Extinction and Re-Ignition in Non-Premixed Flame-Vortex Interactions under Diesel Conditions

In this work, 2-D numerical investigations of extinction and re-ignition during interactions of counter-rotating vortex-pairs with a non-premixed flame are carried out under pressures and temperatures encountered in Diesel chambers. A direct-numerical simulation (DNS) code which employs sixth-order spatial accuracy and fourth-order time-integration is employed to solve the Navier-Stokes equations with chemical reactions. N-heptane is chosen as a representative fuel and irreversible single-step and two-step kinetic models are employed. Several representative values for the vortex circulation and length-scale are chosen from the analysis of the near-field of a Diesel jet. Results show that due to the vortex-induced strain, local extinction occurs along the symmetry axis. This extinction is predicted well by 1-D laminar flamelet libraries provided the time-history of the scalar dissipation rate is taken into account. The flame-edges resulting from local extinction interact leading to re-ignition as the initial vortex size approaches the initial flame thickness. This re-ignition phenomenon is 2-D, involves a partially-premixed flame structure and is not predicted by 1-D diffusion flamelet libraries. The results are summarized in terms of a regime diagram constructed from non-dimensional parameters such as the length-scale ratio and the Damkihler number.