Numerical investigation of edge flame propagation characteristics in turbulent mixing layers

Abstract

The statistical behavior of the displacement speed of an edge flame originating from localized ignition of inhomogeneous reactants in decaying isotropic turbulence is studied based on three-dimensional direct numerical simulations (DNS) with simplified chemistry. The ignition is simulated by introducing a source term in the energy transport equation, which deposits a given amount of energy over a specified time duration. The flame structure is shown to exhibit a tribrachial structure where a diffusion flame is stabilized on the stoichiometric mixture fraction isosurface alongside two premixed flame branches on fuel-rich and fuel-lean mixtures. The speed at which the fuel mass fraction YF isosurface moves with respect to an initially coincident material surface at the point where these three flames meet (commonly referred to as triple point) is taken as the edge flame displacement speed. Its statistics are explained in terms of the reaction, normal diffusion, and tangential diffusion components of the density-weighted displacement speed Sd*. It is shown that the statistical behavior of Sd* and its components in response to the scalar dissipation rate and local flame curvature are strongly dependent on the statistical behavior of the fuel mass fraction gradient ∣∇YF∣ in response to local flame curvature and mixture fraction gradient ∣∇ξ∣. It is shown that the extent of premixed and nonpremixed contributions determines the ∣∇YF∣ variation with ∣∇ξ∣, whereas the flow acceleration due to heat release and dilatation field determines the curvature response of ∣∇YF∣. The variation of Sd* in response to scalar dissipation rate and curvature are shown to be non-monotonic in nature and is found to be consistent with previous results based on two-dimensional DNS with detailed chemistry and experimental studies. This nonmonotonic behavior has been shown here to exist even in the absence of detailed chemistry and is explained in terms of the fluid-dynamical aspects of flame-turbulence interaction.