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Theoretical study of flame initiation by electrical discharge

Theoretical study of flame initiation by electrical discharge

Wednesday 04/08/2021
  • Nir Druker
  • Work towards PhD degree under the supervision of Professor (Emeritus) B. Greenberg and Prof. E. Sher (Technion)
  • Department of Aerospace Engineering
  • Technion – Israel Institute of Technology
  • The talk will be given in English

In Internal Combustion Engines (ICE’s), the combustible mixture is ignited at each cycle by a time-controlled spark discharge. The success of the flame initiation depends to a large extent on the operating conditions (engine speed, engine load, ambient conditions, and spark characteristics). It was found that with traditional ignition systems, the nature of the ignition process induces very significant cycle-to-cycle variations, and as compared to theoretical cycles, these are believed to be the main reason for lower engine torque, high fuel consumption and surplus engine emission. In order to improve the success level of the ignition process, quite a number of different ignition systems have been proposed. Higher discharge energies, longer discharge durations and multi-spark systems have been developed. During the last decade, cold-plasma multi-pulse assisted ignition systems have been proposed. In practice, these systems have shown much better results, though our understanding of how the energy evolves in this complex ignition process is still deficient. Such an understanding is valuable not only for widening and deepening our scientific knowledge, but also to optimize the systems in practical settings.

In this research, we conducted a preliminary, theoretical/numerical investigation into the possibility of use of on-board control of electrically-based cold plasma-assisted ignition and combustion The focus was on diagnostics and enhancement of energy deposition in specific modes, by application of bi-polar short duration voltage pulses in low-pressure air. The physical model couples the electric field, potential and current, with the relevant conservation equations for 24 species via 168 kinetic reactions, including molecules’ rotation, vibration, electronic excitation, dissociation, and ionization inside the electrodes gap. Evaluation using various pulse repetition frequencies and different pulse shapes was conducted. Special attention was given to the overall coupled energy, deposited during the discharge, and to energy channeled to known ignition supportive modes such as nitrogen electronic excitation and oxygen radicals generation.

The results of the analysis show that for the considered conditions, energy deposition can be divided into two main stages, characterized by high and low voltage magnitudes, respectively. It was found for the first time, that the (low voltage) second stage’s energy deposition can be higher than that of the first (high voltage) stage. At the second stage, the deposition of energy into specific modes can be tuned by setting appropriate voltage magnitudes. In addition, the energy deposited in modes important for ignition, exhibits a simple linear relation to the overall energy deposition. Furthermore, based on these findings, we demonstrate how a new sequence of voltage pulses can further increase enhancement of ignition and combustion supportive processes.

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