Edge flaming

Role of Edge Flames for Lifted Flame Stabilization.61... 

Relevance of Nonpremixed Edge Flames to Turbulent Flames.62... 

Observed premixed edge flames (a) Bunsen flame-tip opening, (b) propagating premixed flame in tube (From Jarosinski, J., Strehlow, R.A., and Azarbarzin, A., Proc. Combust. Inst., 19, 1549, 1982. With permission.), (c) slanted counterflow flame (From Liu, J.-B. and Ronney, P.D., Combust. Sci. Tech., 144,21,1999. With permission.), and (d) spinning premixed flames in sudden expansion tube [7]. 

Nonpremixed edge flames (a) 2D mixing layer (From Kioni, P.N., Rogg, B., Bray, K.N.C., and Linan, A., Combust. Flame, 95, 276, 1993. With permission.), (b) laminar jet (From Chung, S.H. and Lee, B.J., Combust. Flame, 86, 62,1991.), (c) flame spread (From Miller, F.J., Easton, J.W., Marchese, A.J., and Ross, H.D., Proc. Combust. Inst., 29, 2561, 2002. With permission.), (d) autoignition front (From Vervisch, L. and Poinsot, T., Annu. Rev. Fluid Mech., 30, 655, 1998. With permission.), and (e) spiral flame in von Karman swirling flow (From Nayagam, V. and Williams, F.A., Combust. Sci. Tech., 176, 2125, 2004. With permission.). (LPF lean premixed flame, RPF rich premixed flame, DF diffusion flame). 

In the following, the propagation characteristics of edge flames will be discussed together with the significance of edge flames in the stabilization of lifted flames in jets and turbulent flames. 

If the strain rate exerted on an edge flame becomes very small, especially for weak intensity flames, the radiative heat loss could influence the edge behavior significantly. Consequently, the propagation speed could decrease appreciably in low-strained flames [29], as marked in fhe dash-dot line. 

Propagation speed of nonpremixed edge flames of propane with concentration gradient. (From Kim, M.K., Won, S.H., and Chung, S.H., Pwc. Combust. Inst., 31, 901, 2007.)... 

Various factors affecting the nonpremixed edge speed, such as flame stretch, preferential diffusion, and heat loss, have also been investigated, including cellular and oscillatory instabilities of edge flames [1,39 3]. 

The existence of tribrachial structure at the base of lifted flame implies that the stabilization of liftoff flames is controlled by the characteristics of tribrachial edge flames. The coexistence of three different types of flames dictates that the edge is located along the stoichiometric contour [52] and the premixed wings have the propagation characteristics, whose speed should balance with the local flow velocity for the edge to be stationary. In the first approximation, the propagation speed was assumed to be constant [10]. 

The relevance of nonpremixed edge flames to turbulent nonpremixed flames can be described in two aspects. One is the mechanism of turbulent nonpremixed lifted flames and the other, the flame-hole dynamics. For turbulent lifted flames in nonpremixed jets, the liftoff height is linearly dependent on jet velocity. There have... 

Furthermore, analytical solutions of premixed edges were studied and the dependence of edge speed with the Damkohler number was reported [61], in which the edge speed ranged from positive to negative values, in a similar fashion as nonpremixed edge flames. 

The relevance of premixed edge flames to turbulent premixed flames can also be understood in parallel to the nonpremixed cases. In the laminar flamelet regime, turbulent premixed flames can be viewed as an ensemble of premixed flamelets, in which the premixed edge flames can have quenching holes by local high strain-rate or preferential diffusion, corresponding to the broken sheet regime [58]. 

J. Buckmaster, Edge-flames, Prog. Energy Combust. Sci. 28 435-475, 2002. 

J. -B. Liu and R D. Rormey, Premixed edge-flames in spatially-varying straining flows. Combust. Sci. Tech. 144 21-45,1999. 

V. Nayagam and F. A. Williams, Curvature effects on edge-flame propagation in the premixed-flame regime, Combust. Sci. Tech. 176 2125-2142,2004. 

V. S. Santoro, A. Linan, and A. Gomez, Progagation of edge flames in counterflow mixing layers Experiments and theory, Proc. Combust. Inst. 28 2039-2046, 2000. 

T. Vedarajan and J. Buckmaster, Edge-flames in homogeneous mixtures. Combust. Flame 114 267-273, 1998. 

Schematic of premixed edge flames in a counterflow burner (a) Mean velocity of the inner tube is greater than that of the outer tube, creating a stretch-induced edge flame, (b) Equivalent ratio of the mixture in the inner tube is different from that in the outer tube, creating a stratification-induced edge flame.

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

Takita, K., Sado, M., Masuya, G., and Sakaguchi, S., Experimental study of premixed single edge-flame in a counterflow field. Combust. Flame, 136, 364, 2004. 

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