In the present study a practical soot model was developed and implemented in the multidimensional computational fluid dynamics code, KIVA-3vr2, for use in conventional diesel and low temperature diesel combustion (LTC) strategies. The model framework is based on four fundamental steps: soot inception from a four-ring polycyclic aromatic hydrocarbon species (pyrene), surface growth through acetylene and benzene, soot coagulation and oxygen- and OH-induced soot oxidation. Diesel combustion was simulated by using reduced n-heptane and primary reference fuel (n-heptane and iso-octane) chemistry mechanisms. Reduced polycyclic aromatic hydrocarbon (PAH) chemistry mechanisms were formulated from the literature and coupled with the n-heptane and primary reference fuels mechanisms. Improvements were made in the chemistry mechanism for better predictions of ignition delay, liftoff length and soot precursor concentrations. The proposed model can be used not only for soot mass predictions, but also for soot number density and soot diameter predictions.
The soot model performance was evaluated by comparing the model predictions with available optical spray chamber experimental data. The model performance was also evaluated by comparing the model predictions with experimental results from an optical engine as well as from light- and heavy-duty metal engines. In particular, a variety of combustion regimes were tested, including conventional diesel, premixed charge compression ignition (PCCI), high exhaust gas recirculation (EGR) LTC and dual fuel reactivity controlled compression ignition (RCCI). In general, the model was able to predict well the trends in soot mass over the range of operating conditions. The model parameters were evaluated by performing a sensitivity analysis of the soot model rates and precursor chemistry. It was seen that the model predicted results were sensitive to the acetylene chemistry, acetylene assisted surface growth rates, and OH assisted oxidation rates.
As future engine technologies aim at higher degrees of premixing by introducing long ignition delay combustion strategies, development of an organic fraction model will be a valuable addition to the current proposed soot model since the organic fraction correlates with the degree of premixing. The basis for an organic fraction model stemming from PAH chemistry is also established in this work.