Simultaneous measurements of OH(A) and OH(X) radicals in microwave plasma jet-assisted combustion of methane/air mixtures around the lean-burn limit using optical emission spectroscopy and cavity ringdown spectroscopy

Abstract

We report a new plasma-assisted combustion system, in which a continuous atmospheric argon microwave plasma jet is employed to enhance combustion of methane/air mixtures in different fuel equivalence ratios (φ) ranging from 0.35 to 1.5. The combustor has three distinct reaction zones along the jet axis (the combustion flame direction): the pure plasma zone, the hybrid plasma-flame zone and the combustion flame zone. Each of the three zones is clearly defined by its emission spectral fingerprints. The plasma zone was featured by strong emissions from OH and NH electronic bands and atomic lines of Ar, Hα and Hβ. In the hybrid zone where the plasma jet met fuel mixtures, emission spectra were dominated by OH, NH and CN transitions and by weak or no atomic transitions. In the combustion flame zone, only weak OH emissions were observed. Simulations of optical emission spectroscopy (OES) yielded gas kinetic temperatures to be 1175 ± 50 K, 1450 ± 50 K and 1865 ± 50 K in each of the three zones, respectively. The plasma-enhancement effect was investigated by comparing the lean-burn limits of the combustion with and without plasma. At the same fuel mixture flow rate of 1.0 standard litre per minute and plasma power of 100 W, the lean-burn limit in terms of the fuel equivalence ratio φ was extended from 0.72 without assistance of the plasma to 0.35 with assistance of the plasma. In addition to OES that was employed to characterize the excited state species including OH(A) in the three different zones, pulsed cavity ringdown spectroscopy was utilized to measure absolute number densities of the ground state OH(X) using the OH A–X (0–0) R2 (1) line in different locations in the flame zone at φ = 0.51, 0.87, 1.10 and 1.45. For rich and lean combustions, significantly different OH(X) number densities and density profiles in the flame zone were observed. At φ = 0.51, the OH(X, V″ = 0, J″ = 0.5) number density increased from 2.29 × 1015 molecule cm−3 at the combustor nozzle to the maximum, 3.13 × 1015 molecule cm−3 at 2 mm downstream, and to the lowest detectable level of 0.12 × 1015 molecule cm−3 in the far downstream where optical emissions were too weak to be detected. Results from the simultaneous measurements of the electronically excited state OH(A) and the ground state OH(X) allow us to discuss the roles of OH(A) and OH(X) in the plasma-assisted ignition and the flame stabilization, respectively.