Micro-scale Mixing in Turbulent Constant Density Reacting Flows and Premixed Combustion

DNS datasets for inert and reactive scalars in statistically homogeneous and stationary turbulence of constant density fluid, and for turbulent premixed flames in an inflow-outflow configuration and in a jet are examined. The objective is to gain a better physical understanding of mixing to use in modeling molecular diffusion terms. Classical descriptions in term of either scalar fluctuation moments or pdf conservation equations can be related to transport of iso-scalar surfaces by turbulent convection and molecular diffusion. The kinematics and propagation speeds of iso-surfaces are theoretically analyzed. The effective normal strain rate, which combines flow and diffusion-reaction induced processes, emerges as an essential variable; scalar gradients and dissipation rates increase for negative effective normal strains and decrease for positive ones. It is argued that the characteristic mixing time should be proportional to the inverse of the effective normal strain rate. The latter displays drastically different behaviors for constant density turbulence, with predominantly positive values, and for flows with significant heat release, with mostly negative ones. The former turbulent flows cause scalars to become less dissipative as time progresses, whereas the latter increase scalar dissipation. Functional dependences of the molecular diffusion rate conditioned on composition are sought. It is suggested that the characteristic mixing time is determined by the different interrelated processes, mainly in the limits of very low and very high Karlovitz numbers.