Large Eddy Simulation of a Reacting Spray Flame under Diesel Engine Conditions

Reynolds-averaged Navier-Stokes (RANS) turbulence model has been used extensively for diesel engine simulations due to its computational efficiency and is expected to remain the workhorse computational fluid dynamics (CFD) tool for industry in the near future. Alternatively, large eddy simulations (LES) can potentially deal with complex flows and cover a large disparity of turbulence length scales, which makes this technique more and more attractive in the engine community. An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a dynamic structure LES model to understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was closed using a delta probability density function (PDF) combustion model. A 103-species skeletal mechanism was used for n-dodecane chemical kinetic model. Significantly different flame structure and ignition processes are observed for the LES compared to those of RANS predictions. LES was observed to have multiple ignition spots in the mixing layer simultaneously while the main ignition initiates in a clearly asymmetric fashion. Multiple realizations for LES were performed to understand the realization to realization variation and to establish best practices for ensemble -averaging diesel spray flames. The relevance index analysis suggests that an average of 5 realizations can reach 99% of similarity to the target average of 16 realizations on the mixture fraction field.