Diffusion flames stability

D. Veynante, L. Vervisch, T. Poinsot, A. Linan, and G. R. Ruetsch, Triple flame structure and diffusion flame stabilization, Proceedings of the Summer Program, Center for Turbulent Research 55-73,1994. 

Alsairafi, A., Lee, S.T., and T ien, J.S., Modeling gravity effects on diffusion flames stabilized around a cylindrical wick saturated with liquid fuel. Combust. Sci. Technol., 176, 2165, 2004. 

If we can decrease the reaction ability of a polymer relative to an oxygen, the critical conditions of the diffusion flame stability would change these polymers would have a different combustibility. 

Investigations of the diffusion flame stabilization occurring at subsonic efflux of a round fuel stream in unperturbed air have proposed two mechanisms of flame blow-off. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

F. Takahashi and V. R. Katta, Further studies of the reaction kernel structure and stabilization of jet diffusion flames, Proc. Combust. Inst. 30 383-390, 2005. 

S. Ghosal and L. Vervisch, Stability diagram for lift-off and blowout of a round jet laminar diffusion flame. Combust. Flame 124 646-655,2001. 

Muniz, L. and Mungal, M. G., Instantaneous flame-stabilization velocities in Ufted-jet diffusion flames. Combust. Flame, 111, 16,1997. 

Buckmaster, J. and Peters, N., The infinite candle and its stability—a paradigm for flickering diffusion flames, Proc. Combust. Inst., 21, 1829, 1988. 

Robson, K. and Wilson, M.J.G., The stability of laminar diffusion flames of methane. Combust. Flame, 13, 626, 1969. 

Takahashi, R, Mizomoto, M., and Ikai, S., Structure of the stabilizing region of a laminar jet diffusion flame, /. Heat Transfer, 110,182,1988. 

Takahashi, R and Katta, V.R., Reaction kernel structure and stabilizing mechanisms of jet diffusion flames in microgravity, Proc. Combust. Inst., 29,2509, 2002. 

COSILAB Combustion Simulation Software is a set of commercial software tools for simulating a variety of laminar flames including unstrained, premixed freely propagating flames, unstrained, premixed burner-stabilized flames, strained premixed flames, strained diffusion flames, strained partially premixed flames cylindrical and spherical symmetrical flames. The code can simulate transient spherically expanding and converging flames, droplets and streams of droplets in flames, sprays, tubular flames, combustion and/or evaporation of single spherical drops of liquid fuel, reactions in plug flow and perfectly stirred reactors, and problems of reactive boundary layers, such as open or enclosed jet flames, or flames in a wall boundary layer. The codes were developed from RUN-1DL, described below, and are now maintained and distributed by SoftPredict. Refer to the website http //www.softpredict.com/cms/ softpredict-home.html for more information. 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

Diffusion Flame. When a slow stream of fuel g s flows from a tube into the atmosphere, air diffuses across the boundary of the stream and Brms an envelope of expl mixture around a core of gas. The core decreases in height until it disappears at some distance above the tube. It thus assumes the shape of a cone. On ignition, a flame front spreads thru the mixture and stabilizes itself around the cooe of fuel gas. The hydrocarbons in common fuel gases crack to form free C H. The shell of carbon-bearing gas so formed gives such flames their luminosity Turbulent Jet Flame. When a gas stream issues from an orifice above a certain critical velocity, it breaks up into a turbulent jet that entrains the surrounding air. The flame of such a jet consists of random patches of combustion and no cohesive combustion surface exists... 

Both imidogen (NH) and nitrous oxide (N20) may subsequently be oxidized to NO. Even though NNH, because of its low stability, never reaches significant concentrations, the NNH mechanism may contribute significantly to NO formation under certain conditions. It seems to be most important in diffusion flames, where NNH may form on the fuel-rich side of the flame sheet and then react with O inside the flame sheet [383]. 

Opposed-flow configurations can be used to establish strained premixed flames. Like the diffusion-flame situation, there are several ways to create the opposed flow, including opposed porous plates [197] or opposed contraction nozzles [349]. As illustrated in Fig. 17.8, two opposed contraction nozzles form a symmetric flow. When the mixture stoichiometry, temperature, and flow rates are equal in both nozzles, twin flames are stabilized near the center. 

JET. The diameters of most jets used today are roughly 0.5 mm. This is because of the type of flame used. In a diffusion flame, as opposed to a flame in which the oxidizer and fuel are premixed, the rate of diffusion of the two gases controls the rate of burning. The velocity of the gas flow out the jet must be set to match the rate of diffusion. If the jet diameter is a little different from 0.5 mm, the detector can be optimized at a proportionately different flow. Much beyond a factor of two differences in diameter leads to some other effects, such as thermal transfer or flow stability problems. 

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