Dmitry V Tereshonok

Initiation of breakdown in bubbles immersed in liquids: pre-existed charges versus bubble size

In this paper we report on results from a computational investigation of streamer evolution in bubbles immersed in liquids. We show that avalanche-to-streamer transition and streamer formation in the uniform electric field are determined by the applied external field, the parameter pR (pressure times bubble size), as well as by the location and amount of initial free charges inside the bubble. We found that streamers are not formed in bubbles with a rather small size, unless pre-existed or injected charges are large enough to initiate the breakdown. The present investigation relates the bubble size and the applied field to the minimal possible amount of charge in a bubble.

Kinetic and electrical phenomena in gas–liquid systems

Disperse systems consisting of a liquid and gas bubbles located in it are considered. Two possible versions of evolution of bubbles under the conditions studied are assessed. In simple liquids, contact between two bubbles causes them to merge, as the separating film breaks. In the case of complex organic liquids, amphiphilic film is formed on the surface of bubbles, and the lifetime of bubbles in contact increases with their size. Under an external electric field, chains of bubbles are formed, lined up along the electric field potential lines. The presence of bubbles in liquid greatly lowers the breakdown threshold, as the critical parameters of the breakdown field in liquids are two to three orders of magnitude higher than those in gases at atmospheric pressure. Various breakdown mechanisms in liquids are discussed from the viewpoint of formation of the gas phase during the passage of an electric current through a liquid medium. The character of propagating a streamer in separate bubbles is studied with their random distribution in liquid and in the case of formation of some structures of bubbles; the critical parameters of disperse systems, that can lead to their electrical breakdown, are presented. Along with the general concepts of electrical breakdown in dispersed systems, experimental studies of these processes are considered, and the nature of electrical breakdown in liquid dielectrics, including transformer oil, is discussed.

Streamer branching on clusters of solid particles in air and air bubbles in liquids

We present the results from a two-dimensional computational investigation of the intersection of a streamer with solid particles and density fluctuations and gas bubbles filled with air and immersed in liquids. We consider the evolution of a streamer propagating along the vector of the applied external electric field. The clusters of particles in air or bubbles in liquids have a symmetric form with branches elongated either in horizontal or vertical direction. The orientation of the cluster determines the branching patterns. We show that clusters having the prevailing horizontal branches facilitate streamer branching in liquids, while clusters in gases with vertical branches promote the splitting or reinitiating of a discharge filament. The phenomenon is mainly due to different polarization patterns of vertical or horizontal clusters in air and liquids and the ratio of dielectric permittivity of medium/particle or medium/bubble.

The bubble method of water purification

The processes of water purification from admixture molecules are analyzed. The purification rate is limited due to a low diffusion coefficient of the admixture molecules in water. At non-small concentrations of the admixture molecules, the water purication can proceed through association of molecules in condensed nanoparticles which fall on the bottom of the water volume. The rate of association may be increased in an external electric field, but in reality this cannot change significantly the rate of the purification process. The bubble method of water purification is considered, where air bubbles formed at the bottom of the water volume, transfer admixture molecules to the interface. This method allows one to clean small water volumes fast. This mechanism of water purification is realized experimentally and exhibits the promises of the bubble purification method.

Cavitation in liquid dielectric under nanosecond high-voltage impulse

We consider the appearance of a cavitation near a needle-shaped electrode under nanosecond high-voltage impulse. It was shown that the gas–liquid boundary moves in an opposite direction to the needle-shaped electrode under the influence of electric field on this boundary. It leads to an increase of the cavity volume. The comparison with experiment shows that the dependence of dielectric permeability on the electric field strength does not dramatically change the result of the numerical simulation whereas accounting for the liquid compressibility and the choice of the high-voltage electrode size are important. It was shown that the existence of the caviation bubbles do not always lead to electrical breakdown in dielectric liquid through the gas phase.

Diffusion and mobility of atomic particles in a liquid

The diffusion coefficient of a test atom or molecule in a liquid is determined for the mechanism where the displacement of the test molecule results from the vibrations and motion of liquid molecules surrounding the test molecule and of the test particle itself. This leads to a random change in the coordinate of the test molecule, which eventually results in the diffusion motion of the test particle in space. Two models parameters of interaction of a particle and a liquid are used to find the activation energy of the diffusion process under consideration: the gas-kinetic cross section for scattering of test molecules in the parent gas and the Wigner–Seitz radius for test molecules. In the context of this approach, we have calculated the diffusion coefficient of atoms and molecules in water, where based on experimental data, we have constructed the dependence of the activation energy for the diffusion of test molecules in water on the interaction parameter and the temperature dependence for diffusion coefficient of atoms or molecules in water within the models considered. The statistically averaged difference of the activation energies for the diffusion coefficients of different test molecules in water that we have calculated based on each of the presented models does not exceed 10% of the diffusion coefficient itself. We have considered the diffusion of clusters in water and present the dependence of the diffusion coefficient on the cluster size. The accuracy of the presented formulas for the diffusion coefficient of atomic particles in water is estimated to be 50%.

Generation of a metal porous film by arc discharge

The copper wire vaporization method is applied to obtain porous copper film on a silicon surface. We determine the distribution of the surface clusters over the sizes and the density. The average size of the clusters under optimal conditions (at a distance of 2 mm from the discharge) is about 0.5 μm, and the deposition density is 3–5 clusters per squared μm.

Hydrodynamical flows in dielectric liquid in strong inhomogeneous pulsed electric field

We consider a hydrodynamical flow of dielectric liquid near a high voltage needle-shaped electrode in a strong inhomogeneous pulsed electric field. It was shown that under a small rise time, a negative pressure area (pressure is less than critical pressure) appears near the electrode leading to the formation of a cavity in which electric breakdown can develop. A comparison of the dependence of the velocity of fluid near an electrode for two cases (taking into account the dependence of dielectric permeability of the liquid on the electric field and without taking it into account) was made. A field-dependent dielectric coefficient leads to the appearance of two local maximums of the velocities and increases the minimum pressure, thus lowering the possibility of cavitation. While under the constant value of dielectric permeability only one local maximum appears.

Computing DC discharges in a wide range of currents with comsol multiphysics: Time-dependent solvers vs. stationary solvers

Summary form only given. The most commonly used solvers in models of DC gas discharges are time-dependent solvers. Solutions are obtained by specifying an initial state and following the evolution of the discharge over time until a steady state is reached. In the last 15 years multiple modes of arc-cathode attachment have been computed in high-pressure arc discharges by means of stationary solvers, see review [1]; a procedure that has now become standard practice. Apart from computing the multiple modes of arc-cathode attachment, stationary solvers have revealed the existence of hysteresis of the modes of arc-cathode attachment in high-pressure arc discharges.Recently, multiple DC glow discharge modes have been computed by means of stationary solvers of the commercial software COMSOL MultiPhysics [1]. Some of the modes computed comprise patterns of cathodic spots which are similar to those observed in experiments [2]. Stationary solvers have also revealed hysteresis in glow discharges, even in apparently simple situations. This work is aimed at finding whether multiple modes and hysteresis can be computed by means of time-dependent solvers in both glow and arc discharges. Capabilities of time-dependent solvers are compared of COMSOL and of a noncommercial code. It has been found that in order to compute the full pattern of multiple modes a stationary solver must be employed. At least one 2D mode of glow discharge can be computed independently with a time-dependent solver.

Initiation of breakdown in strings of bubbles immersed in transformer oil: Paschen curves and proximity of bubbles

Recent experiments1 have shown that bubbling the transformer oil with air or sulfur hexafluoride leads to the decrease of breakdown voltage by 34% and 19%, respectively. To lend an insight to how streamers are initiated in an isolated bubble or strings of bubbles filled with air or SF6 and immersed in transformer oil, we computationally investigated the role of randomly distributed initial seeds of electrons inside bubbles of various size.