Flames turbulent

Turbulent flame speed, unlike laminar flame speed, is dependent on the flow field and on both the mean and turbulence characteristics of the flow, which can in turn depend on the experimental configuration. Nonstationary spherical turbulent flames, generated through a grid, have flame speeds of the order of or less than the laminar flame speed. This turbulent flame speed tends to increase proportionally to the intensity of the turbulence. 

In high speed dusted, premixed flows, where flames are stabili2ed in the recirculation 2ones, the turbulent flame speed grows without apparent limit, in approximate proportion to the speed of the unbumed gas flow. In the recirculation 2ones the intensity of the turbulence does not affect the turbulent flame speed (1). 

In the reaction 2one, an increase in the intensity of the turbulence is related to the turbulent flame speed. It has been proposed that flame-generated turbulence results from shear forces within the burning gas (1,28). The existence of flame-generated turbulence is not, however, universally accepted, and in unconfined flames direct measurements of velocity indicate that there is no flame-generated turbulence (1,2). 

The balanced equation for turbulent kinetic energy in a reacting turbulent flow contains the terms that represent production as a result of mean flow shear, which can be influenced by combustion, and the terms that represent mean flow dilations, which can remove turbulent energy as a result of combustion. Some of the discrepancies between turbulent flame propagation speeds might be explained in terms of the balance between these competing effects. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

F. A. WiUiams, in J. H. S. Lee and C. M. Cuirao, eds., EaminarFlame Instability and Turbulent Flame Propagation, In FuelAirExplosions, University of Waterloo Press, Waterloo, Ontario, Canada, 1982. 

Confirmation of the formation of the radicals during combustion reactions has been made by inuoducing a sample of dre flames into a mass spectrometer. The sample is withdrawn from a turbulent flame which is formed into a thin column, by admitting a sample of the flame to the spectrometer drrough a piidrole orifice, usually of diameter a few tenths of a millimetre. An alternative procedure which has been successful in identifying the presence of radicals, such as CHO, has been the use of laser-induced fluorescence. 

FIGURE 4-2. Sketch of differences in the local direction (upper) and flame front topography (lower) between a laminar and turbulent flame. 

Grumer, J. 1958. Flashback and Blowoff Limits of Unpiloted Turbulent Flames. Propulsion, 28(11), 756-758 (November 1958). 

The article hy Wilson and Flessner gives the dividing line as roughly 50 ft/s between slow flames that can be simply quenched and fast flames that must also be decelerated. Fast flames described in the article have speeds above 60 ft/s as opposed to turbulent flames, which are described as having speeds from 5 to 100 m/s in most venting systems. The test rig described in the article was composed of 6-inch diameter pipe. 

Khitrin, L. N. et al. 1965. Peculiarities of Laminar- and Turbulent-Flame Flashbacks. Proe. 10th Sympos. (Inti.) on Combustion, pp. 1285-1291. 

The solid lines in Figure 4.5 represent extrapolations of experimental data to full-scale vessel bursts on the basis of dimensional arguments. Attendant overpressures were computed by the similarity solution for the gas dynamics generated by steady flames according to Kuhl et al. (1973). Overpressure effects in the environment were determined assuming acoustic decay. The dimensional arguments used to scale up the turbulent flame speed, based on an expression by Damkohler (1940), are, however, questionable. 

Chan, C., J. H. S. Lee, I. O. Moen, and P. Thibault. 1980. Turbulent flame acceleration and pressure development in tubes. Proceedings of the First Specialists Meeting of the Combustion Institute, Bordeaux, France, pp. 479-484. 

Karlovitz, B. 1951. Investigation of turbulent flames. /. Chem. Phys. 19 541-547. 

Lee, J. H. S., R. Knystautas, and C. K. Chan. 1984. Turbulent flame propagation in obstacle-filled tubes. 20th Symp. (Int.) on Combustion, pp. 1663-1672. The Combustion Institute, Pittsburgh, PA. 

Moen, I. O., M. Donato, R. Knystautas, J. H. Lee, and H. Gg. Wagner. 1980b. Turbulent flame propagation and acceleration in the presence of obstacles. Progress in Astronautics and Aeronautics. 75 33-47, AIAA Inc., New York. 

Moen, 1. O., J. H. S. Lee, B. H. Hjertager, K. Fuhre, and R. K. Eckhoff. 1982. Pressure development due to turbulent flame propagation in large-scale methane-air explosions. Comb, and Flame. 47 31-52. 

The only computational approach found in the literature to modeling flash-hre radiation is that of Raj and Emmons (1975), who modeled a flash fire as a two-dimensional, turbulent flame propagating at a constant speed. The model is based on the following experimental observations ... 

The cloud is consumed by a turbulent flame front which propagates at a velocity which is roughly proportional to ambient wind speed. 

The model is a straightforward extension of a pool-fire model developed by Steward (1964), and is, of course, a drastic simplification of reality. Figure 5.4 illustrates the model, consisting of a two-dimensional, turbulent-flame front propagating at a given, constant velocity S into a stagnant mixture of depth d. The flame base of width W is dependent on the combustion process in the buoyant plume above the flame base. This fire plume is fed by an unbumt mixture that flows in with velocity Mq. The model assumes that the combustion process is fully convection-controlled, and therefore, fully determined by entrainment of air into the buoyant fire plume. 

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

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

Chomiak, ]., Dissipation fluctuations and the structure and propagation of turbulent flames in premixed gases at high Reynolds numbers. Proceedings of the Combustion Institute, 16, 1665-1673,1977. 

Tabaczynski, R. Trinker, F. H., and Shannon, B. A. S., Further refinement and validation of a turbulent flame propagation model for spark-ignition engines. Combustion and Flame, 39, 111-121, 1980. 

Klimov, A. M., Premixed turbulent flames-interplay of hydrodynamic and chemical phenomena. Progress in Astronautics and Aeronautics, Volume 88, Bowen, J. R., Manson, N., Oppenheim, A. K., and Soloukhin, R. L, eds., American Institute of Aeronautics and Astronautics, New York, pp.133-146,1983. 

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. 

Based on the flame-hole dynamics [59], dynamic evolutions of flame holes were simulated to yield the statistical chance to determine the reacting or quenched flame surface under the randomly fluctuating 2D strain-rate field. The flame-hole d5mamics have also been applied to turbulent flame stabilization by considering the realistic turbulence effects by introducing fluctuating 2D strain-rate field [22] and adopting the level-set method [60]. 

L. Vervisch and T. Poinsot, Direct numerical simulation of non-premixed turbulent flames, Annu. Rev. Fluid Mech. 30 655-691,1998. 

C. M. Muller, H. Breitbach, and N. Peters, Partially premixed turbulent flame propagation in jet flames, Proc. Combust. Inst. 25 1099-1106,1994. 

These data indicate that thermal losses during unsteady flame-wall interactions constitute an intense source of combustion noise. This is exemplified in other cases where extinctions result from large coherent structures impacting on solid boundaries, or when a turbulent flame is stabilized close to a wall and impinges on the boundary. However, in many cases, the flame is stabilized away from the boundaries and this mechanism may not be operational. 

I.R. Hurle, R.B. Rrice, T.M. Sudgen, and A. Thomas. Sound emission from open turbulent flames. Proc. R. Soc. Lond. A, 303 409 27,1968. 

M. Katsuki, Y. Mizutani, M. Chikami, and Kittaka T. Sound emission from a turbulent flame. Proc. Combust. Inst., 21 1543-1550,1986. 

D.L Abugov and O.I. Obrezkov. Acoustic noise in turbulent flames. Combust. Explosions Shock Waves, 14 606-612, 1978. 

The concept of turbulent flame stretch was introduced by Karlovitz long ago in [15]. The turbulent Karlovitz number (Ka) can be defined as the ratio of a turbulent strain rate (s) to a characteristic reaction rate (to), which has been commonly used as a key nondimensional parameter to describe the flame propagation rates and flame quenching by turbulence. For turbulence s >/ />, where the dissipation rate e and u, L and v... 

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