Characterization of six clustered methane-air diffusion microflames through spectroscopic and tomographic analysis of CH* and C2* chemiluminescence

Abstract The current study presents an emission spectroscopic investigation of clustered microflames established on a burner consisting of six micro fuel nozzles surrounding a center air nozzle which has been designed for flame synthesis and characterization of microflames. We conducted spectrally resolved measurements of the chemiluminescence of C 2 * and CH * and compared these results to a tomographic method applied to filtered emission measurements of CH * emission which can yield spatially resolved three dimensional mapping of the flame front. The analysis of the spatial distribution of the integrated band emission of C 2 * and CH * showed that the emission of both species is generated in the same locations in the flame which are the thin flame sheets shown in the tomography results of CH * . The ratio of the C 2 * and the CH * emission from the emission spectroscopy measurements was used to determine a corresponding local equivalence ratio through empirically derived correlations for premixed flames reported in literature. The resulting equivalence ratio equals one when the flame zone established at the boundary between fuel and air, indicating that diffusion dominated flame zones are established. Under certain nozzle pitch and flow rate conditions, the equivalence ratio above the center air nozzle inside the merged flame became significantly greater than one, which indicates a local fuel rich premixed type flame zone being established. To determine distributions of rotational and vibrational temperatures throughout the entire flame, we used the spectrally resolved emission from C 2 * and CH * . The temperature associated with vibrational excitation was different for both species but remained constant throughout the entire flame region and was found no representation of the flame temperature but an artifact of the combustion reactions producing the chemiluminescence. The rotational temperature of di-atomic molecules, however, equilibrates rather quickly with the translational temperature and can accurately represent the gas-dynamic temperature of the flame. The temperature distributions found from the two species agreed qualitatively, the CH * temperature was consistently higher than the C 2 * temperature by about 200 K. Spatial resolution was assigned to the results from the spectroscopic measurements through a comparison with the tomographically analyzed images of the CH * chemiluminescence.