INTERACTIONS OF TURBULENCE AND SCALARS IN SHEAR-DRIVEN PREMIXED TURBULENT FLAMES USING DNS

The DNS database of modelling lean H2/air combustion in a temporally evolving premixed slot-jet configuration is employed to investigate turbulence-scalar interactions in turbulent reacting flows. In contrast to previous DNS studies of turbulent premixed flames, a mean shear exists in the flow and drives the generation of small-scale turbulence in the shear layer. The orientations of the flame normal and vorticity are examined. It is found that the flame normal preferentially aligns with the most compressive strain rate of the mean flow and the vorticity preferentially aligns with the most extensive mean strain rate. The former is consistent with the fact that the flame front has a tendency to align with the extensive strain rate, while the latter is related to vortex stretching in turbulent flows. The alignment characteristics of the flame normal, vorticity, and turbulent strain rate field conditioned on various progress variable iso-surfaces are investigated. Their influence on the scalar gradients and the vorticity production reflecting the turbulence-scalar interactions are examined quantitatively. INTRODUCTION Premixed turbulent combustion is commonly encountered in practical combustion devices such as gas turbines, and spark-ignition engines. An improved understanding of the interactions between turbulence and chemistry is of great importance for turbulent combustion modelling as well as design of clean and efficient combustion devices. In the present paper, turbulenceflame interactions are studied by analysing the coupled dynamics of the scalar gradient, strain rate and vorticity. The directional preference of the scalar gradients with respect to the strain rate is of substantial importance for modelling premixed turbulent combustion. Specifically, the so-called scalar-turbulence interaction signified by – ρNc(niSijnj) is a source term in the transport equation for the scalar dissipation rate of the progress variable Nc. In the expression signified by –ρNc(niSijnj), ρ is the density, D is the diffusivity, ni is the flame normal component and Sij is the strain rate tensor defined as Sij = 0.5(∂ui/∂xj+∂uj/∂xi). The strain rate Sij can be characterised by the principal eigenvalues λ1, λ2, and λ3 designated by the convention λ1 ≥ λ2 ≥ λ3, which are determined from the characteristic equation. The corresponding eigenvectors are e1, e2 and e3, respectively. The flame normal n is defined in terms of the progress variable c: