Turbulence flame-generated

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). 

Deflagration-to-Detonation Transition (DDT) The transition phenomenon resulting from the acceleration of a deflagration flame to detonation via flame-generated turbulent flow and compressive heating... 

Nevertheless, despite all these remarkable achievements, some open questions still remain. Among them is the influence of the molecular transport properties, in particular Lewis number effects, on the structure of turbulent premixed flames. Additional work is also needed to quantify the flame-generated turbulence phenomena and its relationship with the Darrieus-Landau instability. Another question is what are exactly the conditions for turbulent scalar transport to occur in a coimter-gradient mode Finally, is it realistic to expect that a turbulent premixed flame reaches an asymptotic steady-state of propagation, and if so, is it possible, in the future, to devise an experiment demonstrating it ... 

Another turbulence producing mechanism may occur due to strong velocity gradients parallel to the curved reaction zone which impose shear forces on the interface. This can lead to a roll up of the flame front and, hence, to a larger reaction surface and a higher burning velocity. This kind of flame generated turbulence was found to be important by Karlovitz et al. ° and Scurlock et al. ... 

Fig. 3.9 The linear dependence supporting the Karlovitz hypothesis of flame generated turbulence...

Laminar flame instabilities are dominated by diffusional effects that can only be of importance in flows with a low turbulence intensity, where molecular transport is of the same order of magnitude as turbulent transport (28). Flame instabilities do not appear to be capable of generating turbulence. They result in the growth of certain disturbances, leading to orderly three-dimensional stmctures which, though complex, are steady (1,2,8,9). 

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. 

These mechanisms may cause very high flame speeds and, as a result, strong blast pressures. The generation of high combustion rates is limited to the congested area, or the area affected by the turbulent release. As soon as the flame enters an area without turbulence, both the combustion rate and pressure will drop. 

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which... 

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. 

In elongated confined vessels, with one end closed and the opposite end open or removable, when an explosion begins at or near the closed end, the rapid movement of the flame front caused by the high volume from combustion wall cause displacement of the unburnt mixture ahead of it. Apparently this characteristic is independent of the nature of the combustible material [54], and the velocity can reach 80%-90% of the flame velocity, in part due to the high turbulence generated in the unburnt mixtures. 

R.B. Rrice, I.R. Hurle, and T.M. Sudgen. Optical studies of the generation of noise in turbulent flames. Proc. Combust. Inst., 12 1093-1102,1968. 

R. Clavin and E.D. Siggia. Turbulent premixed flames and sound generation. Combust. Sci. Technol., 78 147-155,1991. 

The so-called turbulent "V-shaped flames" are the flames anchored behind a rod or a catalytic wire in a flow where turbulence is generated by an upstream grid. Trinite et al. [7,40] and Driscoll and Faeth [41] have studied such flames. Instantaneous images of rare beauty have been obtained from which it is very clearly seen that the turbulent flame brush width is continuously increasing downstream of the stabilizing rod, see Figure 7.1.13. 

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