Investigation of soot inception limits and chemiluminescence characteristics of laminar coflow diffusion flames in C/O ratio space

This study investigates soot inception limits of laminar coflow diffusion flames in the carbon-to-oxygen atom (C/O) ratio space, as well as the mechanism of flame structure affecting soot inception. And chemiluminescence characteristics in flames are evaluated. Oxy-ethylene flames with the same adiabatic flame temperature (2500 K) and different stoichiometric mixture fractions (Zst, 0.3–0.6) are investigated by relocating CO2 from oxidant to fuel. The position of soot inception and the chemiluminescence of CH*, C2*, and CO2* are measured by a hyperspectral imager. A numerical model with a detailed kinetic mechanism is used to obtain the temperature and gas-phase species concentration. The results prove that the critical C/O ratio favorable for soot inception is 0.53, and the critical temperatures on the flame centerline and sheet are 1322 K and 1632 K, respectively. The comparatively low temperature on the flame centerline can cause soot inception, as the polycyclic aromatic hydrocarbons (PAHs) concentration and residence time can guarantee the collision and cluster formation of PAHs. Although fuel pyrolysis is promoted on the flame sheet, oxygen-containing species dilutes the concentrations of fuel pyrolysis products and PAHs to limit soot inception, necessitating high temperatures for chemical bond formation. Furthermore, the mixture fraction, local C/O ratio, and net reaction and production rate are calculated, and the key reaction paths are also identified. The results demonstrate that the increase of Zst achieves the suppression of soot inception by lowering the formation of benzene and PAHs in the high C/O ratio (>0.5) zone, improving the oxygenation in the low C/O ratio (<0.5) zone, and compressing the C/O ratio space favorable for soot formation. Meanwhile, the (CH* and C2*) chemiluminescence zone with a C/O ratio of roughly 0.5 exists in the flame. With Zst increases, the CH* and C2* chemiluminescence diminishes at the flame root and enhances downstream, whereas CO2* chemiluminescence remains unaltered.