1998-10-19

Three-Dimensional Simulation of Pollutant Formation in a DI Diesel Engine Using Multiple Interactive Flamelets 982459

Flamelet modeling allows the application of comprehensive chemical mechanisms, which. include all relevant chemical combustion processes that occur in a DI Diesel engine during autoignition, the burnout in the partially premixed phase, the transition to diffusive burning and formation of pollutants like NO, and soot. The highly nonlinear dependencies of the chemistry need not to be simplified, and the complete structure of the flame is preserved.
Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3-D CFD code. The flamelet solution is coupled to the flow and mixture field by the current boundary conditions (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentrations, which are then used by the 3-D CFD code to compute the temperature field.
In this work a multiple flamelets concept is proposed that accounts for spatial inhomogeneities in the boundary conditions for the flamelets. Numerical tracer particles are introduced into the turbulent flow field, where each particle represents a flamelet. The temporal evolution of the flamelets is controlled by the time dependent boundary conditions, which are extracted at the current location of the tracer particle in the flow field. The expected value to find a tracer particle at a given location is calculated by solving an unsteady convection-diffusion equation for each particle.
Applying this model pollutant formation in a n-decane fueled Volkswagen DI1900 Diesel engine is investigated. It is shown that the soot emissions are primarily controlled by the mixing process in the cylinder. Numerical simulations for different injection rates are compared to inhouse experiments, where cylinder pressure and the exhaust gas concentrations of NO, and soot were measured.

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