Experimental and numerical study of variable oxygen index effects on soot yield and distribution in laminar co-flow diffusion flames

We study experimentally and numerically a series of methane-fueled laminar co-flow diffusion flames to investigate the effects of variable Oxygen Index (OI) on soot yield and distribution. OI is defined as the mole fraction of oxygen in the oxidizer. Sixteen flames are studied with OI ranging from 21% (air) to 76.3%, so that OI varies in small increments and its effects are precisely resolved. The soot volume fraction distribution is measured experimentally for all flames using color-ratio pyrometry. Simulations are carried out using an extensively validated chemical kinetic mechanism and an aggregate-based soot model that accounts for all major processes of soot inception, growth, and oxidation. The experimental measurements show that the visible flame height decreases with increasing OI, which is consistent with theoretical estimates and the numerical simulations. The measurements also indicate that increasing OI (from 21% to 36.8%) first results in an increase in the maximum soot concentration, but a further increase in OI leads to a decrease in the soot yield. Additionally, the maximum soot concentration occurs on the flame centerline for low OI flames (below 36.8%), but for higher OI, the peak soot yield occurs in the flame wings. All of these experimental observations are well reproduced by the simulations, with the maximum soot volume fraction magnitudes lying within the error bounds of the experimental measurements. The computational results are used to reveal the underlying physical mechanisms, by examining soot evolution along characteristic Lagrangian trajectories through flame regions. We find that increasing OI leads to higher flame temperature, which results in a stronger soot production rate, but also reduced soot residence time in flame regions, which allows less time for soot production. These competing effects cause the initial increase and subsequent decrease in the maximum soot yield and the shift in the maximum soot yield location with increasing OI.