Physics of electrical discharge transitions in air

About the project

Electrical discharges with a variety of different forms (streamers, glow corona, leaders, etc.) broadly exist in nature and in industrial applications. Under certain conditions, one electrical discharge can be transformed into another form. This thesis is aimed to develop and use numerical simulation models in order to provide a better physical understanding of two of such transitions, namely the glow-to-streamer and the streamer-to-leader transitions in air.

In the first part, the thesis includes the two-dimensional simulation of the glow-to-streamer transition under a fast changing background electric field. The simulation is performed with a fluid model taking into account electrons. An efficient semi-Lagrangian algorithm is proposed to solve the convection-dominated continuity equations present in the model. The condition required for the glow-to-streamer transition is evaluated and discussed. In order to enable such simulations for configurations with large interelectrode gaps and long simulation times, an efficient simplified model for glow corona discharges and their transition into streamers is also proposed.

The second part of the thesis is dedicated to investigate the dynamics of the streamer-to-leader transition in long air gaps at atmospheric pressure. The transition is studied with a one-dimensional thermo-hydrodynamic model and a detailed kinetic scheme for N₂/O₂/H₂O mixtures. In order to evaluate the effect of humidity, the kinetic scheme includes the most important reactions with the H₂O molecule and its derivatives. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a stable leader. The comparison between the proposed model and the widely-used model of Gallimberti is also presented.

Motivation

Electrical discharges in air widely exist in nature and industry. Research on electrical discharges is not only an old, but also a prosperous subject with some discharge phenomena such as transient luminous events discovered only a few decades ago and with emerging applications in industry. One type of electrical discharge can be transformed into another form under certain conditions. The condition required for such a transition to take place is thus of great interest to investigate, not only from the theoretical point of view, but also from the perspective of engineering applications.

Electrical discharges have a long research history. However, our knowledge of electrical discharges is still limited and many questions are still unsolved. Let us take the most common and famous phenomenon of lightning as an example. Several hundreds of years have passed after Benjamin Franklin conducted his experiments on lightning. Nevertheless, one of the most basic and important questions of ‘how lightning is initiated’ is still unsolved. Another basic process poorly understood is the attachment of lightning flashes to grounded objects. Although numerous models have been developed to describe this process, the accurate simulation of the interaction of lightning flashes with structures on the ground is still challenging. Since lightning attachment is a complex physical process, the existing models use rather crude approximations of the different electrical discharges involved in order to reach a practical quantitative evaluation. However, the simplifications assumed by these models, particularly those used to evaluate the transitions between the different discharges, are still controversial. Hence, the debate on the effect of glow corona on the lightning attachment has not been concluded yet, mainly due to the lack of understanding of its transition into streamers. Furthermore, quantitative estimates of the condition necessary for streamers to transform into leaders are still doubtful, especially when evaluating the attachment process after a first lightning strike. Thus, an opportune project to contribute to the research on lightning and other applications is to investigate the physics behind these transitions.

​Publications

PhD thesis (open access): Physics of electrical discharge transitions in air

All publications during PhD: Publications at KTH

Main conclusions

1. The proposed position-state separation method, referred to as POSS, is an accurate and efficient alternative method to simulate electrical discharges.
2. Kaptzov’s approximation does not hold during the glow-to-streamer transition.
3. The assumptions used by the model of Gallimberti is not valid to evaluate the streamer-to-leader transition.
4. The glow space charge cannot hinder streamers to be incepted under the fast changing background electric field

Financial sponsor(s)

China Scholarship Council (CSC), 4 years from 2013 to 2017

Project responsible

Personal experience at KTH

As one of the most famous technical university, KTH provides me with the valuable insight and invaluable assistance from the very international, experienced and widely-acknowledged scientists and engineers. The adequate academic resources, harmonious interpersonal relationships, flexible schedules and the beautiful environment…all these elements and features make me feel relax, confident and energetic when pursuing the PhD. For me, it is just like a journey abroad which gives me a piece of bright and colorful life.

Advisor

Marley Becerra, associate professor at KTH Royal Institute of Technology