Constricted Mode Research Articles

Metallurgical, mechanical and corrosion behavior of Interpulse and pulsed current TIG dissimilar welds of Monel 400 and AISI 316L

This research work explores the weldability, structural integrity, mechanical properties, and corrosion behavior of dissimilar welds of Monel 400 and AISI 316L developed by using Interpulse and pulse arc current modes in TIG welding process. ERNiCrMo-3 filler of 1.6 mm was used to fill the V-groove configuration of 5 mm thick plates in multiple passes. Metallurgical changes were observed in both Interpulse and pulsed current dissimilar weldments using optical and scanning electron microscope techniques. Mechanical properties of the joined structures were evaluated by performing tensile tests on UTM and hardness measurements on weld surfaces using Vickers hardness tester. The resistance against the corrosion especially at the weld area of both welded structures was measured using a cyclic sweep test. From the XRT and visual inspection analyses, the joined structures were free from defects and also seen the uniformity of the filler distribution toward the base metals. The grain structure with well-defined grain structures were identified in PC-TIG weldments whereas fine grains with clear distribution of filler alloying elements were seen in Interpulse-TIG weldments. Interpulse-TIG weldments exhibited with better mechanical and corrosion resistance properties than the PC-TIG weldments vowing to constricted arc mode during the welding process.

Nanosecond plasma actuation by a bending actuator mounted on a sharp edge in quiescent air

An experimental investigation of the actuation characteristics of a repetitive nanosecond pulsed dielectric barrier discharge from bending plasma actuators mounted on a sharp edge is conducted. Four bending actuators with different bending angles are tested and compared with a planar actuator in quiescent air using schlieren imaging and electrical measurements. The results show that when fed by the same pulse, the plasma morphology, current waveform, and energy consumption of the bending and planar actuators are very similar. However, the thermal perturbations and induced flows differ significantly. In this experiment, at a low load voltage (V = 10–14 kV), when the plasma discharge is in the diffuse mode, the bending actuator can induce a stronger vortex and near-wall jet than the planar actuator, and as the bending angle decreases, the strength of the induced jet increases rapidly. At a higher load voltage (V = 18–20 kV), when the discharge is in the constricted mode, the plasma filaments produce hot plumes with both the bending and planar actuators, but the hot plumes from the bending actuator are injected into the air with a larger incidence. During the streamer-to-filament transition (V = 16 kV), induced flows from small-bending-angle (30° and 60°) actuators are characterized by a thickened near-wall jet, while those from the planar actuator are characterized by hot plumes.

Arc dynamics in a vortex-stabilized non-transferred plasma torch with a tangential gas feed

The mode of anode arc attachments in a thermal plasma torch, such as constricted arc, multi-arc or diffused arc attachment, has operational as well as economic consequences on the use of plasma torches in industrial applications. The mode of arc attachment is strongly dependent on the gas flow dynamics, gas vortex formation and wall energy exchange. To gain more insight into arc attachment behaviour, this article presents a transient flow plasma model implemented using OpenFOAM computational fluid dynamics code to study the effect of gas swirl and arc rotation on modes of arc attachments. A tangentially fed plasma gas creates a gas swirl that helps to induce the arc rotation. The investigations were carried out at arc currents between 200–450 A and input gas flow rates in the range of 5–20 slpm. High gas swirl and low arc currents are found to favour constricted attachment on the anode surface. The local gas vortex driven by the plasma swirl creates hot spots inside the torch that dictate the arc attachment. The transition of arc attachment from the diffuse to constricted mode was simulated by imposing a step-change in the gas flow which shows the evolution of different arc attachment behaviour. During the transition from constricted to diffused attachment, the arc tends to undergo a transitionary multi-attachment mode at specific selected current and flow parameters.

Numerical simulation of the effects of protrusion on DC arc anode attachment

The attachment of the DC arc on the anode is usually affected by surface morphology such as protrusions due to ablation or melting deformation. A three-dimensional thermodynamic and chemical non-equilibrium model is used to numerically simulate the effect of artificially assumed surface protrusions on the arc anode attachment. The numerical simulation results show that the arc deflects toward the protrusions on the anode and attaches to them in a constricted mode, resulting in an increase in the temperature of the arc attachment region. The analysis shows that the presence of protrusion on the anode surface changes the electric field distribution, intensifies the degree of thermodynamic and chemical non-equilibrium in its vicinity, further influences the chemical kinetic process of the plasma around it, which is the main reason for the deflection of the arc toward the protrusions and the arc anode attachment in a constricted mode. In order to verify the numerical simulation results, verification experiments are also performed using similar size scale anode protrusion, and the results showed that the presence of protrusion can indeed cause the deflection of the arc and even cause the ablation of the protrusion.

Input-output framework for actuated boundary layers

This work extends the input-output approach to the study of wall-bounded shear flows manipulated using actuators common in experimental flow control studies. In particular, we adapt this powerful analytical framework to investigate the flow response to specified geometric actuation patterns (e.g., different plasma actuators) that can be applied over a range of different temporal input signals. For example, the commonly studied steady-state (time-averaged) flow response corresponds to a superposition of step responses in our modeling framework. The approach takes advantage of the linearity of the transfer function representation to construct the actuated flow field as a weighted superposition of the flow responses to point sources of varying intensity comprising the actuation model. We first validate the proposed method through comparisons with numerical and experimental studies of the time-averaged behavior of a transitional boundary layer actuated using a dielectric-barrier discharge plasma actuator operating in constricted discharge mode. The method is shown to reproduce the streamwise velocity field and the vortical structures observed downstream from the tested plasma actuator configurations. We then demonstrate that the method provides even better agreement with the steady-state response of the boundary layer subject to actuation from arrays of symmetric plasma actuators arranged in both spanwise and serpentine geometries. These results indicate the utility of this extension to the widely used input-output framework in analyzing the effects of certain actuation modalities that have shown promise in flow manipulation strategies for drag reduction. An important benefit of this analytical method is the low computational cost associated with its use in extensive parametric studies that would be cost-prohibitive using experiments or high-fidelity simulations.

Selective synthesis of DC carbon arc-generated carbon nanotube and layered-graphene and the associated mechanism

Herein we report how an anode is eroded and the eroded mass is deposited on the cathode surface at different arc currents () in a DC carbon arc discharge using roughly oriented graphite (ROG) as electrodes. It was found that the nature of anode erosion critically depends on and has profound effects on the morphology of the as-synthesized cathode deposits (CDs), the conversion efficiency of the system to form the CDs and their compositions. By characterizing the as-synthesized CDs in their totality by transmission electron microscopy, Raman spectroscopy, and x-ray diffraction it was found that there exists a critical value of below which the arc remains constricted, and above which the arc becomes intense. It was further found that the system can selectively generate both carbon nanotubes (CNTs) and layered-graphene sheets (LGs) when the carbon arc runs in constricted and intense modes, respectively. By the suitable adjustment of it is possible to switchover between the aforementioned arc-modes. Based on the experimental results, a semi-empirical model encompassing the plausible effects of rapid and random movement of the anode spot on the used ROG anode surface was developed to provide new insights into the growth mechanism of arc-generated CNTs and LGs. The state-of-the-art presented in this paper could facilitate the carbon arc discharge route for the tailored synthesis of highly crystalline CNTs and LGs.

Steady-state simulation of different discharge modes of argon arc under external magnetic field

In this study we performed a numerical investigation on different discharge modes of the magnetically rotating arc at atmospheric pressure. By using a three-dimensional simulation model taking into account of the arc-cathode interaction, it is able to predict the three different kinds of cathode arc attachments. Those modes of arc discharge have been observed in previous experiments. Arc attachments in constricted (i.e. spot) and diffuse mode are almost axisymmetric, however the edge mode offers a non-axisymmetric result. The corresponding arc plasma properties operating in different modes differ greatly from each other. Driven by the non-axisymmetric Lorentz force, a ‘twisted’ arc column emerges between the electrodes in edge mode, while the cathode arc attachment focuses to a corner of the edge. The heavy particle and electron particle temperatures are much lower in diffuse mode than the other two modes. Those plasma features of multi-modes should be considered in the applications as materials treatment.

Numerical simulation of constricted and diffusive arc–anode attachments in wall-stabilized transferred argon arcs

A numerical simulation is conducted to investigate arc–anode attachment behavior, especially the formation mechanism of the constricted arc attachment mode for the water-cooled anode of wall-stabilized transferred argon arcs. Argon molecular ions and the corresponding kinetic processes are included to the finite-rate chemistry model in order to capture the chemical nonequilibrium characteristics of the arc near the anode region. Modeling results show that constricted and diffusive arc–anode attachments can be self-consistently obtained at different arc currents while keeping other parameters unchanged. The dominant kinetic processes contributing to ionization and recombination in the arc center and fringes are presented. The results show that in arc fringes and the arc attachment region, molecular ion recombination plays an important role which leads to the rapid loss of electrons. The radial evolution of the production, loss and transport processes of electrons is further analyzed. It is found that for the constricted arc attachment mode, both the recombination and convection transport caused by the anode jet result in the loss of electrons at the arc fringes, which leads to the shrinkage of the arc column at the anode. The formation of the anode jet is due to the combined action of radial and axial Lorentz forces in the anode region.

Chemical Non-equilibrium Simulation of Anode Attachment of an Argon Transferred Arc

The diffuse and constricted arc attachment modes to the anode of an argon wall-stabilized transferred arc are investigated using a 2D chemical non-equilibrium model. Detailed argon chemical kinetic processes and self-consistent effective binary diffusion coefficient approximation treatment of diffusion are coupled with a single-fluid, two-temperature model to investigate the plasma characteristics of the near-anode region. The diffuse and constricted arc attachment modes to the anode are obtained self-consistently by changing the gas flow rates. The results show that the high current density and associated self-induced magnetic field of the constricted arc attachment give rise to a large Lorentz force near the anode, resulting in an anode jet. The convection process dominated by the cathode jet is conducive to the radial transport of electrons in the diffuse arc attachment mode, while in the constricted arc attachment mode the direction of electron transport due to the convection caused by the anode jet is opposite to that in a diffuse arc attachment. The effect of diffusion on electron transport is relatively small in the arc central region, while it becomes important in the arc fringe. The recombination processes, especially the dissociative recombination reactions, restrict the electron radial diffusion in the arc fringe. The diffusion current density is the main component of current density in front of the anode due to the steep gradient of electron pressure. It is found that when the diffusion current is not considered, the electric field and Joule heating in front of anode promote a more constricted arc attachment.

Operating modes of a magnetized cold cathode plasma in helium 50–6400 mTorr

We characterize the operating modes of a magnetized cold cathode plasma in helium at pressures 50–6400 mTorr (6.7–850 Pa). We find at least three distinct modes: (i) a diffuse magnetron plasma with representative plasma voltage 250 V; (ii) a constricted mode with a bright spot at the cathode surface that moves slowly, with representative plasma voltage 50 V; and, (iii) a constricted mode (CO) with a bright spot at the cathode surface that orbits rapidly around the racetrack region defined by the magnetic field, with representative plasma voltage 80 V. This latter mode is of interest because it appears to be a cold-cathode plasma operating at unusually low voltage, and it is associated with a low rate of cathode mass-loss. We find that the CO mode appears only if the initial rate of current-rise is high, and if the cathode surface is smooth and featureless. It also appears to be correlated with cathode materials that form strong oxide films that can dramatically increase the field emission of electrons from the cathode surface.

Erosion rates of diffuse and constricted magnetron discharges in helium over aluminium, gallium, molybdenum, and tantalum

Magnetically-enhanced discharge plasmas in light monatomic gases such as helium may have applicability in future electric power grid switches. For practical use of a gas discharge switch in power conversion the net cathode erosion rate must be sufficiently low to enable stable operation over an extended life. We have found that magnetically-enhanced He plasmas at pressures of 200–800 mTorr can operate in one of several distinct modes that are characterized by the plasma geometry (diffuse, constricted) and motion (stationary, orbiting, slow). The erosion rates of various cathode materials (aluminium, gallium, molybdenum, and tantalum) were measured in both diffuse and constricted, magnetized He plasmas. The constricted orbiting (CO) mode exhibited a lower erosion rate than either the diffuse stationary or constricted slow modes, even when comparing loss rates against a cathode material that was more liable to sputter (e.g. aluminium versus molybdenum). However, the erosion rate for operation in the CO mode was still found to be significantly higher than required for practical use in a gas discharge switch.

Multicomponent enabled MWCNTs-TiO2 nano-activating flux for controlling the geometrical behavior of modified TIG welding joint process

A comprehensive geometrical behavior against the multicomponent nano-activating flux, multi-walled carbon nanotubes-titanium oxide (MWCNTs-TiO2), was configured in this study. The specimens were investigated for macro-morphologies of the weld seams, distortions, weld depths, and depth to thickness (D/T) ratios. The scrutinized behavior of distortions has reported a decremented effect for every step addition of flux. The traveling trends of vertical distortions individually started from 5.334, 6.146, and 6.455 mm for without flux and eventually ended to 3.896, 4.112, and 4.713 mm respectively up to 2 wt% of MWCNTs-TiO2. A similar beneficial effect on weld depths of flux addition observed from optical micrographs, which revealed the radiating radii of semi-circular profiles with increasing penetration. The centripetal Marangoni convection (change in surface tension) and constriction of tungsten inert gas (TIG) welding arc column found responsible mechanisms for significant uplifting of geometrical behavior. However, the extended drive phenomenon due to the presence of TiO2 created electrical resistance effect, which provokes constricted droplet melting mode of filler. The use of MWCNTs provided heterogeneous nucleation that freezes the weld penetration along with limiting the distortion, attributed to the enormous mismatch of coefficient of thermal expansion (CTE) with AA6061. The experimental results also revealed the better performance of flux at higher welding current (200A), which triggered D/T >1 (at 1.5 wt% and 2 wt% MWCNTs-TiO2) in contrast to 160A and 180A.

Instabilities of a constricted gas discharge with respect to two-dimensional wave perturbations

Modelling of the positive column in the glow discharge of neon was performed under discharge conditions covering the areas of diffuse and constricted states, as well as the transition region between them. An analysis of the stability of the stationary solution for diffuse and constricted regimes of the discharge was made. The spectral problem was solved in order to find the eigenmodes and eigenvalues corresponding to the growth increment of the perturbations of the electron number density and electrical potential. It was shown that the discharge in the diffusion mode is stable with respect to the perturbations under consideration, and the appearance and propagation of essentially two-dimensional spatially periodic perturbations is possible in the constricted mode of the discharge. A two-dimensional structure (topology) of plasma parameters was calculated and illustrated. The results of the theory were compared to the experimental data.

Experimental and numerical investigation of a flow induced by a pulsed plasma column

The paper studies, both experimentally and numerically, a high-speed transient flow induced by a pulsed volume discharge in still air at low pressure. It is shown that, in the constricted mode, the discharge is capable of producing uniform deposition of the electrical energy into a long (24 mm in length), thin (less than 2 mm in radius) plasma column. Flow visualization experiments using particle image velocimetry (PIV) and high-speed shadow imaging indicate that this pulsed localized energy deposition generates a highly symmetrical cylindrical shock wave expanding at an average speed of 550 m/s within the first 40 μs after the discharge. Three-dimensional computational fluid dynamics (CFD) simulations successfully reproduce the experimentally observed flow structures and provide better insight into the complex discharge-induced flow. Modeling the trajectories of “virtual” particles within the CFD-predicted flow yields excellent agreement between numerical and PIV flow velocity profiles, and this comparison is used to quantify the rates of “rapid” energy thermalization in the pulsed discharge.

Electrical and thermal characterization of near-surface electrical discharge plasma actuation driven by radio frequency voltage at low pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 11472306, 51407197, and 51507187).

The electrical and thermal characterization of near-surface electrical discharge plasma driven by radio frequency voltage are investigated experimentally in this paper. The influences of operating pressure, electrode distance, and duty cycle on the discharge are studied. When pressure reaches 60 Torr (1 Torr = 1.33322 × 102 Pa) the transition from diffuse glow mode to constricted mode occurs. With the operating pressure varying from 10 Torr to 60 Torr, the discharge energy calculated from the charge–voltage (Q–V) Lissajous figure decreases rapidly, while it remains unchanged between 60 Torr and 460 Torr. Under certain experimental conditions, there exists an optimized electrode distance (8 mm). As the duty cycle of applied voltage increases, the voltage–current waveforms and Q–V Lissajous figures show no distinct changes.

On transition from diffuse mode to the constricted one with high-current cathode spot in overvoltage open discharge in D2

So called “open discharges” in a narrow gap between the solid cathode and grid anode are widely used for generation of the pulsed high-current electron beams with energy up to 100 keV. The need to get high-energy e-beams leads to the necessity in using of strong overvoltage of the short gas gap with the reduced electric field of the order of 105 Td or higher. The discharge under strong overvoltage is unstable and tends to transit into high-current regime with low voltage. In the case of the open discharge in D2 at low pressure (about 0.5-2 Torr) and powered by stepwise voltage with amplitude up to 25 kV we revealed that this discharge exhibits two diffuse regimes which follow one by one and finally transits into the constricted mode with formation of high-current spots on the cathode. The physical properties of these gas discharge regimes have been explored in detail with the usage of the fast multi-frame camera synchronized with the current and voltage of discharge. Our findings promote more insight into physics of the overvoltage open discharge generating the e-beams with energy up to 25 keV.

High-Current Vacuum Arc Mode Transition of a Horseshoe-Type Axial Magnetic Field Contact With Long Contact Gap

The objective of this paper is to determine the high-current vacuum arc mode transition diagram of a horseshoe-type axial magnetic field (AMF) contact with long contact gap. Drawn arc experiments were carried out in an $L$ – $C$ discharging circuit. Horseshoe-type contacts with contact material of CuCr50 and contact diameter of 120 mm were arranged in a demountable vacuum chamber. Full contact gap was 60 mm. The experiment current level ranged from 4 to 40 kA, stepped by 4 kA. Prospective arcing time was set as 2, 5, and 9 ms, respectively. The evolution of vacuum arc behavior was observed using a high-speed camera. Nonsynchronous evolution phenomenon of twin parallel arc columns was observed. With current level ranged from 24 to 40 kA, the maximum nonsynchronous duration transiting from small-gap constricted column mode to diffuse column mode, from diffuse column mode to diffuse arc mode, and from diffuse arc mode to multicathode-spot arc mode were 0.98, 1.09, and 1.55 ms, respectively. The nonsynchronous duration in various vacuum arc modes was determined as well. Summarizing all arc mode transition points in the experiment, a vacuum arc mode transition diagram of the horseshoe-type AMF contact was proposed. Boundary of uncontrollable arc region was determined, which could provide criteria of maximum arcing contact gap design. Besides, an effective arcing region was defined according to distribution of AMF and arc behaviors, in which AMF on the contact surface plane was higher than 75%Baxial–max.

Homogeneous CO2 conversion by microwave plasma: Wave propagation and diagnostics

A suite of diagnostics is proposed to characterize microwave plasma dissociation of CO2: laser scattering, Fourier transform infrared spectroscopy, and passive emission imaging. It provides a comprehensive performance characterization as is illustrated on the basis of experiments in a 2.45 GHz, 1 kW microwave reactor with tangential gas injection. For example, two operating regimes are identified as function of pressure: the diffuse and constricted plasma mode. Their occurrence is explained by evaluation of microwave propagation, which changes with the electron‐heavy particle collision frequency ve−h. In the diffuse mode, gas temperatures of 1500–3500 K are determined. The measured conversion degree, specific energy input, and temperature are summarized in a two‐temperature thermal model, which is solved to obtain the gas temperature at the periphery of the reactor and the size of the hot zone. Solutions are found with edge temperatures of hundreds of K, and hot zone fractions which agree with the measured behavior. The agreement shows that non‐thermal processes play only a marginal role in the measured parameter space of the diffuse discharge. In the constricted mode, the radial plasma size is independent of power. A skin depth equal to the plasma size corresponds to electron densities of 1018–1019 m−3. Temperatures in the central filament are in the range 3000–5000 K. Both discharge modes are up to 50% energy efficient in CO production. Rayleigh signals increase in the afterglow, hinting at rapid gas cooling assuming that the gas composition remains unchanged.

Investigation of the relationship between arc-anode attachment mode and anode temperature for nickel nanoparticle production by a DC arc discharge

Multiple and constricted arc-anode attachment modes were observed in helium arc discharge to prepare nickel nanoparticles. The electron overheating instability resulted in the formation of multiple attachment modes. The effects of hydrogen concentration and shield gas flow rate on the characteristics of nickel nanoparticles were investigated. The evaporation rate of anode material contributed to forming different arc-anode attachments. The surface temperature of the electrode was measured during the arc discharge by two-color pyrometry combined with a high-speed camera which employs appropriate band-pass filters. The relationship between the arc-anode attachment mode and the temperature behavior of the anode surface was investigated by using two synchronized high-speed cameras. The waveform of anode jet area variation with time follows that of the highest temperature variation of anode surface with time. The fluctuation of the highest anode temperature increased when the arc anode attachment changed from multiple into constricted mode. The highest temperature fluctuation and stability of the arc contributed to nanoparticle size distribution. Nickel nanoparticles with large productivity and narrow size distribution were obtained when shield gas was employed by controlling the residence time of nanoparticle growth. The formation mechanism of different arc-anode attachment modes was explained.

Evolution of magnetically rotating arc into large area arc plasma**Project supported by the National Natural Science Foundation of China (Grant Nos. 11035005, 11475174, and 50876101) and the Science Instrument Foundation of the Chinese Academy of Sciences (Grant No. Y201162).

An arc channel tends to shrink due to its conductivity increasing with the increase of temperature. In this study, to generate large area arc plasma, we construct a magnetically rotating arc plasma generator, which mainly consists of a lanthanide tungsten cathode (13 mm in diameter), a concentric cylindrical graphite anode chamber (60 mm in diameter) and a solenoid coil for producing an axial magnet field. By controlling the cold gas flow, the magnetically rotating arc evolves from constricted mode to diffuse mode, which almost fills the whole arc chamber cross section. Results show that the diffuse arc plasma has better uniformity and stability. The formation mechanism of large area arc plasma is discussed in this paper.