Underwater Pulsed Discharge Research Articles

The propagation of negative pulsed discharges facilitated by artificially injected gas bubble in water

Abstract The artificial injection of bubbles into the electrode gap can effectively enhance the performance of underwater pulsed spark discharge (UPSD). It is crucial to investigate the morphology and propagation characteristics of discharges for a comprehensive understanding of bubble-stimulated UPSD. This paper presented an extensive study of negative discharges facilitated by the injected large bubble (with a diameter of 3 mm) in UPSD. The bridging effect of the bubble and the morphology and propagation of discharges were observed through the shadowgraph images captured by a high-speed camera. A numerical model was built to analyse the influence of the bubble on the initial electric field distribution between the electrodes. The characteristics of discharges were notably influenced by bubble parameters and energization conditions. Not in all cases could the bubble play the bridging effect role. The effective range of the bubble was roughly confined to a small region near the HV electrode tip. The experimental results obtained in this paper have certain guiding significance for the practical application of UPSD.

Formation mechanisms of multiple Si/graphene nanocomposites through underwater pulsed discharge using different wire arrangements

Si/graphene nanocomposite is regarded as an advanced lithium-ion anode material, attributable to its advantageous microstructure and exceptionally high theoretical specific capacity. This study demonstrates a novel approach for the preparation of Si/graphene nanocomposites, achieved through the pulsed discharge method employing graphite and Si strips as precursors. We devised two distinct “one-step” synthetic routes through pulsed discharge of different wire arrangements. The characterization results of the obtained samples unveiled the presence of Si/graphene composite and Si@carbon layer composite. In addition, the formation mechanisms of Si/graphene nanocomposites have been proposed, different wire arrangements lead to the different energy deposition processes, inducing different robust mechanical and thermal effects on the products. The formed Si clusters either deposit on the surface of exfoliated graphene or become encapsulated within curling graphitic layers, forming a core@shell structure. This work not only presents a straightforward and innovative approach for synthesizing Si/graphene nanocomposites but also contributes valuable insights into the underlying mechanisms governing their formation.

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.

Morphological characteristics and cleaning effects of collapsing cavitation bubble in fractal cracks

When a cavitation bubble collapses in a crack, due to the lack of symmetry, spatial uniformity, and obvious correlations during the collapse process, it is difficult to effectively explore the interaction mechanism between the bubble and crack walls. In this paper, a combination of numerical simulation and experimental approaches are used to explore the collapse process of a cavitation bubble in a fractal crack. The numerical model is based on the pseudopotential multi-relaxation-time lattice Boltzmann method (LBM). The experiment platform including an underwater pulsed discharge device combined with a high-speed camera system. Moreover, the morphological analysis method based on Minkowski functionals is used to quantitatively depict the morphological features in this paper. The validity of the numerical model is qualitatively verified by the experimental platform, whereas the influence of the complexity and geometric features of the crack wall on the cavitation bubble collapse process is quantitatively studied by LBM simulation and Minkowski functionals. The research findings indicate that the complexity and geometric features of the crack wall markedly influence the collapse time and behavior of the cavitation bubble. Specifically, the collapse time of the cavitation bubble increases with the increasing complexity of the crack wall. Moreover, the maximum jet direction of cavitation bubble toward the wall is closest to the cavitation bubble. Furthermore, as the fractal dimension increases, the intensity of the pressure wave and jet acting upon the crack wall increase while the radiation range decreases. Cavitation cleaning can be applied to arbitrary complex solid surfaces in various environments.

Spectral diagnosis of underwater microsecond high-current pulsed discharge plasma

Underwater microsecond high-current pulsed discharge can generate powerful shock waves and a significant number of active ions, offering a wide range of applications. To analyse the characteristics of plasma channel formed by the discharge, this study established a comprehensive test platform and conducted spectral diagnosis for underwater high-current pulsed discharge. Considering the randomness of the pulsed arc discharge, the voltage and current waveforms under different discharge cases were measured, and the images and emission spectra of the plasma channel were obtained. The origin of the continuous emission of the initial spectra was analysed. A spectral fitting algorithm was proposed, which could separate the spectral lines into line emission of Hα and continuous background, and correct the self-absorption. The time-varying characteristics of electron density were obtained under three typical discharge cases, the possible errors caused by baseline shift were considered, and the influencing factors of line broadening were analysed. Under the test conditions of this study, the electron density of plasma channel was in the order of 1024–1025 m−3.

A numerical model for the electrical and shock wave characteristics of underwater pulsed spark discharge

Underwater pulsed spark discharge has been widely used in industrial fields as a source of shock waves or acoustic waves, and numerical modeling of the discharge and pressure wave characteristics is necessary to improve the application performance. In this paper, a numerical model is proposed that couples the circuit equation, the mass and energy conservation equations, and a momentum conservation equation based on the Rankine–Hugoniot conditions. A tabulated wide range equation of state and conductivity data of water are used, and various physical processes during the plasma channel expansion are considered, including the energy flow and mass exchange between the channel and the surrounding water due to thermal radiation, evaporation, and condensation. The model self-consistently solves the circuit current and voltage, the plasma channel parameters including composition, temperature, conductivity, pressure, etc., and the pressure profile at a certain distance from the discharge channel. The calculated results show good consistency with the experimental measurements, and three sets of experimental results from other literature are tested to further verify the applicability and effectiveness of the model.

The guiding effect of artificially injected gas bubble on the underwater pulsed spark discharge and its electrical and acoustic parameters after breakdown

The presence of a low density area is beneficial to the facilitation of the underwater pulsed spark discharge, which can be achieved by artificially injecting gas bubble in between the inter-electrode gap. The generation of intensive acoustic waves by such gas-bubble-guided spark discharges makes them promising underwater acoustic sources in multiple practical applications. This paper is aimed at comprehensive investigation of the guiding effect of the injected bubble on the pre-breakdown process of underwater pulsed spark discharges and potential correlations between their subsequent electrical and acoustic parameters with the purpose of optimizing the acoustic emission. The breakdown probability and the pre-breakdown delay were used to evaluate the general facilitation effect brought by the injected bubble. Experimental and numerical works have been conducted and allow observation on the dynamics of the injected bubble under the influence of the applied voltage. Different guiding modes of the injected bubble for plasma streamers' propagation have been observed regarding its relative position. The characteristics of the electrical properties of gas-bubble-guided spark discharges, including the plasma resistance and the plasma energy density, were analyzed by relating them with the breakdown voltage. The dependency of the acoustic wave amplitude and the acoustic efficiency on these electrical parameters was verified, which provides solid regulation principles for the optimization of the plasma-acoustic system for target practical applications.

Analysis of the bubble-pulsation process of underwater high current pulsed discharge

The bubble of underwater high current pulsed discharge forms after the pre-breakdown process, and the change in electrical loop parameters will affect its pulsation characteristics. The relationship between loop electrical parameters and the bubble-pulsation process is unclear, and its influence on the acoustic characteristics of the bubble-pulsation is not apparent. In order to quantitatively analyze the bubble-pulsation characteristics, a comprehensive experimental platform of underwater high current pulsed discharge is built. Under different loop electrical parameters, with the help of experimental data of electrical signals and optical images, the variation characteristics of the bubble-pulsation period and energy are analyzed. The acoustic characteristics of the bubble-pulsation process are analyzed, and the correctness of acoustic characteristic analysis is verified by experimental results. The comparison between experimental and theoretical data shows that the analysis of bubble-pulsation period and energy is correct. The bubble-pulsation characteristics can be predicted utilizing the deposition energy of the plasma channel. The results provide a basis for the quantitative analysis of the bubble-pulsation characteristics and strengthen the connection between the three underwater high current pulsed discharge processes.

Enhanced decomposition of toxic pollutants by underwater pulsed discharge in the presence of hydrogen peroxide and microbubbles

The decomposition of phenol and 4-chlorophenol by underwater pulsed discharge has been examined. The addition of hydrogen peroxide improved the decomposition rate of phenol by about 2.5 times and that of 4-chlorophenol by about 1.5 times. Analysis using a fluorescent probe suggested that the addition of hydrogen peroxide enhanced the production of hydroxyl radicals by about 3 times. We have also examined the effect of using microbubbles on the decomposition of phenol. The use of only microbubbles resulted in a slight increase of the decomposition rate, while the use of both microbubbles and hydrogen peroxide led to a remarkable enhancement of the decomposition rate by about 3.5 times with the conversion of phenol reaching 70% at discharge time of 3 h, which corresponded to the energy yield of 4.39 × 10–9 mol J−1.

Analysis of cavities characteristics of underwater pulsed current discharge

In order to analyze the motion characteristics of pulsed discharge cavities in water, the process of cavities motion were obtained by a combining shadow method and high-speed camera. Through detailed data analysis and calculation, the theoretical motion process of cavities in water is obtained by the Rayleigh–Plesset model. The comparison between the theoretical results and the experimental data shows that the model can reproduce the motion process of cavities well. The variation trend of pressure and temperature in cavities is given, and the characteristics of the cavitiy motion period, radius and initial energy under different voltages are analyzed. Finally, the mass diffusion and heat transfer are calculated quantitatively. The results provide a basis for analyzing the generation process of secondary shock waves and perfecting the theoretical system of pulsed discharge in water.

Propagation Characteristics of Pressure Wave in Cylindrical Reactor Induced by Underwater Pulsed Arc Discharge

Underwater pulsed arc discharge is a promising method for the inactivation of sewage microorganisms. Strong pressure wave plays an important role in the deactivation process. Researchers have conducted numerous studies on it, but there has not been a comprehensive study on its propagation in specific reactor. To achieve the industrial application of underwater pulsed arc discharge on sewage disinfection, this article has conducted a detailed study on the propagation of pressure wave in a cylindrical reactor based on experiments and numerical simulations. Several important conclusions are obtained through experiments. The pressure wave consists of three initial pulses generated by the cavity and their reflected waves. It spreads in the reactor at an average speed of 1655 m/s. The attenuation coefficient of the first initial pressure amplitude is 0.65 as the propagation distance increases. In addition, the maximum pressure amplitude decreases with increasing propagation distance, the attenuation coefficient of which is 0.93. The maximum pressure amplitude is always greater than 1.04 times the first maximum pressure amplitude. The following conclusion is obtained by simulation. When the propagation distance is 370 mm, the diameter of the reactor is reduced by 1/2, and the first maximum pressure amplitude will increase by 2.1 times. These results help further understand the distribution of pressure wave in the reactor and provide better guidance for the design of reactor.

Effect of cavity parameters on the propagation of shock wave generated by underwater pulsed discharge

In order to optimize the shock wave generated by underwater pulsed discharge, the relationship between cavity parameters and shock wave propagation is further studied by three-dimensional numerical simulation. According to the sound pressure field distribution obtained by the simulation, the reflection of the shock wave by the reactor wall can be clearly observed. The reflected pressure wave will reach its maximum value and then gradually attenuate. The study also found that when the deposition energy is constant, when the initial radius of the arc channel increases from 0.1 mm to 2.5 mm, the maximum amplitude of the shock wave will increase from 0.22 × 105 Pa to 1.70 × 105 Pa. When the initial radius of the arc channel is constant, as the deposition energy increases, the time to radiate the shock wave becomes earlier, and the maximum amplitude of the shock wave will increase. This means that a higher pressure can be generated by increasing the input of the deposition energy. When the deposition energy is constant, a higher-pressure level can be obtained by increasing the initial radius of the channel. The excitation frequency also affects the shock wave amplitude. Higher excitation frequency can obtain higher pressure amplitude. These methods will increase the efficiency of underwater pulse discharge treatment of bacteria.

Understanding of TiO2 catalysis mechanism in underwater pulsed discharge system: Charge carrier generation and interfacial charge-transfer processes

Compared with traditional photocatalysis system, TiO2 charge carrier generation and interfacial charge-transfer process may be influenced by various chemical and physical effects in underwater pulsed discharge plasma system. Here, the role of high-energy electron, ozone in TiO2 charge carrier generation and transfer process has been investigated using phenol as the probe molecule. The introduction of electron-trapping agent (KH2PO4) have an inhibiting effect on TiO2 catalytic activity, indicating high-energy electrons played a significant role in TiO2 catalytic process. EPR analysis showed that TiO2 could be activated to initiate pairs of electron-hole by high-energy electrons from plasma, and the electrons on the conduction band (CB) could be trapped on the oxygen vacancies. XPS analysis showed that the Ti3+OH species formed during discharge process due to the capture of CB electrons by Ti4+OH groups located at the TiO2 surface. The CB electrons transfer processes on TiO2 surface was strongly dependent on the redox potential of electron acceptors, which adsorbed on the TiO2 surface. The CB electrons can be transferred to dissolved O3, resulting in more OH production. Meanwhile, the CB electron also transferred to benzoquinone adsorbed on TiO2, resulting in accumulation of hydroquinone.

Analysis of shock wave induced by underwater pulsed discharge using discharge current interception

Electrohydraulic shock wave (EHSW) is seemingly one of the simplest and most common products of microsecond pulsed discharge (μsPD) in water; however, its generation process remains far less clear. To study the influence of current waveforms on the generation of an EHSW, we conducted discharge current interception experiments using a bypass branch in the circuit. The current interception time Δt is properly controlled so that the discharge current through the water gap can be terminated at a chosen time. Results show that the peak pressure Pm is first linearly increasing with Δt, and then Pm reaches a stable value. The expansion of the spark channel with increasing velocities will enhance the peak pressure. This phase can be regarded as the accelerated expansion phase (AEP) of the piston theory. The transition area of the Pm–Δt relationship of this experimental setup shows that the AEP lasts for about tm = 5 μs. After the AEP, the deposited energy will help to maintain a higher pressure in the falling edge of the pressure waves. The full width at half maximum of the waveforms finally approaches 12.5 μs in our tests. The experimental results provide evidence of the piston theory in interpreting the generation of EHSW induced by μsPD.

Comparison and analysis of shockwave characteristics between underwater pulsed discharge and metal wire explosion

In order to study the difference in energy conversion efficiency of the shockwaves induced by the expansion of the plasma channel generated by underwater metal wire explosion (MWE) and subsonic streamer breakdown discharge (SSBD), a test stand was constructed, and the experimental results were obtained based on the condition that the voltages of the two discharge modes were the same at the time of the formation of plasma channels. The results showed that the peak value of shockwaves induced by the expansion of the channel and the energy conversion efficiency of MWE were much higher than those of SSBD. The reason lies in the difference in the injected power at the formation moment of the plasma channel and the difference in the time distribution of energy deposited into channels.

Underwater pulsed spark discharge influenced by the relative position between the top of a pin electrode and an insulating tube

Needle-to-plane geometry has been widely investigated and used in underwater pulsed discharges. The position relationship between the needle tip and insulation layer significantly affects the discharge patterns. We carried out experiments on underwater pulsed discharge with the needle tip protruding from, recessing into, and flushing with the insulating tube. The results are as follows. First, underwater pulsed discharge has a strong randomness under the experimental conditions. Different discharge patterns appeared under the same experimental environment. Second, recession into the insulator surface led to a higher probability of occurrence but a lower strength of spark discharge than protrusion. Third, between the needle tip protruding from and recessing into the insulation material, the average speed of propagation of underwater pulsed spark discharge decreased by an order of magnitude. The study shows that the optimum length of needle tip protruding from the insulation layer is 1 mm to obtain a strong underwater pulsed spark discharge.

Electric characteristic and cavitation bubble dynamics using underwater pulsed discharge

Underwater pulsed discharge is widely applied in medicine, machining, and material modification. The induced cavitation bubble and subsequent cavitation collapse are considered the major motivations behind these applications. This paper presents an underwater pulsed discharge system. The experimental setup is established to induce and investigate the cavitation bubble assisted with a high-speed camera. Three aspects, including the characteristic of the discharge with different applied voltages and conductivities, the evolution of the cavitation bubble profile, and the energy efficiency of cavitation bubble inducing, are investigated, respectively. Especially, the mechanism of pre-discharge time delay in the low field intensity case is explained using the Joule heat effect. The results show the validity of the underwater pulsed discharger and experimental setup. The present underwater pulsed discharger is proved to be a simple, portable, and easy-to-implement device for the investigation of cavitation bubble dynamics.

Effects of electrode parameters on sewage disinfection by underwater pulsed arc discharge

Underwater pulsed arc discharge is a potential method applying on sewage disinfection. The disinfection effect may be influenced by electrode parameters. In this paper, effects of different electrode materials, gap distances and electrode shapes were studied with underwater pulsed arc discharge. The following results were obtained through experiments. Reactive species production of different materials are arranged from largest to smallest is W, Titanium alloy and Ti. However, the difference of pressure amplitude among three materials is less than 0.88%. Under the influence of these physical and chemical effects, with 800 s’ treatment, the residual Escherichia coli content of W, Titanium alloy, and Ti is 103.77 CFU/L, 104.29 CFU/L, and 104.87 CFU/L. W shows the best disinfection effect, which generated 2.2 orders reduction with an energy density of 0.6 J/mL. It can be found that the disinfection effect has a positive correlation with the electron work function of material. When the voltage is constant, the discharge dispersion increases as the gap distance increases. At a discharge voltage of 4.5 kV, with the gap distance changing from 0.25 to 1.50 mm, the total spectral intensity decreases, and the pressure amplitude increases first and then decreases. The study shows disinfection efficiency keeps the same trend with the change of pressure wave. And there is an optimal distance in sewage disinfection. Under the experimental conditions in this manuscript, the optimal distance is 1 mm. For the three electrode shapes, the pressure amplitude and total spectral intensity of rod shape are higher than hemisphere and cone shape. As a result, the rod electrode presented the highest disinfection effect, which residual Escherichia coli content is 103.63 CFU/L after 800 s’ treatment.

Influence of plasma channel impedance model on electrohydraulic shockwave simulation

The simulation of electrohydraulic shockwaves is the key technology for their application and device design. The analytical approach of underwater pulsed discharge is described using the dimensionless analysis method, which combines energy based hydrodynamic equations and the electrical differential equation describing the discharge characteristics of the circuit. A constant plasma channel impedance model based on the experimental results and an improved time-varying plasma channel impedance model based on the Braginskii equation are proposed and presented. The plasma channel impedance has a great influence on the deposited energy into the plasma channel and then affects the calculation and analysis of the shockwave propagation. The electrical characteristics and fluid physical properties are taken into consideration, and the results are compared. The application range of different impedance models is summarized, and the time-varying impedance model can provide a more accurate result in the simulation of shockwave propagation. The dimensionless analysis method, together with a time-varying impedance model, could be a useful tool for evaluation of the electrohydraulic shockwave in practical applications and applied physics research of underwater spark discharge.

Influence of applied voltage and electrical conductivity on underwater pin-to-pin pulsed discharge

A parametric study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration has been performed. The influence of two parameters has been reported, the water conductivity (from 50 to 500 µS cm−1) and the applied voltage (from 6 to 16 kV). Two complementary diagnostics, time resolved refractive index-based techniques and electrical measurements have been performed in order to study the discharge propagation and breakdown phenomena in water according to the two parameters. A single high voltage of duration between 100 µs and 1 ms is applied between two 100 µm diameter platinum tips separated by 2 mm and immersed in the aqueous solution. This work, which provides valuable new results, is of great interest to better understand the mechanisms of initiation and propagation of pin-to-pin discharge in water. For low conductivity (from 50 to 100 µS cm−1) results have confirmed two regimes of discharge (cathode and anode) and the increase of the applied voltage first makes the breakdown more achievable and then favors the apparition of the anode regime. For 500 µS cm−1 results have highlighted cathode regime for low applied voltage but a mixed regime (anode and cathode) for high applied voltage.