Kinetic Studies of Non-equilibrium Plasma-assisted Ignition and Combustion
Authors:   , Wu Liang , Fridman Alexander A. , Miller David Leslie , Cernansky Nicholas Peter 
Published: July 2021

The application of thermal plasmas in combustion has a long history which dates back at least as far as the early spark ignition engines for automobile applications. In recent decades, a great amount of experimental data has demonstrated promising results for non-thermal plasma application in high speed flow and automotive engines. However, the mechanisms of plasma assisted combustion still remain unclear. The first part of this thesis presents a computational study in understanding the physics and chemistry of plasma-assisted combustion at temperatures above the auto-ignition threshold. The energy costs in generation of chemically active species by different discharges were calculated. The optimal physical parameters were determined in terms of the energy efficiency. The role of singlet oxygen molecules was numerically studied in promoting ignition of a hydrogen-oxygen mixture. The major reaction pathways have been identified. The second portion of this study, which is the major emphasis of this thesis, shifted the research focus to the investigation of plasma chemical kinetics at temperatures below auto-ignition threshold. An experimental installation was designed, fabricated and calibrated for this purpose. Three types of laser-based optical diagnostics were applied to investigate the hydroxyl (OH) radical dynamics in the afterglow of a pulsed nanosecond discharge. Experiments were carried out using a premixed lean fuel-air mixture ([phi]=0.1) at atmospheric pressure for temperatures ranging from 300 K to 800 K (below the auto-ignition threshold). The fuels were methane, ethane, propane, butane, hydrogen and hydrogen-carbon monoxide. The nanosecond pulsed discharge was formed in a pin to pin electrode system. During the discharge, atomic oxygen and hydrogen are generated by direct electron impact and dissociative quenching of excited nitrogen. The results from laser induced fluorescence (LIF) have shown that after generation by the plasma the OH persists at significant levels for a long time that lengthens with increasing temperature. The ~100 [mu]s-long plateau clearly indicates the existence of chain reactions at low temperature (starting at 500 K for alkanes mixtures, 400 K for hydrogen mixtures), which are not predicted in current kinetic models. The results from the planar laser-induced fluorescence (PLIF) study have confirmed the unique phenomena and also demonstrated uniform OH radical distribution along the discharge channel. Comparison of OH radical emission dynamics with discharge emission dynamics from excited nitrogen revealed a close similarity in spatial distribution and allowed clarification of the mechanisms of atomic oxygen formation. The third laser diagnostics, Cavity Ring-Down Spectroscopy (CRDS), which is an absorption spectroscopy, demonstrated consistent results and further validated the unpredicted results. The temperature measurements through Optical Emission Spectroscopy (OES) of the second positive nitrogen system revealed no significant heating by the plasma. Further studies were focused on identifying the key reaction pathways to reveal the chemical mechanism of the unpredicted phenomena. The roles of nitric oxide and vibrationally excited nitrogen have been experimentally studied. The results have ruled out the possibility of nitric oxide as the key species and have provided support suggesting that vibrationally excited nitrogen could play a major role in the observed OH plateau effect.