An analysis of the soot formation and oxidation process in a conventional direct-injection (DI) diesel flame was conducted using numerical simulations. An improved multi-step phenomenological soot model that includes particle inception, particle coagulation, surface growth and oxidation was used to describe the soot formation and oxidation process. The soot model has been implemented into the KIVA-3V code. Other model Improvements include a piston-ring crevice model, a KH/RT spray breakup model, a droplet wall impingement model, a wall-temperature heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process.
Experimental data from a heavy-duty, Cummins N14, research DI diesel engine operated with conventional injection under low-load conditions were selected as a benchmark. This experiment reproduced the operating conditions of experimental studies performed on an optically-accessible engine of same class at the Sandia National Laboratories. The predicted ignition timing, in-cylinder pressure and soot emission data were compared to measurements, while the flame and soot formation structures prior to flame-wall impingement were compared to the conceptual diesel model put forth by Dec. The analysis reinforces the conceptual model in many aspects. In particular, spatial correlations between the liquid-spray penetration, the fuel-vapor penetration, and the flame and soot structures agree with the conceptual model. The simulation also provides details on soot-relevant quantities (e.g., particle size, number density, generic precursor and acetylene concentrations) and formation rates (e.g., soot inception, particle coagulation, surface growth and oxidation), which are helpful for a better understanding of the diesel soot formation and oxidation process.