Electrical Breakdown In Liquids Research Articles

Electric Breakdown in Liquids: Faster Ignition Using Less Energy

The formation of vapor layers around an electrode immersed in a conducting liquid prior to generation of a plasma discharge is studied using numerical simulations. This study quantifies and explains the effects of the electrode geometry and applied voltage pulses, as well as the electrical and thermal properties of the liquids on the temporal dynamics of the pre-breakdown conditions in the vapor layer. This model agrees well with experimental data, in particular, the time needed to reach the electrical breakdown threshold. Because the time needed for discharge ignition can be accurately predicted from the model, the parameters such as the pulse shape, voltage, and electrode configuration can be optimized under different liquid conditions, which facilitates a faster and more energy-efficient plasma generation.

Lattice Boltzmann model for simulation of the electric breakdown in liquids

Abstract We investigate pre-breakdown hydrodynamic flows and initial stages of the electric breakdown in dielectric liquids. Three models are considered. The first one represents the purely thermal mechanism. Here, the liquid is simulated by a single-phase lattice Boltzmann equation (LBE) method. The temperature and the electric charge density are described by additional LBE components with zero mass. The permittivity is assumed to be constant. The conductivity increases with the increase of temperature. Electric force acting on a charged liquid is coupled with the hydrodynamics by the exact difference method [Kupershtokh, 2004; Kupershtokh & Medvedev, J. Electrostatics, 2006]. The last process in the model is the Joule heating. In the second model, a possible phase transition is included. To simulate a fluid with an arbitrary two-phase equation of state (such as van der Waals or Carnahan-Starling EOS), the method proposed by Kupershtokh is used [Kupershtokh, 2005; Kupershtokh et al. 2007]. The conductivity increases with the decrease of the fluid density. When the voltage is applied, the charge injection from the surface of electrode begins. The electric force acting on the charged fluid produces negative pressure near electrode leading to a phase transition (evaporation). Conductivity increases leading to enhanced evaporation and growth of a conducting bubble. Thus, the bubble mechanism of breakdown is realized. The last model includes the density-dependent permittivity. For nonpolar liquids, the dependence is given by the Clausius — Mosotti law. In this case, several additional processes are possible. First, dielectrics is pulled into regions with higher electric field which produces rarefaction waves. Second, an anisotropic instability [Kupershtokh & Medvedev, Phys. Rev. E, 2006] can develop producing low-density channels along the electric field. Since these channels can easily become conducting, another mechanism of the breakdown is realized.

Analysis of polarity effects in the electrical breakdown of liquids

Electrical breakdown simulations are carried out for liquids in response to a sub-microsecond (∼100–200 ns) voltage pulse. This model builds on our previous analysis and focuses particularly on the polarity effect seen experimentally in point–plane geometries. The flux-corrected transport approach is used for the numerical implementation. Our model adequately explains experimental observations of pre-breakdown current fluctuations, streamer propagation and branching as well as disparities in hold-off voltage and breakdown initiation times between the anode and cathode polarities. It is demonstrated that polarity effects basically arise from the large mobility difference between electrons and ions. The higher electron mobility leads to greater charge smearing and diffusion that impacts the local electric field distributions. Non-linear couplings between the number density, electric field and charge generation rates then collectively affect the formation of ionized channels and their temporal dynamics.

A new model for the primary process of electrical breakdown in liquids

A new model for the generation of electrical streamers in insulating liquids is proposed, It is based on the mechanical stress generated by the electric field and its influence on the cohesive properties of the liquid. In fields of 10/sup 8/ to 10/sup 9/ V m/sup -1/ the stress is sufficient to enhance significantly the thermal generation of sub-microscopic rupture surfaces (holes) in the liquid which has solid-like properties in the short time of streamer development. Using the well-known Griffith concept of mechanically-generated crack propagation in solids, it is then argued that, when the population of sub-microscopic holes becomes sufficiently large, the same stress encourages macro-crack development which has all the hallmarks of streamer growth, In this model electrical discharges do not have a traditional primary role although they will have an important secondary role once macro-cracks have developed.

Laser triggering of electric breakdown in liquids

Electric breakdown in semihomogeneous geometries has been studied in n-hexane and transformer oil using a laser triggering method for fields far below the spontaneous breakdown field. The method uses a laser-produced plasma to initiate the breakdown. Strong self focusing in the liquids was limited by increasing the numerical aperture of the laser beam to >0.20 prior to focusing. Electrical breakdown could be triggered at fields as low as 50 kV/cm in transformer oil and 25 kV/cm for n-hexane. The time lags are between 30 and 200 /spl mu/s, depending on voltage, liquid, and gap size. Current and light emission measurements of the prebreakdown phenomena are presented and a model for the triggering process is proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mersey and North Wales Electronics Section: Chairman's address. Research on electric breakdown in liquids

Mersey and North Wales Electronics Section: Chairman's address. Research on electric breakdown in liquids