Modeling the Transient Structure of Reacting Diesel Jets using Large Eddy Simulation

Accurate modeling of the transient structure of reacting diesel jets is important as transient features like autoignition, flame propagation, and flame stabilization have been shown to correlate with combustion efficiency and pollutant formation. In this work, the large eddy simulation (LES) technique is used to computationally model a lifted jet flame at conditions representative of those encountered in diesel engines. An unsteady flamelet progress variable (UFPV) model is used for turbulence/chemistry interactions. The UFPV model has been proposed for predicting the averaged/filtered chemistry source terms when modeling turbulent non-premixed combustion. In the model, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate χst, and progress variable Cst by solving the unsteady flamelet equations. In the present study, the progress variable is defined based on the sum of the major combustion products. A 37-species reduced chemical reaction mechanism for n-heptane is used to generate the UFPV libraries. The results show that ignition initiates at multiple points in the mixing layer around the jet, towards the edges of the jet, where the mixture fraction is rich, and the strain rates are within the ignition limits. These ignition kernels grow in time and merge to form a continuous flame front. Lift-off height is determined by the minimum axial distance from the orifice below which the local scalar dissipation rate does not favor ignition. The LES results are compared with Reynolds Averaged Navier-Stokes (RANS) simulation results from prior work. This comparison shows that though there are noticeable differences in the transient phenomena, lift-off heights predicted by both methods are within 25% and the predicted mechanism of lift-off is related to ignition in both cases.