Origin and manifestation of flow-combustion interactions in a premixed shear layer

Abstract

The interactions between the flow field and the combustion process in a premixed shear layer are investigated using the results of numerical simulation. The reaction is governed by a finite-rate Arrhenius kinetics, the flow is compressible and at high Reynolds number, heat release is moderate and molecular heat and mass diffusivities are finite. The thickness of the reaction zone and that of the vorticity layer are approximately the same. Lagrangian simulations are obtained using the vortex and transport element methods. Results indicate that at the early stages, a reacting shear layer behaves like a laminar flame. During the growth of the roll-up eddy, the rate of burning is strongly enhanced by the entrainment fluxes that lead to the swelling of the reaction zone, and the total rate of product formation can be approximated by the unstrained laminar burning velocity times the flame length measured along the line of maximum reaction rate. Following the burning of the eddy core, the strain field along the eddy boundaries causes a noticeable thinning of the reaction zone and reduces the rate of burning. Baroclinic vorticity generation due to the acceleration of fluid elements in the density gradient is the most important mechanism by which combustion affects the flow field. It augments the overall volumetric entrainment into the eddy core, and causes an entrainment asymmetry with a bias towards the products. The generated vorticity extends the growth period of the eddy and imparts on it an extra mean convective motion.