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The Asymmetric Behavior of Steady Laminar Flame Propagation in Ducts

The steady propagation of a premixed flame in an isothermal duct subject to the Poiseuille flow is numerically investigated in the present study. The complete Navier-Stokes equations are used in the mathematical formulation and the flame chemistry is modeled by an one-step overall reaction. The numerical results show that the flame propagating steadily in ducts opposed to the Poiseuille flow can take two distinct shapes: a symmetric mushroom shape and an asymmetrical slant shape. For example, the symmetric mushroom-shaped flame can be found in a small duct, but only asymmetrical slant-shaped flame exists in a larger duct. However, with the symmetric flow assumption, the tulip-shaped flame instead of the asymmetric slant-shaped flame will appear. The behaviors of flame symmetry are examined as a function of the intensity of incoming velocity profile, duct width, duct geometry, and the gravity along the duct. It is found that the above parameters not only modify the flame burning rate but also affect the flame shape and its symmetrical behavior. The validity of a commonly used symmetric flow assumption is also explored. Predictions suggest that the slant-shaped flame is a more robust manifestation than tulip-shaped flame in sufficiently wide channels.