Combustion diffusion flame

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

The formation of carbon black in a candle flame was the subject of a series of lectures in the 1860s by Michael Faraday at the Royal Institution in London (23). Faraday described the nature of the diffusion flame, the products of combustion, the decomposition of the paraffin wax to form hydrogen and carbon, the luminosity of the flame because of incandescent carbon particles, and the destmctive oxidation of the carbon by the air surrounding the flame. Since Faraday s time, many theories have been proposed to account for carbon formation in a diffusion flame, but controversy still exists regarding the mechanism (24). 

Flame Types and Their Characteristics. There are two main types of flames diffusion and premixed. In diffusion flames, the fuel and oxidant are separately introduced and the rate of the overall process is determined by the mixing rate. Examples of diffusion flames include the flames associated with candles, matches, gaseous fuel jets, oil sprays, and large fires, whether accidental or otherwise. In premixed flames, fuel and oxidant are mixed thoroughly prior to combustion. A fundamental understanding of both flame types and their stmcture involves the determination of the dimensions of the various zones in the flame and the temperature, velocity, and species concentrations throughout the system. 

The discussion of laminar diffusion flame theory addresses both the gaseous diffusion flames and the single-drop evaporation and combustion, as there are some similarities between gaseous and Hquid diffusion flame theories (2). A frequentiy used model of diffusion flames has been developed (34), and despite some of the restrictive assumptions of the model, it gives a good description of diffusion flame behavior. 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

Orifice flames can be cliaracterized as either prenii.xed or diffusion flames. In a prenii.xed flame, tlie air for combustion is already nii.xed witli the fuel gas before it leaves the orifice or pipe. In a diffusion flame, fuel e.xiting tlie orifice is piu c and the air needed for combustion diffuses into tlie fuel gas from the surroundings. Orifice flames can also be cliaracterized by tlie fame Reynolds number. Tlie flame lengtli of a diffusion flame can be calculated by Jost s equation. ... 

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). 

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. 

J. Kim and J. S. Kim, Modelling of lifted turbulent diffusion flames in a channel mixing layer by the flame hole dynamics. Combust. Theory Model. 10 21-37, 2006. 

S. R. Lee and J. S. Kim, On the sublimit solution branches of the stripe patterns formed in counterflow diffusion flames by diffusional-thermal instability. Combust. Theory Model. 6(2) 263-278,2002. 

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. 

A. Linan, E. Frenandez-Tarrrazo, M. Vera, and A. L. Sanchez, Lifted laminar jet diffusion flames. Combust. Sci. Technol. 177(5-6) 933-953,2005. 

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

Kempf, A., Flemming, F, and Janicka, J., Investigation of lengthscales, scalar dissipation, and flame orientation in a piloted diffusion flame by LES, Proc. Combust. Inst., 30, 557, 2005. 

Clemens, N.T., Paul, P.H., and Mungal, M.G., The structure of OH fields in high Reynolds number turbulent jet diffusion flames. Combust. Sci, Technol., 129,165,1997. 

Candle and Jet Diffusion Flames Mechanisms of Combustion under Gravity and Microgravity Conditions... 

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