Intrinsic characteristics of asymmetric edge flames: Effects of stoichiometry on edge speed and temperature

Edge flames stabilized in the wake of two merging streams of uniform and equal initial strain rates, one containing fuel and the other oxidizer, are examined within the constant-density diffusive-thermal framework. This configuration models, for example, the stabilization of jet diffusion flames where the edge flame at their base plays a dominant role. The focus is on the effects of stoichiometry, characterized by the initial mixture strength ϕ, on the structure and dynamic properties of the edge flames. Conditions have been specified such that the edge flame when ϕ=1 remains symmetric with respect to the centerline separating the two streams. The Lewis numbers of the fuel and oxidizer are assumed equal, with the common value Le>1, such that the temperature along the diffusion flame remains uniform but considerably lower than the adiabatic flame temperature. The fuel and oxidizer are assumed to play symmetric roles such that it suffices to consider the case ϕ≥1. It is shown that when increasing ϕ, the entire flame leans towards the oxidizer region; the fuel-rich branch of the typical tribrachial edge structure is retained, while for sufficiently large ϕ the fuel-lean branch merges with the trailing diffusion flame giving the entire flame a hook-like appearance. Meanwhile, the edge location moves away from the stoichiometric surface toward the fuel-rich branch and, being less affected by the heat transferred to the diffusion flame, its temperature increases with increasing ϕ. The propagation velocity of the highly asymmetric edge structure is defined to be balanced by the local nonuniform flow at the edge location. To this end, a new methodology has been developed to determine the direction normal to the edge flame, from which both the edge speed and temperature are evaluated. The mechanism accounting for the dependence of these two intrinsic characteristics on stoichiometry is identified.