Measurement of OH(X) in the Microwave Plasma- Assisted Ignition of Methane/Air Mixture by Cavity Ringdown Spectroscopy

Abstract

Cavity ringdown spectroscopy is employed to measure the absolute concentration of the ground state OH(X) radical at the ignition region in the microwave plasma-assisted combustion (PAC) of a premixed methane/air mixture. A 2.45-GHz solid-state microwave source was used to generate the plasma used in this article. The PAC platform consisted of a triple-layered coaxial cylindrical quartz combustor with the argon plasma conducted in the innermost cylinder and the premixed methane/air as the coflow. This configuration allowed for the coupling of the plasma and reactants outside the combustor making the plasma-assisted ignition region accessible to the cavity ringdown beam. Optical emission spectroscopy and visual imaging were used to obtain information about the excited state species along with plasma and flame geometries, respectively. Increasing the plasma power at a constant fuel equivalence ratio resulted in a blue inverted cone-shaped flame anchored to the tip of the plasma. A single peak was observed in the excited state OH(A) emission profile in the hybrid zone, whereas no OH(X) peak was observed in the hybrid zone. The OH(X) number density was measured at $0.12\times 10^{15}$ molecules/cm3 at ${z}\,\,= 2$ mm before increasing to a peak of $1.85\times 10^{15}$ molecules/cm3 at ${z}\,\,= 8$ mm, then subsequently dropping off. The representation of both OH(A) and OH(X) provides a complete picture of the role played by the OH radical, an important intermediate specie in the microwave generated argon PAC of a methane/air mixture. The ability to track the evolution of both states of the OH radical in the ignition zone further sheds light on the role played by this important species in influencing the plasma enhancements of a premixed methane/air combustion. The results obtained further confirmed the hypothesis that OH(X) is more involved in the stabilization reactions, whereas OH (A) is more prevalent in the ignition process.