Underwater Discharge Research Articles

Propagation and branching process of negative streamers in water

Streamer discharges in water, which are a pre-breakdown phenomenon caused by underwater discharge, have gained attention because of the development of plasma-based technologies for water purification and medical treatment. In this study, a single-shot pulsed negative high voltage of –18 to –21 kV with a rise time of 100 ns was applied to a needle electrode in ultrapure water, and streamers were continuously imaged using an optical system combining a high-speed video camera, a streak camera, and an ultra-high-speed camera with a microscope lens. These observations were synchronized with the applied voltage and the discharge current to investigate the effect of the discharge on streamer propagation. Negative streamers propagated with a velocity of 820 ± 50 m/s in the presence of pulsed currents, and 100 ± 20 m/s when pulsed currents were absent. The streak camera detected light emission when the pulsed currents appeared, and it was also observed that 1480 m/s pressure waves were generated during streamer propagation. Furthermore, we developed a simple new method of imaging weak density changes similar to those detected using the Schlieren method or Mach–Zehnder interferometer. This method simply involves inserting a pair of polarizing plates on the optical axis, so that the pressure waves can also be imaged in two-dimensional photographs. Our results indicated that the pressure waves were generated from the propagating streamer head when the pulsed currents appeared in the waveform. Analysis of temporal resolution with nano-second order clarified that the branching phenomenon occurred at different times resulting in the branching streamer propagation with different directions.

Effect of electrical breakdown modes on shock wave intensity in water

Shock waves generated by underwater pulsed discharge have been applied to industrial and bio-medical treatments, and the key technology is to induce high intensity shock waves. Based on the underwater pulsed discharge platform, the effect of the electrical breakdown modes on the shock wave intensity in water is discussed in the positive pin-to-plane configuration. Experimental results demonstrate that the amplitude of applied voltage have a significant influence on the breakdown modes as well as the shock wave intensity. Based on the analysis of discharge images captured by a high-speed camera, it can be concluded that the breakdown modes are classified as subsonic bush-like streamers or supersonic filamentary streamers. The mechanism of subsonic streamer in lower applied voltage is more likely an electro thermal process. The micro-bubbles are firstly generated, and then discharge occurs in the bubble cluster. The bubble volume grows until a breakdown of the underwater gap, and the shock wave is the generated. The time to breakdown of subsonic streamers is commonly within hundreds of microseconds at the experimental condition. In higher applied voltage, the energy loss in the pre-breakdown process is reduced significantly as well. In our experiment, a threshold value of 22.5 kV is necessary for the subsonic streamer mode transition into supersonic streamer mode. The energy conversion efficiency, the time to breakdown and the influence of the applied voltage are discussed at different breakdown modes. The shock wave intensity differs 2–4 times in different breakdown modes even in the same applied voltage, thus increasing the applied voltage may be a remarkable way to enhance the shock wave intensity.

Acoustic impulses generated by air-bubble stimulated underwater spark discharges

Considerable research efforts have been expended in investigating and optimising the emission of acoustic pressure impulses by high power spark discharges in water. Several methods are typically used to generate transient underwater spark discharges, including breakdown of water by stressing electrode(s) with HV impulses, initiation of spark discharges with thin metallic wires placed between opposite electrodes and by injecting gas bubbles into the inter-electrode gap. This paper is aimed at the investigation of the acoustic, hydrodynamic and electrical characteristics of underwater spark discharges initiated by injected air-bubbles. At a constant applied voltage, the injection of air bubbles allows the inter-electrode gap to be increased thus producing stronger acoustic impulses. This paper reports on the impact of injected air bubbles on the characteristics of transient underwater plasma discharges and on the functional relationships between hydrodynamic and electrical parameters of such discharges, including breakdown voltage, plasma resistance, energy delivered into the plasma cavity and the period of gas/plasma cavity oscillation. An analytical model which relates the acoustic efficiency, plasma resistance and energy available in discharge has been developed which can be used for the optimisation and tailoring of underwater plasma-acoustic sources.

Rotary trigger double spark gap switch for repetitive pulse generator and production of silver nanocolloid

High-voltage, repetitive pulse generation technology plays a key role in underwater discharge applications such as water treatment and metal nanocolloid production. For these applications, a high-voltage, high-speed switching technology was crucial to swiftly transfer the energy stored in the capacitor to the electrodes in water. A rotary-triggered double spark gap switch was developed for this purpose in this study. Two spark gaps dedicated for charge and discharge were arranged in this switch, and they eliminated the irregular and unstable pulse generation due to the simultaneous charging and discharging in the pulse-generating circuits. Using this technique, pulses with 12 kV and 100 ns rise time were generated at a repetition rate of 150 pulses per second and applied to underwater discharge to produce silver nanocolloids.

Transient interaction between a particle and an attached bubble with an application to cavitation in silt-laden flow

This study aims to elucidate the complex interaction between a suspended particle and an attached bubble, which is associated with cavitation in silt-laden flow. Systematic experiments are performed with high-speed photography, in which bubbles are generated by underwater electric discharge means. The bubble-particle interactions are found to be strongly dependent on two dimensionless parameters, i.e., the particle-bubble size ratio λL and the particle-liquid density ratio λρ. When λρ equals 2.61, the bubble split phenomenon is universally observed and the particle shooting effect (the particle acceleration during bubble expansion and after bubble-particle detachment) becomes more obvious as λL decreases. If λL < ∼0.34, the particle velocity keeps positive (away from the bubble), otherwise the particle velocity drops below zero (toward the bubble) during the bubble collapse phase. As λρ increases, the particle achieves a lower velocity but a higher impulse, and the bubble necking phenomenon is more pronounced. Our boundary integral simulations reproduce the experiments extremely well, including the particle dynamics, the bubble wrapping the particle, the bubble necking and detachment, and the mushroom-shaped bubble. After the bubble-particle detachment, the liquid around the detachment location is drawn inward and collides on the axis of symmetry, leading to the formation of a localized high pressure region between the bubble and the particle, which accelerates the particle for the second time even in the bubble collapse phase.

Time-resolved diagnostics of a pin-to-pin pulsed discharge in water: pre-breakdown and breakdown analysis

This paper presents an experimental study of an underwater pulsed plasma discharge in a pin-to-pin electrode configuration. Time resolved refractive index-based techniques and electrical measurements have been performed in order to study the pre-breakdown and breakdown phenomena in water (σ = 100 µS cm−1). A single high voltage pulse with amplitude of a dozen of kV, rise time of 20 ns and duration of [0.1–1] ms is applied between two 100 µm diameter platinum tips separated by 2 mm. This novel experimental work reports that different cases of electrical discharge in water occurs for a unique set of experimental conditions such as (i) bush-like channels from the cathode that do not span the electrode gap, (ii) bush-like channels from the cathode leading to breakdown and (iii) filamentary structures from the anode leading to a stronger breakdown. Two breakdown mechanisms, anode and cathode regimes, have been clearly identified and related to the two principal schools of thoughts to explain discharge propagation in liquid.

Studies on mass production and highly solar light photocatalytic properties of gray hydrogenated-TiO2 sphere photocatalysts

In this paper, it is first reported that gray hydrogenated TiO2 sphere photocatalysts (H-TiO2) with high reactivity to solar light are mass produced within a few minutes using an underwater discharge plasma modified sol-gel method at room temperature and atmospheric pressure. This plasma modified system is an easy one-step in-situ synthetic process and the crystallinity, hydrogenation, and spherical structure of H-TiO2 are achieved by the synergy effect between the continuous reaction of highly energetic atomic and molecular species generated from the underwater plasma and surface tension of water. The resultant H-TiO2 demonstrated high anatase/rutile bicrystallinity and extended optical absorption spectrum from the ultraviolet (UV) to visible range. Furthermore, various defects including oxygen vacancies and hydroxyl species on the TiO2 surface permitted the enhancement of the photocatalytic performance. It was demonstrated that H-TiO2 photocatalysts showed significant degradation efficiencies for reactive black 5 (RB 5), rhodamine B (Rho B), and phenol (Ph) under solar light irradiation, up to approximately 5 times higher than that of commercial anatase TiO2 (C-TiO2), which resulted in good water purification. Notably, it was also possible to cultivate HepG2 cells using such well-purified water (to degrees up to 76%), with minimal cytotoxicity. Considering all these results, we believe that this novel plasma technology is promising for important environmental applications.

Graphene nanosheets obtained through graphite powder exfoliation in pulsed underwater electrical discharge

Graphene nanosheets were produced by graphite exfoliation through underwater electrical discharge, also known as plasma underwater. Graphite particles were dispersed in different solvents with different ratios between polar and dispersive surface tension components (pure water and an aqueous solution of isopropyl alcohol (IPA) at different concentrations (1:1 and 7:3 IPA:Water)) under constant stirring and low voltage pulses (about 1.5 kV). Even under relative low voltage, the production of graphene was observed in all solvents with the most of the graphene sheets containing less than 10 monolayers. Results show that further affinity between the surface tensions of the graphite and the solvents influenced in the characteristic of the graphene obtained. The 7:3 IPA/water solution has a similar ratio between polar and dispersive surface tension components to the graphite and the mixture. Exfoliation product obtained with this mixture presented an intense peak at ∼284.3 eV, which is corresponding to sp2 binding, characteristic of the graphene. However, in this solution, it was observed further break of C-C bonds and a significant reduction of its surface area. Smaller graphene flakes sizes present more C-O contents due to the carboxyl groups be more likely to form at graphene edges than at defect sites within the basal plane.

Special issue: Plasma and Liquids

Special issue: Plasma and Liquids

A Compact All-Solid-State Repetitive Pulsed Power Modulator Based on Marx Generator and Pulse Transformer

In this paper, a compact all-solid-state pulsed power modulator was designed based on Marx generator and pulse transformer. It is consisted of a six-stage Marx generator using power MOSFETs as switches to generate primary voltage pulses and a pulse transformer through a closed magnetic core to boost and convert the polarity of the output high-voltage (HV) pulse. A MOSFET driver circuit based on optical–electrical isolation was designed for controlling the Marx generator. It shows that the output pulse shapes, especially the rising and falling phases, are determined by the design of the pulse transformer such as its leakage inductance, distributed capacitance, magnetization inductance, and step-up ratio. The designed modulator is able to generate HV pulses with amplitude ranging from −35 to 35 kV, repetition frequency up to 10 kHz, minimum pulsewidth 250 ns. Various types of plasma discharges, including air microdischarge for local treatments, plate-to-plate air dielectric-barrier discharge, underwater discharge, are able to be driven with good reproducibility by this compact pulse modulator.

Sequential observation of rebound shock wave generated by collapse of vapor bubble in BOS system

The paper reports on a pressure measurement of underwater shock waves generated by collapse of a vapor bubble using the background-oriented schlieren (BOS) technique. In the BOS system, the vapor bubble is induced by an underwater electric discharge. To estimate background-element displacement induced by the underwater shock wave, a spatiotemporal derivative (STD) algorithm is applied in image processing. Furthermore, the accuracy of the STD algorithm is evaluated using the images obtained with our BOS system relative to the cross-correlation method. Subsequently, the pressure distributions are obtained by solving the Poisson equation and applying a filtered-back projection algorithm. An experiment to visualize the behaviors of the underwater shock waves is also carried out using the shadowgraph method. As a result, the pressure waveforms estimated by the BOS technique keep consistent with that in experimental profiles. The pressure attenuations behind the underwater shock wave are also in good agreements with the experimental measurements. It is expected that the BOS technique will probably overcome the problems associated with the conventional method of pressure measurement related to the collapse of microbubbles, and becomes a promising means of quantitatively measuring high-speed phenomena.

Effect of pulse width on streamer propagation of underwater corona discharge

The propagation of pulsed negative corona discharges in water as a function of electrical pulse width ranging from 100 ns to 100 μs has been studied experimentally. Using point‐to‐plane electrodes, it is demonstrated that the propagation of streamers exhibits different behavior under excitations with different pulse width. For pulses with width shorter than 1 µs, the average velocity stays relatively constant at about 1000 m s−1. For pulses with width longer than 1 µs, the average velocity decreases dramatically with increasing pulse width. Numerical fitting shows that the relationship between the velocity and pulse width follows u ∝ τ−0.72. Two different mechanisms are proposed to explain the different discharge characteristics observed at different time scales. At the initial stage before the generation of any significant heat, we show that the electrostatic force may play a major role in supplying the driving force. As the pulse width increases, the gas temperature rises, and the pressure caused by evaporation may become the dominant force to push the head of the filament to propagate. The transition time between two modes was estimated to be in the order of microseconds. Our quantitative predictions are in reasonable agreement with experimental results.

Experimental and numerical study of the effects of a wall on the coalescence and collapse of bubble pairs

Two-bubble interaction is the most fundamental problem in multi-bubbles dynamics, which is crucial in many practical applications involving air-gun arrays and underwater explosions. In this paper, we experimentally and numerically investigate coalescence, collapse, and rebound of non-buoyant bubble pairs below a rigid wall. Two oscillating vapor bubbles with similar size are generated simultaneously near a rigid wall in axisymmetric configuration using the underwater electric discharge method, and the physical process is captured by a high-speed camera. Numerical simulations are conducted based on potential flow theory coupled with the boundary integral method. Our numerical results show excellent agreement with the experimental data until the splashing of the jet impact sets in. With different ranges of γbw (the dimensionless distance between the rigid wall and the nearest bubble center), the interaction between the coalesced bubble and the rigid wall is divided into three types, i.e., “weak,” “intermediate,” and “strong.” As γbw decreases, the contact point of the two axial jets migrates toward the wall. In “strong interaction” cases, only an upward jet towards the upper rigid wall forms and a secondary jet with a larger width appears at the base of the first jet. The collapsing coalesced bubble in a toroidal form splits into many smaller bubbles due to the instabilities and presents as bubble clouds during the rebounding phase, which may lead to a weakened pressure wave because the focusing energy associated with the collapsing bubble is disintegrated.

Enhanced shock wave generation via pre-breakdown acceleration using water electrolysis in negative streamer pulsed spark discharges

This paper presents a method for enhancement of shock waves generated from underwater pulsed spark discharges with negative (anode-directed) subsonic streamers, for which the pre-breakdown process is accelerated by preconditioning a gap with water electrolysis. Hydrogen microbubbles are produced at the cathode by the electrolysis and move towards the anode during the preconditioning phase. The numbers and spatial distributions of the microbubbles vary with the amplitude and duration of each preconditioning pulse. Under our experimental conditions, the optimum pulse duration is determined to be ∼250 ms at a pulse voltage of 400 V, where the buoyancy force overwhelms the electric force and causes the microbubbles to be swept out from the water gap. When a high-voltage pulse is applied to the gap just after the preconditioning pulse, the pre-breakdown process is significantly accelerated in the presence of the microbubbles. At the optimum preconditioning pulse duration, the average breakdown delay is reduced by 87% and, more importantly, the energy consumed during the pre-breakdown period decreases by 83%. This reduced energy consumption during the pre-breakdown period, when combined with the morphological advantages of negative streamers, such as thicker and longer stalks, leads to a significant improvement in the measured peak pressure (∼40%) generated by the underwater pulsed spark discharge. This acceleration of pre-breakdown using electrolysis overcomes the biggest drawback of negative subsonic discharges, which is slow vapor bubble formation due to screening effects, and thus enhances the efficiency of the shock wave generation process using pulsed spark discharges in water.

Ice breaking by a collapsing bubble

This work focuses on using the power of a collapsing bubble in ice breaking. We experimentally validated the possibility and investigated the mechanism of ice breaking with a single collapsing bubble, where the bubble was generated by underwater electric discharge and collapsed at various distances under ice plates with different thicknesses. Characteristics of the ice fracturing, bubble jets and shock waves emitted during the collapse of the bubble were captured. The pattern of the ice fracturing is related to the ice thickness and the bubble–ice distance. Fractures develop from the top of the ice plate, i.e. the ice–air interface, and this is attributed to the tension caused by the reflection of the shock waves at the interface. Such fracturing is lessened when the thickness of the ice plate or the bubble–ice distance increases. Fractures may also form from the bottom of the ice plate upon the shock wave incidence when the bubble–ice distance is sufficiently small. The ice plate motion and its effect on the bubble behaviour were analysed. The ice plate motion results in higher jet speed and greater elongation of the bubble shape along the vertical direction. It also causes the bubble initiated close to the ice plate to split and emit multiple shock waves at the end of the collapse. The findings suggest that collapsing bubbles can be used as a brand new way of ice breaking.

Inactivation of microorganisms by radical droplets from combination of water discharge and electro-spraying

We report the generation of radical droplets activated by an underwater electrical discharge and the sterilization of bacteria cells by spraying the radical droplets to open air. The plasma formed by electrical discharge in a narrow dielectric capillary tube produces atomic oxygen, atomic hydrogen, and hydroxyl radicals from the dissociation of water vapor, which is achieved through Joule heating in the narrow capillary tube. The capillary discharge electrode is placed in a water flow line, and the plasma-activated water is sprayed as fine droplets through a nozzle at 0.08 L/min. In a bactericidal test, the droplets show a sterilization effect of more than 99.99% after an exposure time of 3 h. The proposed radical droplet generator may also be useful for the elimination of airborne microorganisms.

Numerical Simulation of Electrical Discharge Characteristics Induced by Underwater Wire Explosion

The underwater shock-wave phenomenon has been applied in various fields such as manufacturing and food processing and was investigated using many experimental and numerical analyses in the past. An underwater shock-wave is produced by various methods, e.g., underwater wire explosion and pulse-gap electrical discharge. Therefore, clarifying the shock characteristics depending on the stored electrical energy, wire dimension and material is extremely important. However, predicting the pressure and its distribution induced by underwater electrical wire explosion is hard because the phenomena associated with an elementary process are significantly complicated. In this study, to predict the discharge characteristics induced by underwater electrical wire explosion, numerical simulation based on the “simplified model of underwater electrical discharge” was performed. The numerical results show good agreement with the experimental ones.

Generation of underwater discharges inside gas bubbles using a 30-needles-to-plate electrode

Underwater discharges inside helium (He) gas bubbles were generated using a 30-needles-to-plate electrode system with the gas flowing through the needles. The set-up allows continuous treatment of flowing water. The plasma electron temperature and density determined from spectral line intensities and profiles of the Hα and Hβ hydrogen lines were found to be 1.1 ± 0.6 eV and ∼5 × 1016 cm−3, respectively. These parameters are comparable with those of plasmas generated by other underwater gas bubble discharges for the production of OH radicals. The two main long-lived species, hydrogen peroxide (H2O2) and nitrate ions (NO3−), produced in plasma treated water were measured. It was found that without a continuous water flow, the energy yield of H2O2 was comparable with other underwater discharges, while with a continuous flow, the production of H2O2 appeared to be somewhat reduced by nitrites from dissolved air.

Inactivation of Staphylococcus aureus in water by pulsed spark discharge

A pulsed spark plasma discharge system was developed and tested as an energy efficient water sterilization method. A 5 log10 reduction on Staphylococcus aureus concentration of 108 CFU/ml was obtained. Complete inactivation was achieved for concentration of 106 CFU/ml. Of the various factors generated by an underwater spark discharge, ultraviolet radiation plays a major role. The inactivation was completely suppressed by the addition of 30 mg/L of a soluble sunscreen, Benzophenone-9. Results obtained using the pulsed spark plasma discharge showed that this system has several advantages, such as high energy efficiency, absence of harmful by-products and portability, over the conventional sterilization methods.

Experimental and numerical study on bubble-sphere interaction near a rigid wall

This study is concerned with the interaction between a violently oscillating bubble and a movable sphere with comparable size near a rigid wall, which is an essential physical phenomenon in many applications such as cavitation, underwater explosion, ultrasonic cleaning, and biomedical treatment. Experiments are performed in a cubic water tank, and the underwater electric discharge technique (580 V DC) is employed to generate a bubble that is initiated between a rigid wall and a sphere in an axisymmetric configuration. The bubble-sphere interactions are captured using a high-speed camera operating at 52 000 frames/s. A classification of the bubble-sphere interaction is proposed, i.e., “weak,” “intermediate,” and “strong” interactions, identified with three distinct bubble shapes at the maximum volume moment. In the numerical simulations, the boundary integral method and the auxiliary function method are combined to establish a full coupling model that decouples the mutual dependence between the force and the sphere motion. The main features of bubble dynamics in different experiments are well reproduced by our numerical model. Meanwhile, the pressure and velocity fields are also provided for clarifying the associated mechanisms. The effects of two dimensionless standoff parameters, namely, γs (defined as ds/Rm, where ds is the minimum distance between the initial bubble center and the sphere surface and Rm is the maximum bubble radius) and γw (defined as dw/Rm, where dw is the distance between the initial bubble center and the rigid wall), are also discussed.