Combustion with Multiple Flames under High Strain Rates

Abstract

ABSTRACT Structures with multiple flames, nonpremixed and premixed, in strained flow representative of turbulent combustion are analyzed with reacting, viscous, three-dimensional counterflows. There can be differing normal strain rates in each of the three directions. Special attention is given to configurations with a diffusion flame plus one or two partially premixed flames. Reduction of the equations to a similar form is obtained allowing for variations in density due to temperature, heat conduction, and mass diffusion. Solutions to the Navier-Stokes equations are obtained without the boundary-layer approximation. In steady, variable-density configurations, a set of ODEs governs the two transverse velocity components and the axial velocity component, as well as the scalar-field variables. Results for the velocity and scalar fields are found for a full range of the distribution of normal strain rates between the two transverse directions, a very wide range of Damköhler number Da based on strain rate, and various Prandtl number values. As strain rate is increased and/or pressure decreases, the multiple flames will merge into a single diffusion flame. With further changes in that direction, extinction will occur. In the three-flame case, the fuel-rich premixed flame merges first with the diffusion flame as Da decreases; then with further decrease, the fuel-lean premixed flame merges with the diffusion flame, leaving just a diffusion flame that will extinguish with further decrease. Velocity overshoots are seen in the viscous layer, yielding an important correction of theories based on a constant-density assumption. Advection and convection are important in the neighborhood of the flames.