Numerical simulations of flame dynamics in the near-field of high-Reynolds number jets

Recent experiments in diesel jet flames show that flame lift-off has a significant influence on pollutant formation. Lift-off occurs in the near-field of the jet, which is characterized by complex interactions between turbulence and chemistry. Commonly employed modeling approaches based on Reynolds-averaged Navier-Stokes (RANS) simulations are limited in their capability to predict transient and steady lift-off phenomena, as they ignore effects due to unsteadiness and curvature that are inherent in the near-field. In the present work, we perform numerical investigations of localized flame dynamics in the near-field (x/d \u3c 25) of high-Reynolds number (Re) jets encountered in diesel engine applications. The primary focus is on the exploration of unsteady extinction/reignition phenomena. A dual approach involving large-eddy simulation (LES) of a 70,000-Re variable-density isothermal gaseous fuel jet, and studies of flame-vortex interactions and unsteady flamelets, under diesel engine conditions, is employed in this work. Results from flame-vortex interaction studies show that in the near-field (x/d \u3c 25) of the 70,000-Re jet at radial locations close to the centerline (r/d \u3c 1.5), unsteady extinction/reignition phenomena characterize the flame dynamics. Extinction is minimally affected by the vortex-induced curvature, but involves strong unsteady effects leading to extinction limits higher than steady values. On the other hand, reignition is governed by curvature effects, and occurs through the dynamics of edge flames. In the simulated jet near-field, unsteady effects are observed to diminish with increasing axial locations due to relatively weaker vortices, whereas curvature effects increase due to the presence of relatively thicker flames. The unsteady flamelet studies show that in stoichiometric regions at radial locations relatively far from the centerline (r/d \u3e 1.0) in the jet near-field,temporary flame weakening/recovery events are likely to occur. Steady flamelet models provide reasonable estimates of the mean temperature, and mean mass fractions of the major species and unburned hydrocarbons (UHCs), but are inadequate for the prediction of mean NO mass fractions. Extrapolation of the analysis to jets with higher global strain rates shows that unsteady effects on the localized flame dynamics are important for the prediction of transient and steady lift-off behavior