Date of Award

Spring 2026

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Sang Hee Won

Abstract

Atmospheric-pressure plasmas are attractive for practical chemical and energy applications because they can be generated without vacuum systems and can produce reactive environments under relatively mild bulk conditions. However, these discharges are often non-uniform and highly sensitive to operating conditions, especially when water or multiphase flows are introduced. This dissertation investigates how atmospheric- pressure plasma behavior and plasma-driven chemistry change in three challenging but application-relevant regimes: water-mist dielectric barrier discharges (DBDs), multiphase plasma-assisted fuel reforming, and striated AC glow discharges. The work combines optical and electrical diagnostics, imaging, chemical analysis, and time-dependent analysis to connect discharge characteristics with measurable chemical outcomes.

First, atmospheric pressure DBD in air is studied while varying the amount of mist introduced (water vapor). Increasing water loading drives a transition from a more diffuse discharge toward a more filamentary and localized mode. Optical emission spectroscopy shows strong nitrogen features, while excited OH and H emissions are not detected under the tested conditions. This result highlights a practical limitation in water-containing atmospheric discharges: even when plasma–water chemistry may be occurring, key optical features can become weak or obscured due to collisional quenching and competing energy- loss pathways. Electrical measurements are therefore used to quantify changes in discharge activity as water loading increases, demonstrating that water strongly reshapes discharge stability and mode at atmospheric pressure.

Second, a reactor concept is developed to initiate and sustain plasma in a multiphase liquid fuel environment. A cavitation venturi is used to generate a controlled bubble population, creating a bubble-rich region that enables discharge formation downstream of the venturi. Ethanol is used as a baseline fuel, and products are analyzed in both phases using mass spectrometry for gas species and ¹H and ¹³C NMR for liquid composition. Under the investigated conditions, the NMR spectra show negligible change in the bulk liquid phase after plasma exposure, whereas gas-phase measurements show clear evidence of plasma-induced reactions and the formation of lighter species. These results indicate that, in this operating regime, measurable conversion occurs primarily in the vapor region associated with the discharge rather than through broad transformation of the bulk liquid.

Third, the dissertation examines striations in an atmospheric-pressure AC-driven glow discharge in helium and helium–argon mixtures. High-speed imaging is used to quantify striation motion and extract drift behavior under different operating powers. A time-dependent 0-D kinetic model is used to interpret how kinetics within the AC cycle contribute to the observed structure. Traces of argon is used to probe the role of excited- state and metastable pathways in shaping the visible pattern. Together, these results provide new characterization of atmospheric-pressure striations under AC glow conditions and show how combined experiments and modeling can clarify structured discharge dynamics.

Rights

© 2026, Ayuob Al wahaibi

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