Optical Diagnostics of Non-thermal Plasmas and Plasma-assisted Combustion

Abstract: Non-thermal plasma is regarded as a collection of free electrons, ions and neutral particles that are not at local thermodynamic equilibrium. The high-energetic electrons formed in non-thermal plasmas are capable of generating chemically active species and modifying chemical kinetics in practical applications. One promising application of non-thermal plasmas is to achieve plasma-assisted combustion, in which new reaction pathways can be generated to increase chemical reaction rates, as well as to improve combustion efficiency and reduce pollutant emission. Developing and applying optical diagnostic tools help one to understand the underlying mechanisms of non-thermal plasmas and plasma-assisted combustion. A gliding arc discharge is a simple and low-cost technique to provide non-thermal plasmas at atmospheric pressure. The plasma column of gliding arc discharges can be ignited at the narrowest gap between two diverging electrodes by a high-voltage power supply, after which it moves along the electrodes by means of gas flow. In the thesis, optical diagnostics of a gliding arc discharge were carried out with the aim in particular of being able to better understand discharge characteristics involved. High-speed photography at an exposure time of a few microseconds was used to capture the instantaneous structure of the plasma columns, and to record the spatial and temporal evolution of the gliding arc discharge. The plasma column was found to experience short-cutting events and transitions between glow-type and spark-type discharge, as well as a cycle of ignition, extension and extinction. Ground-state OH is an important chemically reactive species that can be generated by gliding arc discharges in humid air. Planar laser-induced fluorescence (PLIF) measurements demonstrated that ground-state OH was distributed around the plasma column and that the thickness of the OH was much greater than that of the plasma column. Turbulent effects played an important role in determining the OH distribution and the dynamics of the gliding arc discharge. Three-dimensional (3D) particle tracking velocimetry (PTV) and 3D reconstructions of the plasma columns were performed, providing a more accurate 3D determination of the slip velocity and the length of the gliding arc discharge. The translational temperature of the gliding arc discharge was measured by planar laser-induced Rayleigh scattering, and the electron temperature was calculated using the measured reduced electric field strength. The rotational and vibrational temperatures were determined by comparing the experimental and simulated spectra. The results are able to contribute to the optimized operation of the gliding arc discharge for practical applications and to a better understanding of the mechanisms governing non-thermal plasmas at atmospheric pressure. The effects of non-thermal plasmas on combustion were investigated experimentally using PLIF. Several plasma sources, such as gliding arc discharges, microwave discharges and products (O3) stemming from dielectric barrier discharges, were employed for stimulating premixed CH4/air flames. It was found that the gliding arc discharge was able to promote the formation of CH2O and OH in the flame. An increase in CH2O was observed by means of PLIF when a high-power-density turbulent low-swirl flame was provided with small amounts of O3. The microwave increased both the chemiluminescence of the high-power-density turbulent flame and the PLIF signals from the OH and CH2O. The distributions of the CH2O signals shifted to be closer to the burner nozzle, indicating an increase in flame speed for the turbulent flame as the O3 and the microwaves were applied to the flame.