Numerical Study of Diesel Lift-Off Flame and Soot Formation under Low-Temperature Combustion

The effects of mixture formation on soot were studied for diesel lift-off flame under low-temperature combustion (LTC) through comprehensive numerical analysis in this work. The predicted results of lift-off length, ignition time, and soot were validated against the experimental data under various ambient conditions. The results indicated that, when the O 2 concentration decreases gradually from 21 to 12% at 14.8 kg/m 3, the distribution ranges of both the fuel/air equivalence ratio (ϕ) and combustion temperature ( T ) of the mixture could be further away from the formation ranges of both high soot and high NO x . However, when the ambient density increased to 30 kg/m 3, the effect of a decreasing O 2 concentration on lowering the combustion temperature of the dense mixture within the range of ϕ > 2.0 was not significantly the same as that at 14.8 kg/m 3 . Furthermore, the ignition time, the lift-off length, and the pressure rising rate during the initial stage after ignition changed much less with the decrease in the O 2 concentration. However, the LTC at 30 kg/m 3 could lower CO significantly and bring higher densities of OH and O 2, contributing to soot oxidation.