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.

我们通过实验和数值研究了一系列甲烷燃料层流同流扩散火焰，以研究可变氧指数（OI）对烟so产量和分布的影响。OI定义为氧化剂中氧气的摩尔分数。研究了16种火焰，OI范围从21％（空气）到76.3％，因此OI以很小的增量变化，并且可以精确解决其影响。使用色比高温法对所有火焰的烟灰体积分数分布进行了实验测量。使用经过广泛验证的化学动力学机制和基于聚集体的烟灰模型进行模拟，该模型解释了烟灰开始，生长和氧化的所有主要过程。实验测量表明，可见火焰高度随OI的增加而降低，这与理论估计和数值模拟是一致的。测量结果还表明，OI的增加（从21％增至36.8％）首先会导致最大烟灰浓度增加，但是OI的进一步增加会导致烟灰产量降低。另外，对于低OI火焰（低于36.8％），最大烟灰浓度出现在火焰中心线上，但是对于较高OI火焰，烟灰峰值出现在火焰翼中。通过模拟可以很好地再现所有这些实验观察结果，最大烟灰体积分数幅度位于实验测量结果的误差范围内。通过检查沿火焰区域沿特征拉格朗日轨迹的烟灰演变，计算结果可用于揭示潜在的物理机制。我们发现，增加的OI会导致更高的火焰温度，从而导致更强的烟灰生成速率，但同时也会减少烟灰在火焰区域的停留时间，从而减少了生成烟灰的时间。这些竞争效应会导致最大烟灰产量的最初增加和随后的下降，以及最大烟灰产量位置随OI的增加而发生的变化。