Investigation of structure and soot formation in laminar inverse diffusion flames by atomic ratio measurements using laser-induced breakdown spectroscopy

Abstract

This experimental study reports the two-dimensional C/O and H/O atomic ratio distributions of oxy-ethylene laminar inverse diffusion flames (IDFs) with elevated stoichiometric mixture fractions (Zst) using laser-induced breakdown spectroscopy (LIBS), and investigates the effects of Zst on the structure and soot formation of IDFs. The LIBS diagnostic system consisting of a pulsed Nd: YAG laser and a fiber optic spectrometer measures the atomic ratios of IDFs with Zst of 0.2–0.5, and the influence of laser energy on atomic ratio calibrations and measurements is corrected to improve accuracy. IDFs are divided into five zones, namely the oxidant zone, the chemiluminescence zone, the gap, the soot zone, and the fuel zone. The C/O atomic ratio of the chemiluminescence zone and the gap remains 0.35–0.5, and the soot zone is 0.4–0.7. Except for the gap, where the H/O atomic ratio remains at 0.55–0.60, the H/O atomic ratio of all other zones is greatly affected by Zst. These atomic ratio results demonstrate that the C/O atomic ratio has the advantage of differentiating the structure of IDFs. Experimental results show a C/O atomic ratio of 0.5–0.6 is required for soot inception in IDFs, so the elevated Zst delays soot inception by lowering the C/O atomic ratio near the flame sheet, and it also compresses the residence time and C/O atomic ratio space favorable for soot formation. Additionally, the decreased local equivalence ratio determined from the H/O atomic ratio further confirms that the increase of Zst makes IDFs unsuitable for soot formation. This study proves that LIBS is an effective flame diagnostic tool and the atomic ratios hold promise for studying a broader range of flame configurations.