  Three-dimensional laminar flow structures with mixing, chemical reaction,
normal strain, and shear strain qualitatively representative of turbulent
combustion at the small scales are analyzed. A mixing layer is subjected to
counterflow in the transverse y- and z-directions. Both non-reactive and
reactive flows are examined. Reduction of the three-dimensional boundary-layer
equations to a one-dimensional similar form is obtained allowing for heat and
mass diffusion with variations in density and properties. In steady
configurations, a set of ODEs governs the three velocity components as well as
the scalar-field variables. A flamelet model for individual diffusion flames
with combined shear and normal strain is developed. Another model with solution
in similar form is obtained for a configuration with a dominant diffusion flame
and a weaker fuel-rich premixed flame. Results for the velocity and scalar
fields are found for ranges of Damkohler number Da, normal strain rate due to
the counterflow, streamwise-velocity ratio across the mixing layer, Prandtl
number, and Mach number. For the flamelet model, a conserved scalar is cast as
the independent variable to give an alternative description of the results. The
imposed normal strain decreases mixing-layer thickness and increases scalar
gradients and transport rates. There is indication of diffusion control for
partially premixed flames in the multi-branched flame situation. The
enhancement of the mixing and combustion rates by imposed normal strain on a
shear layer can be very substantial. Also, the imposition of shear strain and
thereby vorticity on the counterflow can be substantial indicating the need for
flamelet models with both shear strain and normal strain.
