An Analysis of Nanosecond-Pulsed High-Frequency Discharges on Ignition Kernel Formation and Growth

Date of Award

5-5-2024

Degree Name

Ph.D. in Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Joshua Heyne

Abstract

A significant challenge to engine design is the development of combustion systems that meet increasingly strict efficiency, performance, and emissions demands. This pursuit often prompts using conditions at the limits of capability, dictating the need to expand the envelope of robust and reliable operation. The use of nanosecond-pulsed high-frequency discharges (NPHFDs) for ignition has attracted considerable attention because of their ability to produce active radicals and excited species, ultrafast gas heating effects, and unique hydrodynamic behavior. These characteristics are why NPHFDs have proven effective in extending ignition limits and reducing ignition times in quiescent, low-speed, and high-speed turbulent environments. Despite these observations, the community will benefit from additional research on NPHFDs to better understand their kinetics, hydrodynamics, and operational strengths and weaknesses compared to conventional exciter systems. The work within this dissertation provides insight into these topics, starting with results from a two-photon laser-induced fluorescence campaign that reports the spatio-temporal evolution of oxygen atom fluorescence produced from a nanosecond discharge. Results indicated that the evolution of the oxygen atom (O-atom) signal was heavily influenced by the discharge-induced flow field. This work was followed by an exploration of the unique hydrodynamics related to a train of nanosecond discharges, namely, discharge-induced jetting motion and the experimental conditions that maximize/minimize its influence. Overall, the jetting motion was bolstered by larger inter-electrode distances, higher pulse repetition frequencies (shorter inter-pulse times), and larger bursts of pulses. Increasing the bulk flow velocity did not eliminate the jetting motion but reoriented it in the bulk flow direction such that its spanwise magnitude decreased. With the unique attributes of NPHFDs known, a comparison between an NPHFD exciter system and a conventional exciter system was then conducted in terms of ignition kernel size, growth rate, and ignition probability. Results showed that the NPHFD exciter has benefits in terms of ignition kernel size and growth rate compared to the conventional system, as long as the equivalence ratio ensures a 100% ignition probability or the average power is sufficiently high. This dissertation concludes with an investigation of the application of lower-voltage nanosecond discharges to a high-speed turbulent environment, with the conclusion that low-voltage bursts can successfully ignite a cavity flameholder. Overall, the collection of works within this dissertation documents the characteristics of NPHFDs, beginning with their fundamental kinetic and hydrodynamic qualities observed in controlled, low-speed, non-reacting environments.

Keywords

Plasma-assisted ignition, nanosecond discharges, ignition kernel

Rights Statement

Copyright 2024, author

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