Optical and Laser Spectroscopic Study of Microwave Plasma-Assisted Combustion

Abstract

Nonthermal plasma-assisted combustion (PAC) has been demonstrated to be a promising potential method to enhance combustion performance and reduce the pollutant emissions. To better understand the mechanism in PAC, we have conducted a series of studies on the combustion enhancement by plasma using a home-developed PAC platform which employs a nonthermal microwave argon plasma and a suit of optical diagnostic tools including optical imaging, optical emission spectroscopy, and cavity ringdown spectroscopy. A new PAC system in which a continuous atmospheric argon microwave plasma jet is employed to enhance combustion of methane/air mixtures was reported. Reactive species in PAC were characterized in a state-resolved manner including the simultaneously measurements of OH(A) and OH(X) radicals in the PAC flames. Roles of the state-resolved OH(A) and OH(X) radicals in microwave PAC of premixed methane/air mixture were explored. It was concluded that if both OH(A) and OH(X) radicals assisted the ignition and flame stabilization processes, then we may hypothesize that the role of OH(A) was more dominant in the ignition enhancement but the role of OH(X) was more dominant in the flame stabilization. The effect of fuel injection configurations was investigated in the comparative study between PAC of the premixed and nonpremixed methane/air mixtures. It was found that emissions from the CH (A-X) and C2 Swan systems only exist in the nonpremixed PAC which suggest that the reaction pathways are different between premixed and nonpremixed PAC. The PAC of premixed methane/oxygen/argon mixtures was investigated. A U-shaped dual-layer curve of fuel ignition/flame stabilization limit showing the effects of the plasma power on the fuel ignition and flame stabilization was observed and reported. A parametric study of the microwave PAC of the premixed ethylene/air mixtures was conducted. Behavior of the OH, CH, and C2 radicals and their dependence on plasma power, argon flow rate, and total ethylene/air mixture flow rate were also studied.