EFFECT OF NONUNITY LEWIS NUMBER ON PREMIXED FLAME PROPAGATION THROUGHISOTROPIC TURBULENCE

Direct numerical simulations of a passive premixed flame surface propa gating through stationary isotropic turbulence have been performed in three dimensions on a 96(3) mesh with a particular emphasis on charact erizing the effect of Lewis number on the rate of propagation of the f lame surface and flame surface topology. The simulations were based on the flame sheet assumption that implies that the time scale for chemi cal reaction is short as compared with the time scales for the turbule nt fluctuations (so-called flamelet regime). In this limit, the flame surface can be represented by a field equation (Sivashinsky equation) which accounts for local advection of the reaction front due to instan taneous velocity fluctuations and propagation due to reaction. The Nav ier Stokes equations and scalar field equation for the flame surface w ere updated using a pseudo-spectral method with fourth order accuracy in time. Lewis number effects were incorporated into the simulations b y using a modified Sivashinsky equation for the flame surface. At zero turbulence level, the simulation yielded the familiar steady and nons teady cellular structures seen previously by several investigators. At finite turbulence levels the flame speed was augmented above the cell ular flame speed due to additional wrinkling from the hydrodynamic fie ld. Comparisons with experimental measurements in the literature agree with the simulations to within 30% over a wide range of turbulence in tensities and Lewis numbers. In addition spectral analysis of the Siva shinsky equation provides insight into the effect of varying the Lewis number and how that effect may be incorporated into a relatively simp le spectral model for the flame surface.