Intermittency in premixed turbulent reacting flows

Abstract

Intermittency in premixed reacting flows is studied using numerical simulations of premixed flames at a range of turbulence intensities. The flames are modeled using a simplified reaction mechanism that represents a stoichiometric H2-air mixture. Intermittency is associated with high probabilities of large fluctuations in flow quantities, and these fluctuations can have substantial effects on the evolution and structure of premixed flames. Intermittency is characterized here using probability density functions (pdfs) and moments of the local enstrophy, pseudo-dissipation rate (strain rate magnitude), and scalar (reactant mass fraction) dissipation rate. Simulations of homogeneous isotropic turbulence with a nonreacting passive scalar are also carried out in order to provide a baseline for analyzing the reacting flow results. In the reacting flow simulations, conditional analyses based on local, instantaneous values of the scalar are used to study variations in the pdfs, moments, and intermittency through the flame. For low intensities, pdfs of the local enstrophy vary substantially through the flame, with greater intermittency near the products. Changes in the pseudo-dissipation pdfs are, however, less pronounced. As the intensity increases, both the enstrophy and pseudo-dissipation pdfs become increasingly independent of position in the flame and are similar to results from the nonreacting simulations. The scalar dissipation intermittency is largest near the reactants and increases at all flame locations with increasing turbulence intensity. For low intensities and in the reaction zone, however, scalar dissipation pdfs approximately follow a Gaussian distribution, indicative of substantially reduced intermittency. Deviations from log-normality are observed in the pdfs of all quantities, even for intensities and flame locations characterized by strong intermittency. The implications of these results for the internal structure of the flame are discussed, and we also propose a connection between reacting flow intermittency and anisotropic vorticity suppression by the flame.