Turbulent diffusion flames

In practical combustion systems, the predominant mode of heat transfer is usually not molecular conduction, but turbulent diffusion, except at the boundaries and the flame front. Conduction is the only mode of heat transfer through refractory walls, and it determines ignition and extinction behaviors of the flame. Turbulent diffusion, an apparent or pseudo conduction mechanism arising from turbulent eddy motions, will be discussed in Section 4.4. The relations from the theory of conduction heat transfer15-17 can be used to evaluate heat losses through furnace walls and load zones, and through the pipe walls inside boilers and heat exchangers, etc. 

Validation and Application. VaUdated CFD examples are emerging (30) as are examples of limitations and misappHcations (31). ReaUsm depends on the adequacy of the physical and chemical representations, the scale of resolution for the appHcation, numerical accuracy of the solution algorithms, and skills appHed in execution. Data are available on performance characteristics of industrial furnaces and gas turbines systems operating with turbulent diffusion flames have been studied for simple two-dimensional geometries and selected conditions (32). Turbulent diffusion flames are produced when fuel and air are injected separately into the reactor. Second-order and infinitely fast reactions coupled with mixing have been analyzed with the k—Z model to describe the macromixing process. 

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). 

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). 

The various studies attempting to increase our understanding of turbulent flows comprise five classes moment methods disregarding probabiUty density functions, approximation of probabiUty density functions using moments, calculation of evolution of probabiUty density functions, perturbation methods beginning with known stmctures, and methods identifying coherent stmctures. For a thorough review of turbulent diffusion flames see References 41—48. 

When gas concentrations are high, burning is characterized by the presence of a tall, turbulent-diffusion, flame plume. At points where the cloud s vapor had already mixed sufficiently with air, the vertical depth of the visible burning zone is about equal to the initial, visible depth of the cloud. 

T. Baron, "Reactions in Turbulent Free Jets-The Turbulent Diffusion Flame," Cbem. Eng. Prog., 50, 73 (1954),... 

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. 

In the so-called "wrinkled flame regime," the "turbulent flame speed" was expected to be controlled by a characteristic value of the turbulent fluctuations of velocity u rather than by chemistry and molecular diffusivities. Shchelkin [2] was the first to propose the law St/Sl= (1 + A u /Si) ), where A is a universal constant and Sl the laminar flame velocity of propagation. For the other limiting regime, called "distributed combustion," Summerfield [4] inferred that if the turbulent diffusivity simply replaces the molecular one, then the turbulent flame speed is proportional to the laminar flame speed but multiplied by the square root of the turbulence Reynolds number Re. ... 

Complex strain flame-front regime. Where the flame fronfs are still lamella-like but thickened due to enhanced turbulent diffusivity. Scalar transport is expected to be counter-gradient in this regime. 

Blake, T. R., Gas Jets in Fluidized Media, Turbulent Diffusion Flames, and Condensing Vapor Jets in Liquids, Powder Technology (1996)... 

Although fire mainly involves the study and consequences of diffusion flames, premixed flames are important precursors. In order to initiate a diffusion flame, we must first have a premixed flame. In regions where a diffusion flame is near a cold wall, we are likely to have an intermediary premixed flame. Even in a turbulent diffusion flame, some state of a premixed flame must exist (see Figure 4.1). 

Smith, D.A. and Cox, G., Major chemical species in buoyant turbulent diffusion flames, Combustion and Flame, 1992, 91, 226-38. 

Hasemi, Y. and Tokunaga, T., Flame geometry effects on the buoyant plumes from turbulent diffusion flames, Combust. Sci. Technol., 1984, 40, 1-17. 

Heskestad, G., Luminous heights of turbulent diffusion flames, Fire Safety J., 1983, 5, 103-8. 

Jet or Spray Fires - Are turbulent diffusion flames resulting from the combustion of a liquid or gas continuously released under pressure in a particular direction. 

Bilger, R. W. (1982). Molecular transport effects in turbulent diffusion flames at moderate Reynolds number. AIAA Journal 20, 962-970. 

Turbulent diffusion flames. Annual Reviews of Fluid Mechanics 21, 101-135. 

Mobus, H., P. Gerlinger, and D. Briiggemann (1999). Monte Carlo PDF simulation of compressible turbulent diffusion flames using detailed chemical kinetics. In Paper 99-0198, AIAA. 

Modeling of extinction in turbulent diffusion flames by the velocity-dissipation-composition PDF method. Combustion and Flame 100, 211-220. 

This expression reveals that the height of a turbulent diffusion flame is proportional to the port radius (or diameter) above, irrespective of the volumetric fuel flow rate or fuel velocity issuing from the burner This important practical conclusion has been verified by many investigators. 

The Merryman-Levy sequence could explain the experimental results that show high N02/N0 ratios. For the experiments in which these high ratios were found, it is quite possible that reaction (8.85) is quenched, in which case the N02 is not reduced. Cemansky and Sawyer [29], in experiments with turbulent diffusion flames, also concluded that the high levels of N02 found were due to the reactions of NO with H02 and O atoms. 

Heskestad, G. 1981. Peak Gas Velocities, and Flame Heights of Buoyancy-Controlled Turbulent Diffusion Flames. Eighteenth Symposium on Combustion. The Combustion Institute, Pittsburgh, PA. 

Heskestad, G. 1983. Luminous Height of Turbulent Diffusion Flames. Fire Safety Journal, Volume 5, No. 2. 

DesJardin, P. E., M. J. Zimberg, and S. H. Prankel. 1997. Towards large eddy simulation of strongly radiating turbulent diffusion flames. In Advanced computation and analysis of combustion. Ed. G. D. Roy, S. M. Frolov, and P. Givi. Moscow, Russia ENAS Publ. 503-19. 

Sivathanu, Y., and G.M. Faeth. 1990. Soot volume fractions in the overfire region of turbulent diffusion flames. Combustion Flame 81 133-49. 

Coppalle, A., and D. Joyeux. 1994. Temperature and soot volume fraction in turbulent diffusion flames Measurements of mean and fluctuating values. Combustion Flame 96 275-85. 

Toqan, M. A., J. M. Beer, P. Jansohn, N. Sun, A. Testa, A. Shihadeh, and J. D. Teare. 1992. Low NOa, emission from radially stratified natural gas-air turbulent diffusion flames. 24th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 1391-97. 

There is general agreement that both fronts accelerate, and that the process is cumulative by virtue of their mutual interaction. This is promoted by turbulence, diffusion of hot particles ahead of the flame front, and passage of reflected shocks and rarefactions thru the "processed medium (Ref 3, p 130). Advance of the flame front thru it differs more and more from the initial laminar burning... 

Many practical industrial processes are diffusion limited (i.e., have a high Damkohler number), and the assumption that the chemistry is fast is often sufficient to predict the overall characteristics of the process. For instance, in turbulent diffusion flames, the rates of fuel oxidation and heat release are often governed by the turbulent transport and mixing. 

Because thermal NO is the dominating source under conditions with high temperatures and excess air, it was once assumed that prompt NO formation is negligible in most practical applications. This assumption is hardly valid, however. Turbulent diffusion flames are the most common practical flame configuration. In these flames the reaction zone is typically somewhat fuel rich, providing favorable conditions for prompt NO formation. While the relative contributions of the two formation mechanisms is still in dispute, there is little doubt that prompt NO is an important source of NO in most practical gas-diffusion flames. 

Figure 4. Probability density functions of temperature for Ht-air turbulent diffusion flame determined at various radial positions 134 mm downstream of the fuel line tip according to procedures indicated in Figure 3. The measurement positions are drawn schematically in the center of the figure to correspond to the radial positions r on the scale at the RHS.

We have chosen the latter approach, and show in Fig. 6 a sketch of a movable fan-induced co-flowing turbulent jet combustion tunnel. The working section is a 15 cm x 15 cm square pipe with large glass windows giving clear optical access to the turbulent diffusion flame produced on a 3-mm-diameter fuel tube. 

Figure 5. Probability density function (pdf or histogram) for temperature X velocity for turbulent diffusion flame. These data correspond to a test zone along the axis, 50 fuel-tip diameters downstream from the fuel line tip.

Figure 8. Nitrogen concentration vs. temperature, determined from Raman data at position shown in Hi-air turbulent diffusion flame. The solid theoretical curve, corresponding to adiabatic conditions, was obtained by replotting the information in Figure 7. The theoretical point for stoichiometric combustion ( = 1) is shown on this curve as a filled-in circle. These Raman data were not corrected for optical background at the Raman spectral band position.

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