Structure of nonpremixed reaction zones in numerical isotropic turbulence

Results from direct numerical simulation of low heat release, turbulent nonpremixed reacting flows modeled using single-step reactions with constant and temperature-dependent rate laws are discussed, and compared with laminar predictions. The mixture fraction and its dissipation rate are statistically independent in regions of intense reaction, partially supporting a commonly made assumption in flamelet-based models. In the presence of a finite rate reverse reaction, the reaction zone spans the entire range of mixture fraction. The joint pdf of the reactive scalars evolves to an equilibrium that is dictated by a balance between the reactive and mixing fluxes in composition space. When the temperature-dependent rate law is implemented, strain-induced extinction is observed for a Zel'dovich (modified) number of 10. As the ratio of local flow to chemical time scale is decreased below unity, a larger fraction of the flow field experiences this mode of extinction. The critical turbulent scalar dissipation rate is compared with laminar values and asymptotic predictions.The first two authors express their acknowledgment to the donors of The Petroleum Research Fund, administered by the American Chemical Society for partial support of this work through a type G grant. We also acknowledge the Council for Research and Creative Writing at the University of Colorado for providing partial financial support.