Flamelets

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]. 

E. W. Price, Effect of multidimensional flamelets in composite propellant combustion, /. Propulsion Power 11 717-728,1995. 

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. 

Experimentally, two modes of extinction, based on the separation between the twin flames are observed. Specifically, the extinction of lean counterflow flames of n-decane/02/N2 mixtures occurs with a finite separation distance, while that of rich flames exhibits a merging of two luminous flamelets. The two distinct extinction modes can be clearly seen in Figure 6.3.2. As discussed earlier, the reactivity of a positively stretched flame with Le smaller (greater) than unity increases (decreases) with the increasing stretch rate. Therefore, the experimental observation is in agreement with the... 

More recently, experimental studies have been carried out using a similar device but with an annular external hot coflow of burned gases that allowed one to operate within a much larger velocity range. Chen et al. [26] and more recently Chen and Bilger [27,28] have studied the perturbations that the smallest scales of turbulence can impose to the local flamelet structures. Those studies are of paramount importance, first because they have allowed to get deeper insights into the local structure... 

Perturbed flamelet structure as obtained from 2D instantaneous Rayleigh temperature fields for the case FI (jet exit velocity is 65m/s). (Reproduced from Dumont, J.P., Durox, D., and Borghi, R., Combust. Sci. Tech., 89,219,1993. With permission. Figure 3.1, p. 233, copyright Gordon Breach Science Publishers (Taylor and Francis editions).)... 

The results reproduced from Ref. [28] and presented in Figure 7.1.9 show that the internal structure of the "flamelets" within the studied flames displays strong departures from both unstretched laminar flamelet and stretched counterflow flamelets. Figure 7.1.9 supports the picture of the perturbed flamelet model recently introduced in Ref. [29]. In this model, depending on the local value of the ratio of laminar flame thickness and... 

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.)... 

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... 

A. Mura, F. Galzin, and R. Borghi 2003, A unified PDF-flamelet model for turbulent premixed combustion. Combust. Sci. Technol. 175 (9) 1573-1609. 

N. Peters 1986, Laminar flamelet concepts in turbulent combustion, Proc. Combust. Inst. 21 1231-1250. 

Qualitative comparison of the inclined structure of thin layers of high scalar dissipation in a piloted CH4/air jet flame as revealed by (a) mixture fraction imaging, (b) LES with a steady flamelet library (a and b are adapted from Kempf, A. Flemming, F., and Janicka, ]., Proc. Combust. Inst, 30, 557, 2005. With permission.), and (c) LES with unsteady flamelet modeling. (Adapted from Pitsch, H. and Steiner, H., Proc. Combust. Inst., 28, 41, 2000. With permission.)... 

Meneveau, C. and T. Roinsot, Stretching and quenching of flamelets in premixed turbulent combustion. Combust. Flame, 1991. 86 311-332. 

A model must be introduced to simulate fast chemical reactions, for example, flamelet, or turbulent mixer model (TMM), presumed mapping. Rodney Eox describes many proposed models in his book [23]. Many of these use a probability density function to describe the concentration variations. One model that gives reasonably good results for a wide range of non-premixed reactions is the TMM model by Baldyga and Bourne [24]. In this model, the variance of the concentration fluctuations is separated into three scales corresponding to large, intermediate, and small turbulent eddies. 

A flame is a chemical reaction producing a temperature of the order of at least 1500 K and generally about 2500 K at most in air. Fire is generally a turbulent ensemble of flames (or flamelets). A flamelet or laminar flame can have a thickness of the order of 10-3 cm and an exothermic production rate of energy per unit volume of about 108W/cm3. However, at the onset of ignition, the reaction might only possess... 

At present, there exists no completely general RANS model for differential diffusion. Note, however, that because it solves (4.37) directly, the linear-eddy model discussed in Section 4.3 can describe differential diffusion (Kerstein 1990 Kerstein et al. 1995). Likewise, the laminar flamelet model discussed in Section 5.7 can be applied to describe differential diffusion in flames (Pitsch and Peters 1998). Here, in order to understand the underlying physics, we will restrict our attention to a multi-variate version of the SR model for inert scalars (Fox 1999). 

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