DNS: A Comprehensive Tool to Investigate the Autoignition in Diesel Engines

In this work, the study of the autoignition process in Diesel-type environment has been performed by using the Direct Numerical Simulation (DNS). The simulation of combustion in its own complexity is certainly a challenging problem and DNS is probably the most suitable tool to deal with it. In fact, differently from the Reynolds Averaged Navier-Stokes (RANS) equations, the DNS solves the instantaneous flow variables and the smallest length scales of turbulence. With regards to Diesel engine combustion, the investigation of autoignition is of primary interest, as the ignition delay affects the overall operating characteristics of the engine, its efficiency and performance, as well as pollutant emissions. In the present work, a detailed kinetic mechanism has been implemented in a DNS code and its capabilities have been assessed. In the computations fuel n-heptane has been used because its cetane number is comparable to that of standard Diesel fuels and the reaction mechanism of n-heptane is well-understood. A simplified 4-step reaction mechanism for n-heptane has been implemented to reduce the computational cost. The code has been used to study the reacting mixing layer and the results have been compared with those reported in literature. This work shows that the DNS code is a powerful tool, especially to deal with problems involving the local and instantaneous flow variables and the composition of the gas mixture.