Premixed flames describing

In Chapfer 7.2, J.H. Frank and R.S. Barlow describe the basic characteristics of non-premixed flames wifh an emphasis on fundamenfal phenomena relevant to predictive modeling. They show how the development of predictive models for complex combustion systems can be accelerated by combining closely coupled experiments and numerical simulations. 

The complexity of the turbulent reacting flow problem is such that it is best to deal first with the effect of a turbulent field on an exothermic reaction in a plug flow reactor. Then the different turbulent reacting flow regimes will be described more precisely in terms of appropriate characteristic lengths, which will be developed from a general discussion of turbulence. Finally, the turbulent premixed flame will be examined in detail. 

For the flames described in Table 8.7, Harris and Weiner [74] obtained the results shown in Fig. 8.20, where the increase of soot volume fraction is plotted as a function of time. Thus, temperature measurements revealed that Flame 3 has the lowest temperature, Flames 2 and 4 are of equal, and somewhat higher temperature, and Flame 1 has the highest temperature. These results supply quantitative proof that in premixed flames the tendency to soot decreases with increasing flame temperature. Also of importance is the fact that Flame 2, which has toluene as a fuel constituent and the same equivalence ratio as that of Flame 4 for pure ethene, gives the same soot volume fraction as Flame 4... 

As additives to reduce soot output in flames, metal and organometallic compounds, such as ferrocene, have attracted the attention of many investigators (see Ref. [113]). The effect in premixed flames is best described by Bonczyk [114], who reported that the efficiency with which a given metal affects soot production characteristics depends almost exclusively on temperature and the... 

COSILAB Combustion Simulation Software is a set of commercial software tools for simulating a variety of laminar flames including unstrained, premixed freely propagating flames, unstrained, premixed burner-stabilized flames, strained premixed flames, strained diffusion flames, strained partially premixed flames cylindrical and spherical symmetrical flames. The code can simulate transient spherically expanding and converging flames, droplets and streams of droplets in flames, sprays, tubular flames, combustion and/or evaporation of single spherical drops of liquid fuel, reactions in plug flow and perfectly stirred reactors, and problems of reactive boundary layers, such as open or enclosed jet flames, or flames in a wall boundary layer. The codes were developed from RUN-1DL, described below, and are now maintained and distributed by SoftPredict. Refer to the website http //www.softpredict.com/cms/ softpredict-home.html for more information. 

In the early investigations of hydrodynamic instability [9], [190], it was presumed that the instability evolves to a chaotic state characteristic of turbulence. Thus self-turbulization of premixed flames was attributed to hydrodynamic instability (analogous, in a sense, to the development of turbulence in shear flows). This viewpoint must be revised if the instability evolves to stable nonplanar structures, as suggested above. From numerical experiments with equations describing the self-evolution of flame surfaces in the limit of small values of the density change across the flame, it has been inferred [152], [198]-[200] that the hydrodynamic instability evolves... 

As noted earlier, waste destruction occur as a consequence of premixed and diffusion flames. In premixed flames, the fuel and oxidizer are mixed at the molecular level, and the relative amounts of the reactants are described by the equivalence ratio (4>), defined below ... 

Searby and Rochwerger [9] developed a model describing the effect of an acoustic field on the stability of a laminar, premixed flame, treated as a thin interface between two fluids of different densities and under the influence of a periodic gravitational field. Their model is an extension of the work by Markstein [8] and is consistent with the more recent flame theory of Clavin and Garcia-Ybarra [16]. Bychkov [17] later solved the problem analytically, presenting the following linear equation for the perturbation amplitude, /, of a flame under the influence of an acoustic field [17] ... 

For flames that exhibit the parametric instability, the velocity at which the exponential growth of velocity fluctuations started for each experiment was noted. These critical velocities are shown in Fig. 7.5, normalized by the laminar-flame speeds reported in [13]. All points shown on this plot represent the ensemble average of measurements from five experiments, and the error bars indicate the standard deviation about the mean value. The other curve on this plot was calculated using the analytical model of a premixed flame under the influence of an oscillating gravitational field by Bychkov [17], ris described above. Each point represents the smallest normalized acoustic velocity at the most unstable reduced wave number that resulted in the parametric instability. The experimental results show the same trend as the theoretical model mixtures with an equivalence ratio of 0.9, which require the smallest normalized acoustic velocity to trigger the parametric instability while flames on either side require larger values. 

When a microscopic flammable gas mixture formed between neighboring droplets is coimected throughout the spray element, a premixed flame can propagate along this corridor and consume oxygen contained in the spray element. It should be noted that the flame propagation, which can be described by the conventional two-phase flow theories is restricted to this case. 

We are now interested in the numerical simulation of a two-dimensional premixed flame propagating in gaseous mixture. Some simplifications of the underlying physical processes lead to the so-called thermo diffusive model described by the reaction-diffusion equations... 

The lifetime is here weighted to both the diffusion time in a manner similar to which is described for the manual reduction procedure by Equation (19), and the flame time, rp suitable for premixed flames. It can also be weighted to the scalar dissipation rate, a, in diffusion flames. 

In the 1980s reduced mechanisms for premixed and non-premixed flames of methane were described and, shortly after, these mechanisms were used in the asymptotic and numerical analysis [11]. In the 1980s, some research groups focused their attention on methane flames and developed useful techniques for the systematic reduction of the detailed kinetic mechanism. It was found that kinetic models for hydrocarbons have a logical hierarchical stractuie, where the kinetic mechanism of any fuel contains, as a subset, the mechanism of smaller molecules [12]. 

Combustion occurs with a large number of intermediate steps and even simple processes, such as the ones listed in Table 10.1, occur through dozens of coupled elementary reactions. With computer simulations it is possible to describe the interaction between the reactions, and concentration profiles can be calculated. In order to perform the computer calculations it is necessary to know the rate constants for the individual elementary reactions. Comparisons between theory and experiments are best made for a flat, premixed flame, which in its central part can be considered to have only onedimensional (vertical) variation, allowing computer calculations to be performed comparatively easily. The most important reactions are included in the computer description. In Fig. 10.1 experimental and theoretically calculated concentration curves are given for the case of low-pressure ethane/ oxygen combustion. As examples of important elementary processes we give the reactions... 

In the flame system which has been described, Eq. (4.9) defines two imaginary surfaces, b w = 0) and u(co = 1), which bound the flow region of interest. In the particular context of laminar premixed flames, three types of boundary are of practical importance ... 

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