Effects of Ambient Oxygen and Density on Primary Soot Size under Diesel-Like Conditions Using a Lagrangian Soot Tracking Model

This article investigates the effect of ambient oxygen (O₂) levels and ambient density on the primary soot size under diesel engine-like conditions via the Lagrangian soot tracking (LST) method. The numerical studies and soot analysis are carried out for an n-heptane spray flame in the Sandia constant volume combustion chamber. Numerical studies are carried out at two O₂ levels of 15% and 12%, as well as two ambient densities of 14.8 kg/m³ and 30 kg/m³. The LST model involves treating the soot particles formed in the spray flame as Lagrangian particles, and their individual soot information is stored. Based on the primary soot size distribution for soot particles in the core of the spray jet, an increase in ambient density from 14.8 kg/m³ to 30 kg/m³ is shown to increase the peak and mean soot size by a factor of 1.5. Furthermore, the peak and mean primary soot size decreases with decreasing O₂ levels from 15% to 12%. The larger primary soot size at higher O₂ levels and ambient densities can be attributed to the higher net growth rate experienced by the soot particles. At low density, the span of the soot cloud is shorter O₂ level is low. In contrast, the high-density cases show a comparable soot cloud span at both O₂ levels before steady-state is reached. Soot age is introduced to predict the soot residence time in the spray flame. The results show that the soot residence time is dependent on both the span of the soot cloud and the initial onset location of the soot formed.