3D simulation of Dl diesel combustion and pollutant formation using a two-component reference fuel

Abstract

By separating the fluid dynamic calculation from that of the chemistry, the unsteady flamelet model allows the use of comprehensive chemical mechanisms, which include several hundred reactions. This is necessary to describe the different processes that occur in a DI Diesel engine such as autoignition, the burnout in the partially premixed phase, the transition to diffusive burning, and formation of pollutants like NO(x) and soot. The highly nonlinear reaction rates need not to be simplified, and the complete structure of the combustion process is preserved. Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3D CFD code. The flamelet solution is coupled to the flow and mixture field by several time dependent parameters (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentations, which are then used by the 3D CFD code to compute the temperature field and the density. The density is needed in the 3D CFD code for the solution of the turbulent flow and mixture field. Pollutant formation in a Volkswagen DI 1900 Diesel engine is investigated experimentally. The engine is fueled with Diesel and two reference fuels. One reference fuel is pure n-decane. The second is a two-component fuel consisting of 70% (liquid volume) n-decane and of 30% (liquid volume) α-methylnaphthalene (Idea-fuel). The experimental results show good agreement for the whole combustion cycle (ignition delay, maximum pressures, torque and pollutant formation) between the two-component reference fuel and Diesel. The simulations are performed for both reference fuels and are compared to the experimental data. Nine different flamelet calculations are performed for each simulation to account for the variability of the scalar dissipation rate, and its effects on ignition is discussed. Pollutant formation (NO(x) and soot) is predicted for both reference fuels. The contributions of the different reaction paths (thermal, prompt, nitrous, and reburn) to the NO formation are shown. Finally, the importance of the mixing process for the prediction of soot emissions is discussed.