Numerical Predictions of Diesel Flame Lift-off Length and Soot Distributions under Low Temperature Combustion Conditions

The lift-off length plays a significant role in spray combustion as it influences the air entrainment upstream of the lift-off location and hence the soot formation. Accurate prediction of lift-off length thus becomes a prerequisite for accurate soot prediction in lifted flames. In the present study, KIVA-3v coupled with CHEMKIN, as developed at the Engine Research Center (ERC), is used as the CFD model. Experimental data from the Sandia National Labs. is used for validating the model predictions of n-heptane lift-off lengths and soot formation details in a constant volume combustion chamber. It is seen that the model predictions, in terms of lift-off length and soot mass, agree well with the experimental results for low ambient density (14.8 kg/m³) cases with different EGR rates (21% O₂ - 8% O₂). However, for high density cases (30 kg/m³) with different EGR rates (15% O₂ - 8% O₂) disagreements were found. Accordingly, the reduced chemistry mechanism was updated to include intermediate cool flame reactions involving ketohydroperoxide and formaldehyde radicals for improving the lift-off length predictions. Also, a comparison of the predictive capabilities of a two-step and a detailed nine-step phenomenological soot model was done. Overall, the present CFD model is able to replicate the qualitative trends of lift-off and soot formation at different EGR rates for both ambient densities.