Interactions between turbulence and flames in premixed reacting flows

Abstract

The interactions between turbulence and flames in premixed reacting flows are studied for a broad range of turbulence intensities by analyzing scalar (reactant mass-fraction) gradient, vorticity, and strain rate fields. The analysis is based on fully compressible, three-dimensional numerical simulations of H2-air combustion in an unconfined domain. For low turbulence intensities, a flame reconstruction method based on the scalar gradient shows that the internal flame structure is similar to that of a laminar flame, while the magnitudes of the vorticity and strain rate are suppressed by heat release and there is substantial anisotropy in the orientation of intense vortical structures. As the turbulence intensity increases, the local flame orientation becomes increasingly isotropic, and the flame preheat zone is substantially broadened. There is, however, relatively little broadening of the reaction zone, even for high intensities. At high turbulence intensities, the vorticity and strain rate are only weakly affected by the flame, and their interactions with the scalar gradient are similar to those found in nonreacting turbulence. A decomposition of the total strain rate into components due to turbulence and the flame shows that vorticity suppression depends on the relative alignment between vorticity and the flame surface normal. This effect is used to explain the anisotropy of intense vortices at low intensities. The decomposition also reveals the separate effects of turbulent and dilatational straining on the flame width.