||Flame behaviors and stability limits of the two-dimensional steady premixed flame in one-step tubes are investigated numerically in this study. In this study, the complete Navier-Stokes equations are used in the mathematical formulation and the flame chemistry is modeled by a simplified four-step reduced reaction mechanism of methane-air mixture. The steady premixed flames in the one-step tubes are studied. Parametric investigations are carried out to understand the effect of average inflow velocity, mixture equivalence ratio, expansion ratio of the tube and wall-temperature on reattachment length, flame position, flame shape, distribution of gas mole fraction and flame stability limits.|
It was observed that the recirculation zone, created due to sudden flow expansion, at the backward step significantly modifies the flow velocity profile and directly changes the combustion reaction mechanism, however the flame position will indirectly affect the distribution of the flow field. The inflow velocity is varied so as to cover all possible flame positions inside the domain. Three expansion ratio, 1.67, 2.5, 5, have been considered. Three flame behaviors have been observed when ER is 5, which are mushroom-shaped flame, tulip-shaped flame and asymmetrical tulip-shaped flame. Four flame behaviors have been observed when ER is 2.5, which are mushroom-shaped flame, tulip-shaped flame, asymmetrical tulip-shaped flame and slant-shaped flame. Five flame behaviors have been observed when ER is 1.67, which are weak flame, mushroom-shaped flame, tulip-shaped flame, asymmetrical tulip-shaped flame and slant-shaped flame.
Results show that the lower the wall-temperature and the mixture equivalence ratio, the smaller the flame stability limits range are, the fewer the types of flame shape are and the larger the increments of flame displacement become, and vice versa.