Underwater Arc Research Articles

Modeling the coupled bubble-arc-droplet evolution in underwater flux-cored arc welding

For the numerical simulation of underwater wet flux-cored arc welding, the most crucial issue is to investigate the intense interactions between the underwater bubble, arc plasma, and molten metal. However, it is a great challenge to couple them into a single numerical model. In this study, a 3D three-phase flow model is originally established, which successfully couples the dynamic bubble, arc, and droplet. A modified Lee model is employed to realize spontaneous phase transition from liquid water to bubble gas. According to the simulation results, the bubble evolution is divided into four main stages, and two bubble separation modes are recognized. It is found that, both the changing pressure field around the bubble and the varying flow direction of surrounding water contribute to the unique bubble evolution patterns. As for the droplet, its violent up-and-down oscillation at the wire tip is mainly caused by the opposite gas drag forces, which are produced by the turbulent gas flow inside the bubbles. The upward gas drag force can even make the neck of the droplet disappear. With different droplet detaching angles, two predominant droplet transfer modes are numerically produced; when the angle reaches 147°, the droplet is pushed away and becomes a spatter. Furthermore, the underwater arc is found to be subjected to compressions from both the radial direction due to bubble necking and the axial direction due to droplet growth. The arc temperature and velocity vary significantly not only during the whole droplet transfer period, but also within each bubble evolution cycle. To verify the reliability of the model, underwater welding experiments and visual sensing are conducted. The simulated results match well with the experimental ones, with an 8% error in the droplet transfer period and only a 1.4% error in the bubble evolution cycle.

One-step synthesis of carbon-onion-supported Pt Co alloy by underwater arc discharge for pH-universal hydrogen evolution reaction

One-step synthesis of carbon-onion-supported Pt Co alloy by underwater arc discharge for pH-universal hydrogen evolution reaction

Repetitive shock waves generated by a single long pulse underwater arc discharge

The electrohydraulic effect induced by underwater arc discharge is an efficient way to generate controllable, high-intensity shock waves. However, the development process of underwater arc discharge involves the complex coupling of plasma arc, gas bubble, and liquid medium, of which the evolution mechanism is not well understood. In this paper, the underwater arc discharge process at a millisecond pulse (>50 ms) was investigated by high-speed shadow imaging and colorimetric temperature measurement, and a simulation model of bubble pulsation was proposed to quantitatively estimate the state variation and energy transfer of the gas bubble. The results indicate that the whole arc discharge process can be categorized into three successive stages: short-period oscillation, long-period oscillation, and quasi-steady state. The vapor inside the bubble can reach a supercritical state (827 K and 140 MPa) at the minimum bubble radius. The simulation shows that the light radiation absorption and the heat conduct loss through metal electrodes are the two dominant factors influencing the pulsation of the bubble, and further analysis indicates that the dynamic evolution of the arc determines the bubble pulsation mode. Our findings demonstrate why and how repetitive electrohydraulic shock waves can be generated by a single long pulse underwater arc discharge, providing a low-cost way of shock wave generator based on an AC/DC high-voltage power source.

Characteristic simulation of underwater microsecond high-current pulsed arc discharge plasma

Modeling analysis of underwater pulsed arc discharge can predict the characteristics of plasma channels, providing theoretical guidance for the practical application of underwater pulsed discharge. Due to the complexity of experimental diagnostics for ‘kA’-level underwater pulsed discharge, there is currently a lack of precise experimental data to support the initial value selection and result optimization of the modeling. This paper established a plasma channel model for underwater pulsed arc discharge. In conjunction with the Saha ionization equilibrium equation, the model was capable of simulating the current, pressure, temperature, and electron density of the channel after gap breakdown. By utilizing spectroscopic diagnostic data and a multi-objective optimization algorithm, the initial values and key parameters of the model were reasonably determined. The simulation results were in good agreement with the experimental diagnostic results, reasonably representing the trends in electron density and blackbody radiation temperature. Moreover, the model was applicable for reasonably explaining the emission spectral mechanism of the arc channel and shock waves prediction under different discharge conditions.

Study on thermal penetration process of oil casing in shallow water environment by plasma arc based on heat source simulation and experiments

Abandoned offshore oil wellheads must be removed and plugged to protect the marine environment. Underwater plasma arc cutting holds the promise of playing a significant role in this work. However, much remains unknown about the thermal burn-off process of underwater plasma arcs for oil well casing. This work investigated the thermal penetration process of oil casing in a shallow water environment through experiments and heat source modeling simulations. A workpiece made of mild steel plate and cement block was considered as a small segment of the oil casing to be penetrated. The hole diameters, depths, and removal volumes of the workpieces were investigated experimentally. A combined heat source consisting of an elliptical cone heat source and a semi-ellipsoidal heat source was established based on the hole shape. The simulated and experimental hole profiles matched well. The results showed that the evolution of the hole shape of the workpiece was related to the motion state of the plasma arc and could be reflected by the variation of the arc voltage. Compared to steel plates, cement materials were easily removed by heat. These findings are favorable to promote the development of underwater plasma arc cutting in oil casing removal.

A highly effective N2 fixation method based on reverse vortex flow gliding arc plasma under water

A gliding arc plasma has the advantages of strong reactivity, high energy density, and easy scale-up, so it is very competitive in the field of plasma-based N2 fixation. In this work, we designed an underwater transfer arc discharge plasma N2 fixation system based on a reverse vortex flow gliding arc. In full contact with air and water, the plasma could realize the efficient conversion of N2, O2, and H2O, obtaining gaseous NO, NO2, and activated water rich in NOx*. The experiment showed that an underwater transfer arc discharge was formed when the current was greater than 0.6 A. Under this operating condition, the conversion rate of N2 reached 1.63%, the concentration of NOx reached 2.4%, and the production rate of nitrogen-containing products was about 3.41 mol/h. In addition, the energy consumption was as low as 2.21 MJ/mol, the lowest reported value for atmospheric-pressure rotary gliding arc plasma-based N2 fixation. The generated activated water had abundant NO3− and NO2− ions, which could be directly applied as N2 fertilizer in agriculture, promoting plant germination and growth. This paper describes an environmentally friendly and efficient technical route toward developing industrial N2 fixation/green agriculture, strongly supporting the carbon neutrality concept.

Formation characterization and dynamic mechanism of underwater wet wire and arc directed energy deposition

For the first time, the underwater wet wire and arc directed energy deposition (UW-WADED) multi-tracks deposition layers with favorable morphology were obtained. The forming characteristics of single-track UW-WADED were determined through response surface methodology. It was found that the power function curve provided the best fit for the profile of single-track UW-WADED owing to its changing plumpness value. Subsequently, the formation characteristics of multi-track UW-WADED with varying intervals were investigated. The deposition layer exhibited the flattest surface and the least root-mean-square deviation, measuring 321.79 μm when the interval was 8 mm. The dynamic process of UW-WADED was unveiled through X-ray high-speed images, leading to the formulation of UW-WADED droplet force model. As the arc bubble expanded, the horizontal component of the air drag force pointed to the opposite side of the molten pool and was the principal factor for the droplets to become spatters in UW-WADED. The mechanism of the defect formation was also revealed. The accumulation of the molten pool on the previous deposition layer coupled with the downflow of the molten slag were the inducers of slag inclusion defect while the exorbitant molten pool increased the likelihood of molten pool explosion and subsequent spatter formation.

Influence of NIR-laser radiation on the underwater arc welding process with flux-cored wire

The combination of laser radiation and electric arc as hybrid welding is used successfully in industry. In the underwater area, this technology is currently being researched but is not yet part of the industry. This paper demonstrates laser-stabilized arc welding with flux-cored electrode for underwater applications where the laser power density of the laser radiation does not result in a deep penetration welding effect. Steel samples (S235-JR) with a thickness of 10 mm were examined as bead-on-plate welds with a laser power density of 0.7-2.8 x 104 W/cm² at a wavelength of 1030 nm. The addition of laser radiation at low laser power density stabilizes the arc and produces a homogeneous weld seam. The process behavior and stability are characterized by measuring the current and voltage signals and analyzed regarding the laser beam and current source parameters. The metallographic analyses are used to compare the weld seam geometry and the microstructure properties.

A novel strategy to improve melting efficiency and arc stability in underwater FCAW via contact tip air chamber

The conventional method of underwater flux-cored arc welding (UWW-FCAW) is characterized by water around the welding torch's electric parts, the tubular wire, the molten pool and the workpiece. In the present work, underwater FCAW was performed by keeping the contact tip dry, inside a specially developed torch, and as in conventional underwater welding. The welds were carried out in a flat position at simulated depths of 0.3 m, 10 m and 30 m using a hyperbaric chamber. Welding electrical signals of voltage and current and contact tip temperature were acquired and processed to determine process behavior. As a result, the welding current was reduced when the contact tip was kept dry inside the torch because of the higher temperature achieved by resistance heating when it was insulated from the water. The ambient surrounding the contact tip interfered with the coefficient of heat transfer and, consequently, with its temperature. Welds performed with the dry contact tip also presented slight variations in bead shape parameters in different water depths. Higher arc stability was achieved by welding with the contact tip inside the air chamber, as minor variations of electric signals and fewer arc extinction events were observed compared to conventional underwater welds. The combination of improved arc stability with higher electrode temperature may also have contributed to minor porosity and smaller variation of the reinforcement along the weld bead.

Study of underwater plasma arc ignition and cutting process for surface nonconductive steel structures for offshore decommissioning

Plasma cutting is expected to play a significant role as a highly efficient thermal cutting method in offshore decommissioning. Offshore steel structures get covered with nonconductive substances such as rust, mud, and barnacles, which affect the ignition and maintenance of the plasma arcs. This study aimed to determine a suitable method for cutting steel structures covered with nonconductive substances. The entire process of the underwater plasma arc, including ignition, piercing, and cutting, was systematically studied experimentally. A multi-arc root state occurred during the ignition of the transferred plasma arc. In this state, the workpiece current is a significant signal for characterizing the piercing of nonconductive substances on the steel structure. The composite plasma arc exhibits better resistance to interference and a longer ignition distance. In lateral water flow, which was used to simulate ocean currents, the composite plasma arc pierced through 12-mm thick mild steel. Based on the experimental results, a novel cutting strategy was proposed, and a double-layered workpiece consisting of 8-mm thick cement and 12-mm thick mild steel was successfully cut. These findings may drive the application of plasma arc cutting in offshore decommissioning.

Experimental study of underwater plasma arc penetration parameters for oil wellheads

Dismantling abandoned oil wellheads is a central aspect of the decommissioning of production facilities. A wellhead consists of multiple layers of conductive oil casing nested with nonconductive cement rings. Plasma-arc cutting is expected to be used in abandoned wellhead demolition owing to its high processing efficiency for conductive materials. Currently, no reports are available on the operating parameters of plasma-arc cutting in oil wellhead dismantling. This study experimentally investigated the penetration parameters of an underwater plasma arc in abandoned wellheads. The stand-off distance, penetration angle, nozzle diameter, plasma arc current, and gas flow rate were considered as the main factors affecting the penetration effect. A composite workpiece consisting of 10 mm thick mild steel and 30 mm thick cement blocks was used to simulate a small segment of an oil wellhead. The penetration depth, removal volume, penetration specific energy, and nozzle loss were used as comprehensive evaluation indicators for optimizing the penetration parameters. Based on the results, the experimental phenomena during penetration were explained and recommended values of the underwater plasma arc processing parameters were provided. This study may advance the development of plasma arc cutting for the demolition of abandoned wellheads.

Underwater Arc Discharge Treatment Using a High-Voltage Arc-Robust Supply

Underwater arc discharge is one of the most effective water treatment methods because of the effects of intense shockwaves and high ultraviolet radiations. In this paper, a high-voltage underwater arc discharge system is proposed to study the inactivation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Escherichia coli</i> bacteria, which has an arc-robust feature. It consists of a high-voltage capacitor charger and a discharge circuit. The high-voltage capacitor charger is developed by an LCCL resonant converter to reach a current supply scheme that is inherently protected and robust against arc discharge. The detailed mathematical analysis and the design optimization of the resonant converter are presented. The discharge circuit is developed by a two-step spark gap scheme using an ultrafast highvoltage trigger circuit, providing a reliable system operation independent of the water conductivity. The proposed underwater arc discharge system is applied to 500ml of pollutant water to study the system’s performance on the inactivation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E. Coli</i> bacteria. The proper microbiological tests are carried out, showing the effectiveness of the proposed underwater arc discharge system. The complete inactivation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E. Coli</i> bacteria is achieved in an extremely short treatment time. Moreover, the experimental results are presented, which validate the desired performance of the proposed underwater arc discharge system.

Experimental study on the underwater DC arc behaviors regulated by an external transverse magnetic field

An arc discharge in water can generate extremely high voltage, which has great potential to be used in DC fault protection. The voltage characteristic is closely related to the arc behaviors. However, due to the complexity of the gas–liquid mixed environment, the arc shape is difficult to observe directly. In this letter, an observation device is specially designed and underwater arc motion with millisecond-scale duration is clearly photographed. The corresponding relationship between the arc behaviors and the varied voltage is revealed. Additionally, the random behaviors of underwater arc are controlled stably by applying an external transverse magnetic field. The enhanced cooling mechanism and the directional arc motion characteristics are discussed. We demonstrate that fast elongation of the arc column and continuous heat exchange between phase surfaces are the keys to realizing a rapid increase of underwater arc voltage.

Research on the underwater cutting mechanism of flux-cored arc cutting for aluminum alloy process

With the fast-growing demand of aluminum alloys in ships and deep-sea pressure structures, an accurate and efficient underwater cutting operation for aluminum alloy is significant. However, its underwater arc cutting mechanism is still not clear enough, which limits its further application. The challenges lie in poor underwater visibility and complex underwater environment. In this study, process experiments, underwater sensor, and numerical simulations were conducted during cutting process to investigate the cutting mechanism of 5,052 aluminum. Firstly, the effect of parameters were investigated on cutting current, voltage, and water depth on the underwater kerf formation. In addition, three typical kerf formations, including “V” type, “II” type and “∧” type, were found; Secondly, visual sensing system was setup to monitor the cutting arc trajectory and the combustion process during the cutting process. Specially, the underwater burning phenomenon was observed. Finally, Finite Element Analysis was performed to further analyze the underwater arc cutting kerf formation of aluminum alloy. A semi-ellipsoidal composite heat source was applied to simulate the underwater arc, and the aluminum thermal reaction-generated heat was introduced. A dynamic method named “birth and death elements” was utilized to simulate the removal of molten metal. The temperature test results show that the simulation process is feasible. All results showed that different cutting parameters led to different cutting mode and affected the kerf forming. 5,052 aluminum alloy in the deep water environment (≥50 m) kerf significantly narrowed. The kerf cross-section produces an inward concavity and the kerf surface is as wide as the cutting wire. The aluminum kerf forming process is greatly impacted by the process parameters and self-propagating high-temperature synthesis reaction during aluminum alloy cutting. The periodicity of the cutting process was computed by the numerical simulation with the arc motion trajectory monitored by the high-speed camera. The numerical results of temperature distribution and kerf shape were consistent with the experimental data, which revealed the cutting mechanism.

Reservoir Porosity Improvement Device based on Underwater Pulse Arc Fracturing and Frequency Resonance Technology

SummaryThe low porosity of the reservoir has a significant impact on the production of unconventional oil and gas, hence a device to increase reservoir porosity and enhance unconventional oil and gas recovery was developed. The device can be lowered to 3000 m and operate continuously for more than 30 minutes under a discharge voltage of 11 kV in the frequency range of 0–60 Hz to improve reservoir porosity by causing reservoir resonance. The equipment structure includes an energy storage circuit, trigger switch, and energy transducer. The theoretical model of the energy storage circuit was established by the state space averaging method to obtain the time constant which was verified by a simulation experiment. The gas spark switch with an adjustable gap was used, the frequency control of the discharge pulse was achieved by rectifier voltage regulation, and the underwater pulse arc fracturing experiment was performed to confirm the accuracy and stability of the frequency control. Additionally, the effect of frequency resonance on reservoir porosity improvement was examined through comparative experiments, and the images of the distribution of pore texture in shale obtained by the X-ray computed tomography (CT) system demonstrate that resonance excitation can significantly promote the development of fractures and the improvement of shale samples’ porosity. The stimulation operation field experiment was carried out on coalbed methane wells in Shanxi Province, and the multipole array acoustic logging image verified that the equipment has a good reservoir porosity improvement effect. Experimental results indicate that this study has a potential application value in the field of unconventional reservoir stimulation.

Numerical modeling of coupled arc plasma, metal transfer and molten pool evolution for underwater flux-cored arc welding

Numerical modeling of coupled arc plasma, metal transfer and molten pool evolution for underwater flux-cored arc welding

Effect of Prebreakdown Time on Shock Wave Generation Characteristics of Underwater Plasma Sound Source

Acoustic array bunching is an effective method to realize the directional radiation of underwater plasma sound source (UPSS). The prebreakdown time is one of the decisive factors for the performance of acoustic array bunching. The working characteristics of the underwater plasma sound source and the prebreakdown process of underwater plasma arc discharge are deeply analyzed. The precondition of plasma sound source array bunching is given by analyzing the waveforms of electrical signals and acoustic signals. Through the experiments of arc discharge based on different aqueous solutions, a feasible scheme of underwater plasma sound source array bunching in salt water is proposed. Through the arc discharge experiments based on different system discharge parameters, the variation of prebreakdown time under the influence of different charging voltage, conductivity, and other parameters was studied. The experimental results show that the prebreakdown time decreases with the increase of charging voltage and conductivity. At the same time, the discharge current increases, the oscillation time decreases, and the energy injection rate on the electrode accelerates. The research results are helpful to understand the prebreakdown mechanism of plasma sound source and optimize the underwater plasma sound source array bunching scheme and the acoustic shock wave waveform.

Heat Transfer and Thermohydraulic Characteristics of Arc‐Discharge Argon Plasma Plume Injected into Water

The flow and thermohydraulic characteristics of an atmospheric-pressure argon arc jet plume injected into water were investigated. The temperature of the underwater plasma plume was measured using optical emission spectroscopy, and the water with a thermocouple. The velocity field of the water induced by the arc-jet injection was observed using the particle image velocimetry method, which demonstrated that as the discharge current increased, the water flow velocity also increased. The diameter of the plasma plume was observed to be approximately the same as that of the anode nozzle, 6.7 mm. The heat transfer characteristics of the injected arc jet plume to the surrounding water were also investigated. The average velocity of the arc jet plume in the water was estimated based on mass balance to be in the range of 280–2500 m/s as a monotonically decreasing function of the flow distance. The velocity of the underwater arc is found to be subsonic over the entire volume of the plasma plume, however, it is larger than the sound velocity of the water up to 10 mm downward from the plasma nozzle. The resultant Reynolds number of the plasma flow was found to be between 1000 and 2800, indicating the flow was turbulent only at the most upstream region up to 3 mm downward from the nozzle, while it was almost laminar over the whole observed area of the underwater arc plasma plume. The heat transfer characteristics were discussed in terms of the dimensionless numbers. The Nusselt number ranges from approximately ∼3.8 to ∼6.5, which is a reasonable value. Although the existing theories of correlation heat transfer cannot fully explain the relationship between these dimensionless numbers, the correlation proposed by Fiszdon and the one by Lee–Pfender can qualitatively explain the observed results for the upstream region, which include the effects of the variation in the mass density, the viscosity, and the specific heat for a constant pressure of the argon arc plasma. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Probing ionization characteristics of under-water plasma arc discharge using simultaneous current and voltage versus time measurement in carbon nanoparticle synthesis

Abstract This study investigated the characteristics of plasma ionization that occurs during plasma arc discharge in water using carbon electrodes. The characterization of plasma ionization begins by observing the arc shape using a digital camera associated with voltage oscillation using current and voltage vs. time (I-t and V-t) characteristics. Furthermore, the ionization energy calculation is performed using simultaneous I-t and V-t measurement data. These calculations are then compared to the photon energy from the optical emission spectroscopy (OES). The results from the digital camera indicate four types of arc discharge in terms of shape and intensity related to voltage oscillation. Moreover, the energy calculations show that by using the I-t and V-t methods is possible to measure the plasma ionization detected not only in the range 1–4 eV but also in the outside range, including smaller than 1 eV and greater than 4 eV. These ranges correspond to the emission lines in visible light, infrared, and ultraviolet, respectively. In addition, the I-t and V-t methods can also measure the excitation and recombination energy of the arc discharge plasma ionization. This study reveals the possibility of investigating the characteristics of plasma ionization using simultaneous I-t and V-t measurements, which comprise an easier and more practical method in the diagnostics of carbon nanoparticle synthesis • A novel plasma diagnostic using I-t and V-t measurement were demonstrated as a probing method for ionization of arc discharge. • I-t and V-t measurement measurement reveals voltage oscillation during arc discharge for different states. • The difference in arc intensity and shape is due to the difference in plasma ionization characteristics of arc discharge. • Numerical analysis of single discharge energy is calculated from I-t and V-t data to obtain plasma parameters.

Synergistic effect of underwater arc discharge plasma and Fe2O3-CoFe2O4 enhanced PMS activation to efficiently degrade refractory organic pollutants

This study employed a self-designed underwater arc discharge (UAD) plasma reactor, which combined advanced oxidation process (Fe2O3-CoFe2O4/PMS) and low-temperature plasma technology to efficiently degrade refractory organic pollutants (i.e. phenol, p-chlorophenol, p-nitrochlorobenzene). The results illustrated that the plasma/PMS/Fe2O3-CoFe2O4 system possessed high degradation efficiency and TOC removal rate towards three target pollutants (phenol: 99.8%, 96.12%; 4-CP: 98.4%, 86.48%; p-NCB: 99.9%, 88.31%). With the help of in-situ variable temperature EPR and Indigo colorimetry method, ·OH, SO4·-, O2·-, 1O2 and O3 were identified as the main reactive oxygen species (ROS) for the degradation of target pollutants, revealing the synergistic catalytic mechanism between plasma and Fe2O3-CoFe2O4 for PMS activation and inferring the degradation pathways of target pollutants. The results confirmed that the plasma/PMS/Fe2O3-CoFe2O4 system was feasible for the degradation of refractory organic pollutants, has the advantages of high efficiency and low energy consumption, and could provide an effective strategy for the treatment of refractory organic pollutants in groundwater/soil.