Turbulent flames flamelets

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

Stud5ting the influence of increased operating pressure on Bimsen turbulent flames, Kobayashi and coworkers [38,39] have recently put into evidence possible effects of flamelets instability, including modification of length scales, in particular. Figure 7.1.12 shows this remarkable... 

Bray, K. N. C. and N. Peters (1994). Laminar flamelets in turbulent flames. In P. A. Libby and F. A. Williams (eds.), Turbulent Reacting Flows, pp. 63-113. New York Academic Press. 

An alternative view [36] is that if l/d becomes too small, then extinctions of laminar flamelets are reflected in extinction of the turbulent flame. According to this idea, there is a region to the upper left in Figure 10.5 in which turbulent flame propagation cannot occur. It seems physically that phenomena of this type may pertain to confined turbulent flows in reactors of small volume, where they would reflect influences of turbulence properties... 

Michel, J.-B., Colin, O., Veynante, D. Modeling ignition and chemical structure of partially premixed turbulent flames using tabulated chemistry. Combust. Flame 152, 80-99 (2008) Michel, J.-B., Colin, O., Angelberger, C., Veynanteb, D. Using the tabulated diffusion flamelet model ADF-PCM to simulate a lifted methane-air jet flame. Combust. Flame 156, 1318-1331 (2009)... 

Parametric investigations of turbulent combustion are based on the definition of a reduced flame radius 7 l (0 that corresponds to the slope of the pressure - t time curve. When the chemical reaction is localized in narrow flamelets (see. Borgi diagram. Fig. 1.4), the spherical volume of radius 7 l (0 is close to the balanced turbulent combustion products volume. For this case, the pressure ratios P = P t) obtained experimentally during the time of burning are used for calculating / l (0-The turbulent flame velocity 5t (often designated as Ut) can be denoted by... 

The relevance of premixed edge flames to turbulent premixed flames can also be understood in parallel to the nonpremixed cases. In the laminar flamelet regime, turbulent premixed flames can be viewed as an ensemble of premixed flamelets, in which the premixed edge flames can have quenching holes by local high strain-rate or preferential diffusion, corresponding to the broken sheet regime [58]. 

This recent attempt differs from the previous classification where the wrinkled flamelet regime has been considered up to rj = (5l- Chen and Bilger have proposed to tentatively classify the different turbulent premixed flame structures they observed among four different regimes ... 

Wrinkled laminar flamelet regime. The well-known ideal regime where the laminar flame structure is only wrinkled by turbulence without any modification of ifs internal structure. 

Premixed turbulent combustion regime diagram proposed by Chen and Bilger. Two intermediate regimes are delineated between distributed flame front and wrinkled laminar flamelets. (Reprinted from Chen, Y.C. and Bilger, R., Combust. Flame, 131, 400, 2002. With permission. Figure 9, p. 411, copyright Elsevier editions.)... 

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