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

Abstract

Radiation is important for the propagation of streamers in dielectric liquids. Photoionization is a possibility, but the effect is difficult to differentiate from other contributions. In this work, we model radiation from the streamer head, causing photoionization when absorbed in the liquid. We find that photoionization is local in space (µm-scale). The radiation absorption cross section is modeled considering that the ionization potential (IP) is dependent on the electric field. The result is a steep increase in the ionization rate when the electric field reduces the IP below the energy of the first electronically excited state, which is interpreted as a possible mechanism for changing from slow to fast streamers. By combining a simulation model for slow streamers based on the avalanche mechanism with a change to fast mode based on a photoionization threshold for the electric field, we demonstrate how the conductivity of the streamer channel can be important for switching between slow and fast streamer propagation modes.

Cerman: Software for simulating streamer propagation in dielectric liquids based on the Townsend-Meek criterion

“…The model is based on the Townsend-Meek criterion in which an electron avalanche have to obtain a given size for the streamer to propagate. The software was developed and used for simulating our models for electron avalanche growth [14], conductance and capacitance in the streamer channels [15], as well as photoionization in front of the streamer [16]. From the examples on how to set up, run, and evaluate simulations, others can recreate our previous results or create their own set of simulations.…”

“…The electric field and potential is calculated by considering electrostatic shielding [14], as well as the conductance in the channel and the capacitance towards the planar electrode [15]. The streamer undergoes a transition into a fast propagation mode when radiation from the streamer head can ionize molecules directly in front of the streamer [16]. More details on the model is given in section 3.…”

“…The experimental conditions specifies an overvoltage applied to medium size needle-plane gap. The values of the physical parameters in seeds and avalanches and streamer structure are justified in our previous work[14,15,16]. Parameter values related to the simulation algorithm, or the model in general, have also been discussed in previous work.…”

Electromagnetic and Thermal Phenomena Modeling of Electrical Discharges in Liquids

“…Discharges in liquids are similar, under certain conditions, to discharge process in gas [16]. In gaseous environments most important factor for discharge process is impact ionization that depend on kinetic energy of electrons in the discharge area [17,18]. In most cases it has been assumed that the discharges can be initiated by the absorption of radiation quantum [19].…”

“…The CFD model was used for heat and mass transfer analysis in the time between next discharges. Coupled equations for continuity of flow (18), momentum (19) (here presented in one dimension of Cartesians system), and energy (20) balances were computed in Ansys program. A typical turbulent k-ε model was in use.…”