Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

Abstract

A simulation model for second mode positive streamers in dielectric liquids is presented. Initiation and propagation is modeled by an electron-avalanche mechanism and the Townsend-Meek criterion. The electric breakdown is simulated in a point-plane gap, using cyclohexane as a model liquid. Electrons move in a Laplacian electric field arising from the electrodes and streamer structure, and turn into electron avalanches in high-field regions. The Townsend-Meek criterion determines when an avalanche is regarded as a part of the streamer structure. The results show that an avalanche-driven breakdown is possible, however, the inception voltage is relatively high. Parameter variations are included to investigate how the parameter values affect the model. visible light [3], re-illuminations, from one or more of its branches. Above the breakdown voltage, streamers may change between the 2nd, the 3rd, and the 4th mode during propagation. There are usually more reilluminations in the 3rd mode than the 2nd mode. The inception of the 4th mode is associated with a drastic increase in speed and fewer, more luminous, branches [2].There are numerous mechanisms that can be involved in the streamer phenomena, the challenge is identifying their importance during initiation and propagation. Applying a potential to a needle can cause charge injection, giving a space-charge limited current [16] causing Joule heating [16], which in turn can cause bubble nucleation [17]. A breakdown in the gas bubble can then propagate the needle potential, and the process may repeat. This is one way to explain 1st mode propagation. Electric fields can also cause electrohydrodynamic flow, which could cause streamer formation through cavitation [18]. Electrostatic cracking has also been proposed as a cavitation mechanism [19]. A main topic of discussion is whether a lowering of the liquid density is needed before charge generation can occur. Electron avalanches are important in gas discharge, but their importance in liquid breakdown is still disputed. In water, strong scattering could prevent electrons from forming avalanches in the liquid phase [20]. Therefore, discharges in micro-bubbles can be important for charge generation [10,14,20]. The same mechanism was also proposed for non-polar liquids [19], however, the relative permittivity is about 80 in water and about 2 in a typical oil, and this difference can prove important since the field enhancement within a bubble in oil is much lower than in water. Contrary to water, there are indications of electron avalanches in non-polar liquids [16,21,22], furthermore, while the initiation and the propagation length of 2nd mode streamers are dependent on the pressure, their propagation velocity is not pressure dependent [16,23]. This implies that the mechanism responsible for propagation occurs in the liquid phase and that the gaseous channel follows as a consequence. In very high electric fields, field-ionization can occur [24,25], and this mechanism has been proposed for the fast 3rd and 4th propagation mode...

Photoionization model for streamer propagation mode change in simulation model for streamers in dielectric liquids

“…The present work does not aim to improve on these limitations for slow streamers, but rather demonstrate how changes between slow and fast propagation can occur in different parts of the gap. The propagation speed for slow-mode streamers is about ten times of that predicted by figure 7, but the difference can be removed by assuming a higher electron mobility or a higher seed density [18].…”

“…E γ > E fdip . Prolate spheroid coordinates are used to calculate the Laplacian electric field magnitude and direction [18], in order to calculate σ by (5). The radiance B in (2) and the ionization rate w in (3) can then be calculated, assuming low density (ρ ≈ 0) within the streamer head and constant density in the liquid.…”

“…Whenever a new head is added, the streamer structure is optimized, possibly removing one or more existing heads. Streamer heads within 50 µm of another head closer to the plane, and heads with k i < 0.1, are removed [18]. Each streamer head is associated with a resistance in the channel towards the needle and a capacitance in the gap towards the planar electrode [19].…”

Branching Initial Streamers to Inhibit the Streamer Propagation in Natural Ester-based Nanofluid

“…The function of f(ρ np,sat -ρ np ) was defined as the charging process of the nanoparticles, and the saturation charge density ρ np,sat was 500 C/m 3 in this work [11]. The function of H(l) was defined to simulate the carrier fluctuation caused by additional carrier injection (e.g., photoionization).…”

“…Simulation analysis is an effective way to study the streamer propagation and breakdown mechanism of dielectric liquid [10,11]. Hwang et al established a streamer model for nanofluids by considering the electron adsorption of nanoparticles [12][13][14][15].…”