Search

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. [ABSTRACT FROM AUTHOR]

The x-ray spectrum of a flash radiography source based on wire-shorted rod-pinch diode (RPD) is measured by an absorption spectrometer and calculated by the Geant4 Monte-Carlo code. The RPD is driven by a two-stage linear transformer driver (charging voltage ±45 kV per stage), which delivers ∼300 kA current within ∼200 ns on a short-circuit inductive load (∼30 nH). The spectrometer consists of LiF thermoluminescence dosimeters and metal filters of different materials and thicknesses, and the spectrum is reconstructed based on the Geant4 simulated dosimeter responses to various photon energies through different filters. The measured spectrum reaches an endpoint photon energy of ∼130 keV. The radiation characteristics of the RPD in the experiment are also modeled in Geant4 using measured voltage and current waveforms, and the results show an overestimation in photon number and endpoint energy (∼147 keV) compared to the measured spectrum, which indicates that during the bremsstrahlung process, the effective electron current is ∼27% of the total current, and the effective voltage is ∼12% lower than the directly reconstructed resistive voltage. A rough agreement is found between the spectra results and the non-relativistic Bethe–Heitler model with Sommerfeld correction; therefore, the analytic result can serve as an initial guess for spectrum reconstruction methods such as the perturbation method. [ABSTRACT FROM AUTHOR]

The gas mixture CO2–O2 has been considered as an insulation and arc-quenching medium in gas-insulated switchgears. In this paper, the dielectric breakdown properties of CO2–O2 mixtures at different O2 concentrations and gas pressures were studied theoretically by considering ion kinetics in a spatial–temporal growth avalanche model. A kinetic scheme that includes all the main reactions likely to occur in CO2–O2 mixtures is presented. An improved method to calculate the dielectric strength of the gas mixture is developed, based on an avalanche model that considers both spatial growth and temporal processes. Next, the reaction rates of ionization, attachment, detachment and ion conversion, the effective ionization Townsend coefficient αeff/N, and reduced critical electric field strength (E/N)cr in CO2–O2 mixtures at different mixing ratios and gas pressures are analyzed in detail. Finally, a pulsed Townsend experiment is performed to verify the validity and accuracy of the calculation method. Based on this, one detachment reaction rate is modified to yield more accurate results. Better consistency between the results and the experimental values supports the validity of the kinetic system, reaction rates, and the improved calculation method. [ABSTRACT FROM AUTHOR]

Underwater electrical wire explosion is usually used as a source of underwater shock waves, and the generated shock wave intensity can be regulated by adopting a wire array. In this paper, the dynamics of the electrical explosion of two wires with different diameters connected in parallel were studied. The current flowing through each wire was measured to study the current distribution, and the self-emission image and laser backlight shadow image was obtained to observe the evolution of the wires. A numerical model was built to make explanations and predictions. The results showed that due to the presence of wire inductance and contact resistance between the wire and electrode, the wires are with different current densities and undergo asynchronous heating from the beginning of the discharge. Furthermore, the variation of current distribution among the wires was divided into six stages according to the moment the wire melted and exploded, and analyzed stage by stage. The unbalanced energy deposition results in obvious differences in thermal dynamic parameters, which explains the gradual "extinguishment" of the thinner wire in the self-emission image. [ABSTRACT FROM AUTHOR]

Recently, C4F7N mixtures have attracted attention as a promising SF6 alternative due to their low global warming potential and excellent electrical performance. In this study, we introduce a newly built experimental setup for pulsed Townsend measurements as well as the physical model for the acquisition of electron swarm parameters. The effective ionization rate coefficients, electron drift velocity, and density-normalized longitudinal diffusion coefficients in C4F7N and its mixtures with CO2 and N2 were obtained. The reduced critical electric field for pure C4F7N at 100 Pa is determined to be 1002 Td, while it decreases with increasing pressure because of the effect of ion kinetics during the discharge process. C4F7N shows good synergism with CO2 and N2. To have a dielectric strength equivalent to that of SF6, the mixing ratio of C4F7N in its mixtures with CO2 and N2 should be 18% and 12%, respectively. The results provide fundamental data for modeling discharges in C4F7N and its mixtures. [ABSTRACT FROM AUTHOR]

Recently, fluoronitrile C4F7N is regarded as a promising alternative gas for SF6, and its global warming potential (GWP) is extremely high. C4F7N has good insulation properties and a relatively low GWP, so it can be used as an insulation medium for gas-insulated switchgears. Some studies have been conducted on the decomposition pathway of C4F7N gas that may produce possible neutral molecules and free radicals, which helps understand the thermal decomposition of C4F7N. However, anions and cations will be produced under discharge conditions. In these cases, the decomposition pathway of C4F7N will be different. In this paper, the ionization decomposition pathway and the attachment decomposition pathway of C4F7N were first calculated, while the recombination processes of the cations and anions were also considered. The results can reveal the decomposition mechanism of C4F7N and the recombination mechanism of the stable products under discharge conditions. The energy changes of all the reactions were calculated, which reveals the difficulty of chemical reactions. To better investigate the formation mechanism of stable products, the partial discharge experiment was also carried out, and the decomposition products of C4F7N mixed gas were detected. Finally, the ionization energy and electron affinity energy of the stable products were also calculated, which can determine the difference in the total electronic energy between the neutral molecule and the ion. The results provide an important reference for studying the decomposition characteristics of C4F7N gas under discharge conditions from the theoretical aspect. [ABSTRACT FROM AUTHOR]

Iso-C4 perfluoronitrile (C4F7N) is one of the most promising alternatives to SF6 for use in power equipment, such as high-voltage circuit breakers, due to its excellent electrical properties and environmentally friendly characteristics. The use of SF6 is being reduced because of its high global warming potential. To describe the physical and chemical processes occurring in the arc plasma in circuit breakers, both local thermodynamic equilibrium (LTE) and nonlocal chemical equilibrium (LCE) conditions have to be considered. The plasma composition of the arc core region can be calculated under the assumption of LTE by the method of minimization of the Gibbs free energy. The plasma composition of the arc boundary region or during the arc ignition period is not in LTE or LCE, so the use of a chemical kinetic model that considers the effects of the energy barrier in chemical reactions is required. Calculations for both conditions are presented for C4F7N. To develop the chemical kinetic model, the complete decomposition pathway and transition states were first reexamined and further developed. Based on the decomposition pathway, the rate constants of reactions were obtained according to the variational transition state theory method. The results obtained for the two cases provide a reference for the systematic understanding of the decomposition characteristics of C4F7N gas and for related engineering applications. [ABSTRACT FROM AUTHOR]

We investigate the evolution of the species from both the target and the air, and the plasma parameter distribution of the nanosecond laser-produced plasmas in atmospheric air. The technique used is spatially resolved optical emission spectroscopy. It is argued that the N II from the air, which is distributed over a wider region than the target species in the early stages of the discharge, is primarily formed by the shock wave. The ionized species have a larger expansion velocity than the excited atoms in the first ∼100 ns, providing direct evidence for space-charge effects. The electron density decreases with the distance from the target surface in the early stages of the discharge, and both the electron density and the excited temperature variation in the axial direction are found to become insignificant at later stages. [ABSTRACT FROM AUTHOR]

The dielectric breakdown properties of SF6-CF4 mixtures were investigated at different ratios of SF6, 0.01-1.6 MPa, and gas temperatures up to 3000 K. Initially, the equilibrium compositions of SF6-CF4 mixtures were calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. Then the electron energy distribution function was obtained based on those data by solving the Boltzmann equation under the zero-dimensional two-term spherical harmonic approximation. Finally, the critical reduced electric field strength (E/N)cr of SF6-CF4 mixtures, which is defined as the value for which total ionization reaction is equal to total attachment reaction, were determined and analyzed. The results confirm the superior breakdown properties of pure SF6 at relatively low gas temperatures. However, for higher gas temperatures (i.e., T > 2200 K at 0.4 MPa), the (E/N)cr in SF6-CF4 mixtures are obviously higher than that in pure SF6 and the values of (E/N)cr increase with the reduction of the ratio of SF6. [ABSTRACT FROM AUTHOR]

Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images. [ABSTRACT FROM AUTHOR]

The dielectric breakdown properties of SF6-N2 mixtures were investigated at different concentrations of N2, 0.01-1.6 MPa, and 300-3000 K. The equilibrium compositions of different SF6-N2 mixtures at several gas pressures and temperatures up to 3000 K were first calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. The electron energy distribution function was then obtained using the composition data by Boltzmann equation analysis. It was found that adding N2 to SF6 gas can markedly reduce the kinetic energy of electrons at relatively high gas temperatures, which enhances the dielectric field strength. Finally, the critical reduced electric field (E/N)cr, defined as the value for which ionization is equal to attachment, of hot SF6-N2 mixtures was determined. The results indicate that in the gas temperature range around 2000-3000 K, increasing the concentration of N2 effectively enhances the (E/N)cr of SF6-N2 mixtures, and the (E/N)cr of SF6-N2 mixtures at a wide range of concentrations of N2 (5%, 50%, and 95%) are all higher than that of pure SF6 gas. Further, this trend exists at all the gas pressures considered (0.01-1.6 MPa). [ABSTRACT FROM AUTHOR]

The development of electrothermal chemical launch technology and the effective ignition of energetic thermoplastic elastomer (ETPE) propellants all call for a thorough understanding of the plasma propellant interaction mechanism. Therefore in this work, a numerical model is developed to study this process. In this model, special attention is paid to deal with the regression of propellant surface, which is caused by plasma ablation, and is assumed to have considerable influence on the interaction process. By this model, interaction features of four propellants, including two ETPE propellants are studied with the aid of experiments, which are carried out to obtain plasma parameters utilized in the model. Surface temperature, ablation rate, and surface regression rate are obtained for different propellants under plasma impingement. Finally, the influence of propellant composition and charging voltage are analyzed. [ABSTRACT FROM AUTHOR]

A two-dimensional magneto-hydrodynamic (MHD) model of an ablation-dominated capillary discharge is developed. Special attention is paid to the interaction between the plasma and the capillary in two-dimensional geometry. The mass exchange process is taken into account by adopting a kinetic ablation model and a revised deposition model. The momentum and energy exchange processes associated with the ablated and deposited mass are also considered. By solving this model, key plasma parameters are calculated and presented in the paper. It is shown that distinct two-dimensional features exist in the distribution of these plasma parameters. Further, the calculated temperature results are consistent with previously presented measurements [ABSTRACT FROM AUTHOR]