| Direct numerical simulation (DNS) of initially nonpremixed reactants under homogeneous decaying velocity field has been used to study local extinction and reignition. Besides using Eulerian data, this research applies flame element tracking and investigates the time history of individual points (‘flame elements’) along the stoichiometric surface. This study identifies three major scenarios of reignition: independent flamelet scenario, reignition via edge (triple) flame propagation, and reignition through engulfment by a hot neighborhood. The results give insight into the role different scenarios play in the reignition process, reveal the physical processes associated with each scenario, and provide the relative frequency of reignition for each scenario. The data are used to investigate the possibility of using the flamelet and the conditional moment closure approximations for prediction of extinction and reignition. This study also compares a series of simulations associated with different values of kinematic viscosity and molecular diffusivities, but with approximately identical large-scale dynamics and initial scalar blob sizes. With increasing Reynolds number, extinction takes place faster and results in lower temperatures. Reignition, however, appears to take place approximately at similar rates in the different cases. Although, the amount of extinction increases as the kinematic viscosity is decreased, the increase appears to slow down with increasing Reynolds number values. |
