Soot formation modelling for n-dodecane sprays using the transported PDF model

Soot formation in an n-dodecane spray flame under diesel engine conditions, known as Spray A, is modelled with the transported probability function (TPDF) method. The approach employs an acetylene-based two-equation soot model coupled with a Reynolds-averaged turbulence model and a Lagrangian discrete phase spray model. The aims are to evaluate, in the context of soot, the predictive capability of the model, the effects of turbulence–chemistry interactions (TCI), and various available chemistry mechanisms. TCI effects are evaluated by comparisons between the TPDF model and simulations using a well-mixed model neglecting turbulent fluctuations. Five test cases having variations in ambient temperature and oxygen concentration are considered. Five chemical mechanisms are first compared to experiments in terms of their ignition delay (ID) and lift-off length (LOL) under ambient O2 and temperature variations. Three relatively new mechanisms exhibit good ID performance (with both TCI approaches), while two short mechanisms also provide good LOL performance in conjunction with the TPDF approach. The two short mechanisms are considered for further comparisons. The auto-ignition process is analysed by comparing TPDF simulations with measurements from schlieren and 355 nm planar laser-induced fluorescence, detecting CH2O and polycyclic aromatic hydrocarbons, with overall good qualitative agreement, though with some differences on low-temperature reactivity. The experimental comparisons for soot consider transient soot mass and KL in the baseline condition, and steady state soot volume fraction (SVF) fields and total soot masses for all five ambient conditions. In terms of comparisons to experiment, the transient stage of the soot mass development is not well captured. An analysis of the transient KL in the baseline case shows the soot-containing region is larger in the experiment than the model, with soot extending in the experiment much closer to the jet boundary, suggesting that the model underestimates gradients around the jet head. During the steady period, however, the SVF agrees quite well. The soot models with both chemistry and TCI approaches were able to reproduce the overall soot trends with varying ambient temperature and oxygen, though the effect of the ambient temperature on the soot mass was under-predicted, in particular in its variation from 900 to 1000 K. TCI effects on soot were in overall terms relatively minor, in part due to compensating errors. Neglecting TCI showed generally higher peak soot amounts, narrower soot distributions, and more downstream soot onset and soot peak locations. These differences between the models are explained with the help of a detailed analysis of the soot phenomena.