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Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensities are simulated for global equivalence ratios < φ >= 0.7 and < φ >= 1.0 using three-dimensional Direct Numerical Simulations (DNS) with simplified chemistry. For the simulations of partially premixed flames, a random distribution of equivalence ratio following a bimodal distribution of equivalence ratio is introduced in the unburned reactants ahead of the flame. The simulation parameters in all of the cases were chosen such that the combustion situation belongs to the thin reaction zones regime. The DNS data has been used to analyse the behaviour of the dissipation rate transports of both active and passive scalars (i.e. the fuel mass fraction Y F and the mixture fraction ξ ) in the context of Reynolds Averaged Navier-Stokes (RANS) simulations. The behaviours of the unclosed terms of the Favre averaged scalar dissipation rates of fuel mass fraction and mixture fraction (i.e. ε Y = ρ D∇Y F · ∇Y F /ρ and ε ξ = ρ D∇ξ · ∇ξ /ρ) transport equations have been analysed in detail. In the case of the ε Y transport, it has been observed that the turbulent transport term of scalar dissipation rate remains small throughout the flame brush whereas the terms due to density variation, scalar-turbulence interaction, reaction rate and molecular dissipation remain the leading order contributors. The term arising due to density variation remains positive throughout the flame brush and the combined contribution of the reaction and molecular dissipation to the ε Y transport remains negative throughout the flame brush in all cases. However, the behaviour of scalar-turbulence interaction term of the ε Y transport equation is significantly affected by the relative strengths of turbulent straining and the straining due to chemical heat release. In the case of the ε ξ transport, the turbulent transport term remains small throughout the flame brush and the density variation term is 2 Flow Turbulence Combust (2011) 86:1-44 found to be negligible in all cases, whilst the reaction rate term is exactly zero. The scalar-turbulence interaction term and molecular dissipation term remain the leading order contributors to the ε ξ transport throughout the flame brush in all cases that have been analysed in the present study. Performances of existing models for the unclosed terms of the transport equations of ε Y and ε ξ are assessed with respect to the corresponding quantities obtained from DNS data. Based on this exercise either suitable models have been identified or new models have been proposed for the accurate closure of the unclosed terms of both ε Y and ε ξ transport equations in the context of Reynolds Averaged Navier-Stokes (RANS) simulations.

The scalar gradient alignment statistics for active and passive scalar gradients (e.g., fuel mass fraction gradient and mixture fraction gradient) are presented here for turbulent stratified flames. It has been shown that, under some conditions, the active scalar gradient may align with the most extensive principal strain rate in contrast to the passive scalar gradient that remains aligned with the most compressive principal strain rate even when the active scalar gradient aligns preferentially with the most extensive principal strain rate. Physical explanations are provided for the observed behaviors.