Combustion with Multiple Flames under High Strain Rates

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.

KEYWORDS:

• flamelet theory
• three-dimensional counterflow combustion
• multi-branched flames

Nomenclature

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Disclosure statement

No potential conflict of interest was reported by the author.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This research was supported by the Air Force Office of Scientific Research under Grant FA9550-18-1-0392 with Dr Mitat Birkan as the scientific officer.