Two dimensional OH radical measurements in argon plasma-assisted combustion flame of premixed and nonpremixed methane/air mixtures using cavity ringdown spectroscopy

Abstract

We report a comparative study of plasma-assisted combustion (PAC) flames of the premixed and nonpremixed methane/air mixtures using a suite of optical diagnostic tools including imaging, optical emission spectroscopy (OES), and cavity ringdown spectroscopy (CRDS). Image study showed that the premixed PAC flame had a jet-shape but was split into two branches whereas the nonpremixed PAC flame had a paper-clip-shape. The OES spectra of the premixed PAC flame were characterized by emissions from OH A-X only. However, the nonpremixed PAC flame featured with emissions from the CH A-X band and the C2 swan band, in addition to the OH A-X band. Spectral simulations of OES spectra showed a higher temperature in the nonpremixed PAC flame than in the premixed PAC flame. Furthermore, two-dimensional (2-D) number densities of the ground state OH(X) in different locations in the premixed and nonpremixed PAC flames were measured by CRDS. Results showed that different OH(X) number density profiles exist among separate regions in the same type of flame and that OH(X) number density patterns differ between the premixed PAC flame and the nonpremixed PAC flame.