Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented.

Large and smallscale stirring of vorticity and passive scalar in a 3D temporal mixing layer

Using direct numerical simulations of homogeneous isotropic turbulence, the temporal evolution of small-scale vorticity and passive scalar structures has been examined. Visualization of small-scale vorticity structures indicates a process where vortex sheets may roll up into intense vortex tubes through a Kelvin–Helmholtz-type instability. The ability of these vortex structures to mix a passive scalar has also been examined. The regions of intense scalar gradient form sheets which are found to occur in regions of persistent straining flow, such as between neighboring vortex structures of approximately equal circulation. The magnitude of vorticity of these structures is usually not large, as persistent straining is most commonly found in connection with regions of moderate vorticity. Scalar gradient sheets are also found between low intensity regions of antiparallel vorticity, which are also observed to produce persistent straining.