抄録
Three dimensional calculations of a split injection in DI Diesel engine were performed using the KIVA3V, release 2, CFD code. The detailed chemistry approach used involves the Partially Stirred Reactor (PaSR) model for the turbulence-chemistry interaction coupled with the chemical mechanism of a diesel fuel surrogate represented by a mixture of n-heptane and toluene (68 species, 270 reactions). When simulating the Volvo NEDS DI Diesel engine in a split injection mode, it was found that the droplet collisions play a considerable role in predicting the rate of heat release during the pilot injection. It was found that if a collision probability is over-predicted, it causes a droplet cluster formation and too fuel lean conditions resulting in a decrease in combustion intensity. The default collision model in the KIVA3V code, formulated by O'Rourke, is replaced by the modified model proposed in Nordin. The O'Rourke model formulation defines collision frequency inverse proportional to the cell volume that causes grid dependence and does not discriminate droplet trajectories which are not intersecting each other. In a new grid independent formulation, two vital requirements have to be met for collision between two droplets to occur: first, they have to travel towards each other being in a so-called "collision" cone and second, all colliding droplets are constrained to a certain exponential time-decaying initial probability in the Poisson law. Using the new formulation, higher combustion intensity was achieved during the combustion.
