Plasma-Assisted Combustion: Kinetics and Control

Abstract

Plasma-assisted combustion has drawn significant focus over the last several years for novel engines requiring ultra-short flow residence times, fuel flexibility and lean mixtures. Previous studies of non-equilibrium plasmas have demonstrated significant phenomenological effects in extending burning limits, changing flame regimes, accelerating low-temperature chemistry and fuel cracking/reforming at reduced temperatures. However, understanding of key elementary plasma-assisted chemical pathways remains unknown, especially for more engine relevant liquid fuels. This dissertation seeks to utilize advanced diagnostics to identify and quantify key reactions of plasma-assisted oxidation kinetics and to demonstrate potential uses of plasma-assisted oxidation for future control of combustion.

To meet these aims, a number of experiments are undertaken with increasingly complex plasma chemistry. First, a single plasma produced molecule, ozone, which is a prevalent byproduct of oxygen-containing plasmas, is studied using synchrotron photoionization molecular beam mass spectrometry. Ozone reacts with C-C double bonds in a process called ozonolysis that is important for accelerating low temperature as well as atmospheric chemistry where reactions with the ozone layer can generate significant secondary aerosol pollutants. This ozonolysis process is studied with ethylene in a jet-stirred reactor from 300 to 1000 K to bridge the gap between atmospheric studies and temperatures relevant for combustion control. At atmospheric temperatures, studies of the Criegee Intermediate, a highly reactive intermediate in ozonolysis, reveals a network of adduct species with up to nine oxygen atom additions. Second, for applications, ozone’s effects on deflagration to detonation transition (DDT) is explored in microchannels. Ozone drastically accelerates both the onset time and distance for DDT as well as extend the lean limits of this process. Third, the chemical kinetic effect of direct, plasma-assisted, low-temperature oxidation on liquid fuels is also investigated using a nanosecond repetitively pulsed dielectric barrier discharge flow reactor. Using time-dependent laser absorption diagnostics, the plasma coupling effect on n-heptane oxidation is explored and compared to a computational model. A second study with n-pentane is conducted with the aim to develop a predictive kinetic model of this plasma-assisted oxidation. With the addition of electron impact dissociation reactions, the branching ratios of fuel dissociation are derived and validated, demonstrating significant improvement.