Restrike Mode Research Articles

Experimental Study on the Restrike Mode of a DC Arc Anode Attachment

The restrike mode is an important arc anode attachment mode under the cross flow in DC arc device, which often causes a large amplitude of arc voltage fluctuations and therefore has an important impact on the performance of arc thermal plasma device. A transferred arc device with planar anode parallel to gas flow direction is used to experimentally study the restrike mode characteristics under different operating conditions. It is found that for the case of a fixed gas flow, the arc restrike frequency increases while the amplitude of arc voltage jump decreases with the increase of arc current. For the case of a fixed arc current, both arc restrike frequency and amplitude of arc voltage jump increase with the increase of gas flow rates. Further analysis shows that the restrike process in one period can be divided into two phases. The first phase corresponds to the generation and development of upstream new arc roots and the disappearance of downstream old root, and its time scale is less than the order of 1 ms. The second phase corresponds to the newly formed, single arc root moving downstream under the combined action of gasdynamic drag force and Lorentz force until new arc root appears upstream. The time-resolved temperature field measurement based on the relative intensity of emission spectrum method shows that the temperature at the junction of the arc column and the anode arc root is relatively low, while it still able to maintain the current conduction, which indicates the significant deviation from thermodynamic equilibrium state.

MHD modeling of rotating arc under restrike mode in ‘Kvaerner-type’ torch: II. Dynamics and stability at 20 bar pressure

This paper focuses on the 3D MHD modeling of a very high-pressure (20 bar) arc plasma at industrial level; the torch is a ‘Kvaerner-type’ one, made of two concentric graphite electrodes and where spots are displaced with an external magnetic field. Different phenomenologies in arc movement were observed, ranging from a gliding arc on electrodes surfaces to hopping arc foots and notably impacting electrodes erosion and the overall efficiency in the plasma process. Besides those dynamics which are indebted to torch configuration, current and flow rates, arc stability is also investigated with a theoretical model based on an energy principle. This research work provides new insight on the dynamics and stability of the arc inside the Kvaerner-type torch along with an unprecedented understanding of the very high pressure arc under concentric-electrodes-configuration, highlighting strong deformations due to pressure-driven instabilities for the high power arc.

MHD modeling of rotating arc under restrike mode in ‘Kvaerner-type’ torch: part I. Dynamics at 1 bar pressure

This paper focuses on the 3D MHD modeling of an arc plasma at industrial level; the torch is a ‘Kvaerner-type’ one, made of two concentric graphite electrodes and where spots are displaced with an external magnetic field. Different phenomenologies in arc movement were observed, ranging from a gliding arc on electrodes surfaces to hopping arc foots, agreeing with previous experimental works at atmospheric pressure. These dynamics have non-negligible consequences in the plasma process: the hopping mode induces more erosion due to a higher heat flux at the electrodes surfaces in comparison to the gliding arc; the latter mode results also in a strong gas swirling inside the torch, thus longer particles residence time is predicted.

Experimental Observations of Constricted and Diffuse Anode Attachment in a Magnetically Rotating Arc at Atmospheric Pressure

At atmospheric pressure, the anode attachment can appear in two different modes: constricted and diffuse. In this paper, a magnetically rotating arc plasma generator is built to investigate dynamics of anode attachment. A constricted and diffuse anode attachment, as well as the transformation between the two modes, are obtained. Results show the constricted anode attachment to be preferable for low arc currents and a low magnetic field, whereas the diffuse anode attachment is favored by the opposite conditions. With an increase in arc currents/magnetic field, the constricted anode attachment can transfer from a “restrike mode” with high voltage fluctuation to a “sliding mode” with low voltage fluctuation and then to a diffuse anode attachment with a long arc-shaped structure. An expanding arc column and high-temperature anode are suggested as essential conditions for the diffuse anode attachment because a thinner and hotter anode boundary layer may be promoted. In addition, the time-resolved transformation of different modes exhibits a strong mutual influence between the arc column and anode attachment, which indicates complex coupling mechanisms exist in the near-anode region.

Time-Resolved Emission Spectroscopy Measurements during the Restrike Period in Arc Plasma Torch

Simultaneous optical, spectroscopic, and electrical measurements in the region of the arc anode attachment of the water-argon plasma torch are presented. A movement of the arc attachment along the anode surface together with its restrike mode is monitored. Temporal evolution of temperature during one cycle of the restrike mode is obtained in three different axial positions in the plasma column. Resulting temperature profiles show how the position of the arc attachment influences the plasma properties.

Experimental study on ignition characteristics of kerosene–air mixtures in V-shaped burner with DC plasma jet igniter

In this paper, we investigate the effects of DC plasma jet igniter on ignition characteristics of the kerosene–air mixtures in V-shaped burner. The primary principles of DC plasma igniter design have been put forward to prolong the igniter life and improve ignition performance. The main characteristic of plasma igniter is that the anode arc root is fixed at anode nozzle outside surface other than the anode inner surface in conventional design. The electric characteristics of the plasma jet igniter including voltage–current characteristics, the arc column propagation, emission spectrum and anode arc root movement in a quiescent environment are investigated with high speed CCD together with a recording of the voltage and current. The entire ignition process of the plasma jet igniter is recorded by high speed CCD and the ignition delay times of the kerosene–air mixtures defined by the interval between the onset of the plasma igniter and the establishment of a stable flame are measured with two photomultipliers with one of 10 mm band pass filter centered at 780 mm (the plasma igniter emission spectrum of OI) and the other of 10 mm band pass filter centered at 430 mm (direct flame emission of CH(A2Δ→X2Π)). Results show that the plasma igniter works in a restrike mode and the anode spot movement is irregular. The plasma igniter jet contains abundance active particles inferred from the emission spectrum. The ignition delay times increase at first and then decrease with the increase of excess air coefficient ranging from 0.91 to 6.38. The delay times increase quickly when the mixtures are extremely lean or rich.

Dynamic behavior of a rotating gliding arc plasma in nitrogen: effects of gas flow rate and operating current

The effects of feed gas flow rate and operating current on the electrical characteristics and dynamic behavior of a rotating gliding arc (RGA) plasma codriven by a magnetic field and tangential flow were investigated. The operating current has been shown to significantly affect the time-resolved voltage waveforms of the discharge, particularly at flow rate = 2 l min−1. When the current was lower than 140 mA, sinusoidal waveforms with regular variation periods of 13.5–17.0 ms can be observed (flow rate = 2 l min−1). The restrike mode characterized by serial sudden drops of voltage appeared under all studied conditions. Increasing the flow rate from 8 to 12 l min−1 (at the same current) led to a shift of arc rotation mode which would then result in a significant drop of discharge voltage (around 120–200 V). For a given flow rate, the reduction of current resulted in a nearly linear increase of voltage.

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.

Direct Observation of Anode Arc Root Behaviors in a Non-transferred Arc Plasma Device with Multiple Cathodes

To investigate the anode arc root behaviors, a non-transferred arc plasma devices with multiple cathodes is designed in this paper. A novel setup to investigate the dynamics of arc root is developed. Based on high speed photography and voltage waveform analysis, anode arc root behaviors are captured for the steady, takeover and restrike arc modes. The transition of a single arc root, the coexistence of multiple arc roots and the evolution of multiple arc roots are herein investigated. Results show that anode arc root behaviors for these three arc modes are quite different. The restrike, takeover and steady modes correspond differently to the longitudinal motion of single root, circumferential motion of a single root and stationary multiple arc roots, respectively. Moreover, contrary to past experimental results, the stationary multiple arc roots which exist on the anode just within the nozzle entrance and the process by which they form, are demonstrated for the first time.

Effects of Arc Discharge Mode on the Efficiency of Biogas Reforming in an AC-Pulsed Arc Plasma System

The effects of arc discharge mode on biogas reforming performance in an AC-pulsed arc plasma system were investigated for different pulse rising times. The discharge mode changed from the re-strike arc mode to constant arc discharge mode on increasing the pulse rising time from 2 to 5 μs at a fixed discharge power. The length of the arc column periodically changed under re-strike mode, while the arc column was always longer under constant arc mode. The production yield of synthesis gas and the energy efficiency significantly improved when the arc discharge was operated under constant arc mode. The conversion rates of CH4 and CO2 increased from 15 to 80 and 12 to 27%, respectively. Under constant arc mode, the production yield of synthesis gas was 6.5 times higher than that under re-strike arc mode. In addition, the energy cost for synthesis gas production decreased remarkably from 52 to 9 kJ/mol. This is because the biogas reforming reactions, including thermal cracking, dry reforming, and partial oxidation, proceeded effectively in an area of the arc plasma, wherein the gas could react in the high-enthalpy region for a duration sufficient to ensure efficient reforming reactions. Therefore, the arc discharge mode significantly affected the chemical reactions and energy efficiency of biogas reforming.

Characteristics of Atmospheric Pressure Rotating Gliding Arc Plasmas**supported by National Natural Science Foundation of China (No. 51576174), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120101110099) and the Fundamental Research Funds for the Central Universities (No. 2015FZA4011)

In this work, a novel direct current (DC) atmospheric pressure rotating gliding arc (RGA) plasma reactor has been developed for plasma-assisted chemical reactions. The influence of the gas composition and the gas flow rate on the arc dynamic behaviour and the formation of reactive species in the N2 and air gliding arc plasmas has been investigated by means of electrical signals, high speed photography, and optical emission spectroscopic diagnostics. Compared to conventional gliding arc reactors with knife-shaped electrodes which generally require a high flow rate (e.g., 10–20 L/min) to maintain a long arc length and reasonable plasma discharge zone, in this RGA system, a lower gas flow rate (e.g., 2 L/min) can also generate a larger effective plasma reaction zone with a longer arc length for chemical reactions. Two different motion patterns can be clearly observed in the N2 and air RGA plasmas. The time-resolved arc voltage signals show that three different arc dynamic modes, the arc restrike mode, takeover mode, and combined modes, can be clearly identified in the RGA plasmas. The occurrence of different motion and arc dynamic modes is strongly dependent on the composition of the working gas and gas flow rate.

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.

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

The 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 µs. 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 µs 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.

Investigation and control of dc arc jet instabilities to obtain a self-sustained pulsed laminar arc jet

The uncontrolled arc plasma instabilities in suspension plasma spraying or solution precursor plasma spraying cause non-homogeneous plasma treatments of material during their flight and also on coatings during their formation. This paper shows that the arc motion in dc plasma torches mainly originates in two main modes of oscillation (Helmholtz and restrike modes). The emphasis is put on the restrike mode in which the time component is extracted after building up and applying a numerical filter to raw arc voltage signals. The dependence of re-arcing events on experimental parameters is analysed in the frame of a phenomenological restrike model. It is shown that when the restrike frequency reaches the Helmholtz one, both modes are locked together and a pulsed arc jet is generated.

Experimental observations of arc-anode attachment in steam-argon-air environment

DC arc plasma torches performance depends strongly on processes near anode. Arc-anode interaction not only defines lifetime of the electrode but also influences power distribution in the plasma and flow structure of the jet. This paper presents an experimental investigation of effects of gas properties and flow conditions on an arc-anode attachment. Several optical and electric diagnostic means were used to provide complex image of behaviour of the attachment and resulting state of anode surface. It is shown that even in one particular regime of the arc-anode attachment, which is called a restrike mode, dynamic behaviour and resulting erosion of anode can be significantly different. For example with argon in arc-anode gap the anode surface melt more intensively than with air environment. Despite increased melting weaker radiation of copper vapour was found in plasma. It was also shown that control of arc-anode attachment position in air environment had substantial effect on anode erosion pattern and plasma flow behaviour.

Discharge Characteristics of DC Arc Water Plasma for Environmental Applications

A water plasma was generated by DC arc discharge with a hafnium embedded rod-type cathode and a nozzle-type anode. The discharge characteristics were examined by changing the operation parameter of the arc current. The dynamic behavior of the arc discharge led to significant fluctuations in the arc voltage and its frequency. Analyses of the high speed image and the arc voltage waveform showed that the arc discharge was in the restrike mode and its frequency varied within several tens of kilohertz according to the operating conditions. The larger thermal plasma volume was generated by the higher flow from the forming steam with a higher restrike frequency in the higher arc current conditions. In addition, the characteristics of the water plasma jet were investigated by means of optical emission spectroscopy to identify the abundant radicals required in an efficient waste treatment process.

On the dynamic behavior of the anode–arc–root at the nozzle surface in a non-transferred plasma torch

The dynamic behavior of the anode–arc–root at the nozzle surface of a plasma torch was experimentally investigated in this work. A gas (N2) vortex–stabilized non-transferred arc torch with a thoriated tungsten rod (2wt %) cathode (3.2 mm diameter) and a coaxial anode (5 mm diameter, 30 mm length) was used in the experiment. By using a sweeping Langmuir probe in floating condition, the voltage of the plasma jet outside the nozzle was inferred. Arc voltage waveforms were also obtained. Data have been obtained for an arc current of 100 A and a gas flow rate of 30 Nl min−1. A typical sawtooth shape (i.e., restrike mode) (with a fluctuating level of ≈ ± 25 %) and a dominant frequency of ≈ 6.5 kHz was observed in the arc voltage waveforms, which is attributed to anode–arc–root movements along the anode surface followed by a restrike at a certain point close to the cathode. By performing a time correlation between the probe and arc voltage oscillograms together with simple estimations, the amplitude of the movement of the arc–root along the anode surface as well its velocity were inferred.

Characterization of a steam plasma jet at atmospheric pressure

An atmospheric steam plasma jet generated by an original dc water plasma torch is investigated using electrical and spectroscopic techniques. Because it directly uses the water used for cooling electrodes as the plasma-forming gas, the water plasma torch has high thermal efficiency and a compact structure. The operational features of the water plasma torch and the generation of the steam plasma jet are analyzed based on the temporal evolution of voltage, current and steam pressure in the arc chamber. The influence of the output characteristics of the power source, the fluctuation of the arc and current intensity on the unsteadiness of the steam plasma jet is studied. The restrike mode is identified as the fluctuation characteristic of the steam arc, which contributes significantly to the instabilities of the steam plasma jet. In addition, the emission spectroscopic technique is employed to diagnose the steam plasma. The axial distributions of plasma parameters in the steam plasma jet, such as gas temperature, excitation temperature and electron number density, are determined by the diatomic molecule OH fitting method, Boltzmann slope method and Hβ Stark broadening, respectively. The steam plasma jet at atmospheric pressure is found to be close to the local thermodynamic equilibrium (LTE) state by comparing the measured electron density with the threshold value of electron density for the LTE state. Moreover, based on the assumption of LTE, the axial distributions of reactive species in the steam plasma jet are estimated, which indicates that the steam plasma has high chemical activity.

3D MHD modelling of low current–high voltage dc plasma torch under restrike mode

We present in this paper a magnetohydrodynamic (MHD) modelling of the gliding arc behaviour of a dc plasma torch operating with air under low current and high voltage conditions. The low current leads to instabilities and difficulties with simulating the process because the magnetic field is not sufficient to constrict the arc. The model is 3D, time dependent and the MHD equations are solved using CFD software Code_Saturne®. Although the arc is definitively non-local thermodynamic equilibrium (LTE), the LTE assumption is considered as a first approach. The injection of air is tangential. A hot gas channel reattachment model has been used to simulate the restriking process of the arc root. After the description of the model, the most appropriate electrical voltage breakdown parameter has been selected in comparing with experimental results. A typical operating point is then studied in detail and shows the helical shape of the arc discharge in the nozzle. Finally, the mass flow rate and the current have been varied in the range 0.16–0.5 g s−1 and 100–300 mA, respectively, corresponding to typical glidarc operating points of our experimental plasma torch. The model shows good consistency with experimental data in terms of global behaviour, arc length, mean voltage and glidarc frequency.

Examination of Initial Strike and Restrike Modes in Exploding Metallized Films

The discharge modes of a capacitive exploding metallized film setup are discerned. Long exposure still images of the plasma formation during the initial strike and restrike are presented.