V. S. Vorob'ev

The nucleation mechanism of wire explosion

This study deals with the nucleation mechanism of electric explosion of wires allowing estimation of wire parameters at the start of the explosion for a wide range of experimental conditions. We analyse the dependence of the limit value of the energy deposited during the initial resistive phase of heating of the wire on the parameters of the wire and circuit as well as the size distribution of metal particles formed on electrical explosion of the wire. We discuss the correspondence of these results with previously published experimental data.

Metastable States of Liquid Tungsten Under Subsecond Wire Explosion

A numerical simulation of the initial stage of tungsten wire self-heating by a high-power microsecond current pulse was carried out. A wide-range semiempirical equation of state to account for the effects of melting and evaporation of tungsten at high temperatures was used. The metastable states were included in the process model, and the results of the simulation are in good agreement with experimental data.

Metastable States of Liquid Metal under Conditions of Electric Explosion

The paper deals with the simulation of the initial stage of tungsten wire explosion under the effect of a high-power nanosecond current pulse. The calculations involve the use of a semiempirical equation of state for tungsten, which allows for the effects of melting and evaporation at high temperatures. The laws of conservation that take into account the presence of a magnetic field and relative motion of the media are written at the liquid–vapor interface. It is demonstrated that a description fitting adequately the available experimental data is possible if one takes into account the possibility of realizing metastable states of liquid metal in the process of evolution of the system.

Mechanisms of Impulse Breakdown in Liquid: The Role of Joule Heating and Formation of Gas Cavities

The impulse dielectric behavior of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction, and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes that could be responsible for dielectric breakdown by submicrosecond and microsecond impulses. While for short submicrosecond impulses ionization (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, the thermal effects associated with Joule heating start to play an important role. This paper provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high-voltage (HV) impulses with duration sufficient to cause local heating, evaporation, and formation of prebreakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles, and the criterion of formation of these percolation chains is obtained. To test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n-hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show good agreement when compared with the experimental data available in the literature.

Breakdown processes in gas micro-bubbles in liquids under electric stress

The present work is concerned with a theoretical analysis of the breakdown characteristics of gas-filled micro-bubbles formed in insulating liquids stressed with electric field. It is assumed that the gas inside these bubbles is air which allows the use of experimental Paschen curve data for air in this analysis. Two main discharge mechanisms have been considered, the Townsend discharge and impulse breakdown. The combination of bubble diameter, D, gas pressure, p, and duration, τ, of the field stress determines the type of breakdown. Parameters which are required for the Townsend mechanism of breakdown and impulse breakdown to occur inside gas bubbles have been obtained and these conditions have been represented as boundary lines in the (Dp, τp) coordinate system. It is shown that there are such combinations of these parameters which satisfy neither Townsend nor impulse breakdown conditions. Experimental data on breakdown in air for these intermediate values of (Dp, tp) between the Townsend and the impulse discharges are not available in the literature and the breakdown behavior under such conditions is not well defined.

Cavity initiation through an evaporating mechanism for the pulse breakdown in liquid

This paper presents an analytical model which describes the dielectric strength of insulating liquids stressed with the impulse electric fields. The Joule heating by the space charge saturated current may cause over-heating of pure liquids or may result in generation of nucleation centers associated with impurities in the case of practical liquids. Evaporation of the liquid from these impurities has been analysed in the paper and formation criterion for percolation chains of gas bubbles in impure dielectric liquids has been established. Based on this percolation condition, the dielectric behavior of n-hexane has been studied. Breakdown volt-time characteristics of liquid n-hexane have been calculated for different temperatures, and its dielectric strength has been obtained as a function of externally applied pressure.

Phase transitions in metal under fast selfheating by high-power current pulse

A numerical simulation of the initial stage of tungsten wire selfheating by a high-power microsecond current pulse has been carried out. A wide-range semiempirical equation of state to account for the effects of melting and evaporation of tungsten at high temperatures was used. The simulation results demonstrate that the phase transitions in metal, as well as possibility of realization of metastable liquid states have a considerable influence on the dynamics of the initial stage of electrical explosion under the effect of high-power current pulse.

Phase Explosion of Conductor with Current

The paper deals with the effect of a magnetic field developed by the current flowing in a liquid-metal conductor on the parameters of its phase equilibrium with own vapors. A phase explosion occurs as a result of loss of thermodynamic stability of a two-phase system, when, as the conductor is heated with current, the vapor in equilibrium with the liquid metal phase compressed by ponderomotive forces reaches the state of limiting supersaturation (spinodal). This instability initiates the dispersion and scattering of the conductor upon its electric explosion. The parameters of electric explosion of a copper conductor are calculated. It is demonstrated that the conductor temperature at the moment of explosion depends considerably on the parameters of the electric circuit and may vary in a wide range almost from the boiling point to a close-to-critical temperature without, however, reaching the critical temperature.