A PRIORI DNS MODELING OF THE TURBULENT SCALAR FLUXES FOR LOW DAMKOHLER NUMBER STRATIFIED FLAMES

Statistically planar turbulent stratified flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios <phi > = 0.7 and <phi > = 1.0 using three-dimensional simplified chemistry-based direct numerical simulations (DNS). The simulation parameters are chosen in such a manner that all the cases considered here represent low Damkohler number thin reaction zones regime combustion. The DNS data have been used to analyze the statistical behaviors of the turbulent fluxes of fuel mass fraction Y-F and mixture fraction xi, and their variances and covariances. It has been found that turbulent scalar flux of fuel mass fraction predominantly exhibits gradient-type transport in all cases considered in the current study, but countergradient-type transport has also been observed in the globally stoichiometric flame for small values of turbulence intensity where the flame normal acceleration due to chemical heat release overcomes the effects of turbulent velocity fluctuation. Models have been identified for the turbulent fluxes of fuel mass fraction YF and mixture fraction xi, and their variances and covariances, which are shown to satisfactorily capture the qualitative and quantitative behaviors of the corresponding quantities extracted from DNS data. It has been shown that the accurate modeling of the scalar fluxes of fuel mass fraction and mixture fraction plays a key role in the modeling of scalar fluxes of the fuel mass fraction variance and the covariance of fuel mass fraction and mixture fraction.