interest for industrial applications such as plasma spraying. Emphasis has been on the central and downstream regions of the plasma flame. The entrainment of air into the jet was found to be very high, even close to the axis of the jet. Gas samples analyzed with a gas chromatograph showed demixing of the air, i.e., nitrogen is more abundant in the jet than at room temperature. The high air entrainment has a strong cooling effect on the plasma, resulting in a rapid temperature drop along the axis. The influence of the argon flow rate and of the arc current on the jet's conditions was parametrically studied. Matching of the quantities measured in the jet with the torch input confirmed the validity of the results, and the relevance of enthalpy probe diagnostics in thermal plasma jets.     

Heat Generation and Particle Injection in a Thermal Plasma Torch

The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions.     

Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.     

Effects of Nozzle Length and Process Parameters on Highly Constricted Oxygen Plasma Cutting Arc

The influence of nozzle length and two process parameters (arc current, mass flow rate) on the plasma cutting arc is investigated. Modeling results show that nozzle length and these two process parameters have essential effects on plasma arc characteristics. Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase with the nozzle length. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage and power, therefore increases plasma velocity, temperature and heat flux.