Premixed edge flames

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]. 

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

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. 

Figure 4.1.2 is a photograph of a coimterflow burner assembly. The experimental particle paths in this cold, nonreacting, counterflow stagnation flow can be visualized by the illumination of a laser sheet. The flow is seeded by submicron droplets of a silicone fluid (poly-dimethylsiloxane) with a viscosity of 50 centistokes and density of 970 kg/m, produced by a nebulizer. The well-defined stagnation-point flow is quite evident. A direct photograph of the coimterflow, premixed, twin flames established in this burner system is shown in Figure 4.1.3. It can be observed that despite the edge effects.

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). 

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]. 

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. 

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

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. 

A flame edge can be defined as the boundary between the burning and the nonbuming states along the tangential direction on a flame surface, which could exist in both premixed and nonpremixed flames [1,2]. The base of a nozzle-attached flame, either premixed or nonpremixed systems, is a typical example. 

When a premixed or a nonpremixed flame has an edge, fhe neighborhood of edge exhibifs premixed or... 

R. Daou, J. Daou, and J. Dold, The effect of heat loss on flame edges in a no-premixed counterflow within a thermo-diffusive model, Combust. Theory Model. 8(4) 683-699, 2004. 

R. W. Thatcher and J. W. Dold, Edges of flames that do not exit Flame-edge dynamics in a non-premixed counterflow, Combust. Theory Model. 4 435-457, 2000. 

In this method premixed gases flow up a jacketed cylindrical tube long enough to ensure streamline flow at the mouth. The gas bums at the mouth of the tube, and the shape of the Bunsen cone is recorded and measured by various means and in various ways. When shaped nozzles are used instead of long tubes, the flow is uniform instead of parabolic and the cone has straight edges. Because of the complicated flame surface, the different procedures used for measuring the flame cone have led to different results. 

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