Karlovitz number effects on velocity and scalar statistics in turbulent premixed combustion

Velocity and scalar statistics are obtained from a series of Direct Numerical Simulations of low Mach number spatially-evolving turbulent premixed planar jet flames. In these simulations, the bulk Reynolds number and stoichiometric equivalence ratio are fixed, and the Karlovitz number is adjusted by varying the global strain rate. Hydrogen combustion with detailed transport is modeled using a detailed nine-species chemical kinetic mechanism, and coflows of combustion products are used to ensure flame stability at uniform equivalence ratio. From these simulations, velocity and scalar statistics are obtained conditioned on progress variable. The analysis of these statistics focuses on two key areas. First, theoretical arguments indicate that pressure-dilatation work becomes a significant source of turbulent kinetic energy in low Karlovitz number flames, which has been confirmed previously. In this work, the relative importance of the pressure-dilatation term versus other terms in the turbulent kinetic energy budget is expected to decrease with increasing Karlovitz number, and this will be directly assessed with the DNS databases. Second, the degree of alignment between the Reynolds stresses (scalar fluxes) and the strain rate (scalar gradient), the foundation of a majority of the turbulence models used in reacting flows, is expected to depend on Karlovitz number and will be assessed with the DNS databases.