Turbulent flame stabilization

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]. 

CONFINED TURBULENT FLAMES STABILIZED ON BLUFF BODIES... 

The Presumed Probability Density Function method is developed and implemented to study turbulent flame stabilization and combustion control in subsonic combustors with flame holders. The method considers turbulence-chemistry interaction, multiple thermo-chemical variables, variable pressure, near-wall effects, and provides the efficient research tool for studying flame stabilization and blow-off in practical ramjet burners. Nonreflecting multidimensional boundary conditions at open boundaries are derived, and implemented into the current research. The boundary conditions provide transparency to acoustic waves generated in bluff-body stabilized combustion zones, thus avoiding numerically induced oscillations and instabilities. It is shown that predicted flow patterns in a combustor are essentially affected by the boundary conditions. The derived nonreflecting boundary conditions provide the solutions corresponding to experimental findings. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

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. 

D. Veynante, L. Vervisch, T. Poinsot, A. Linan, and G. R. Ruetsch, Triple flame structure and diffusion flame stabilization, Proceedings of the Summer Program, Center for Turbulent Research 55-73,1994. 

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. 

Darrieus and Landau established that a planar laminar premixed flame is intrinsically unstable, and many studies have been devoted to this phenomenon, theoretically, numerically, and experimentally. The question is then whether a turbulent flame is the final state, saturated but continuously fluctuating, of an unstable laminar flame, similar to a turbulent inert flow, which is the product of loss of stability of a laminar flow. Indeed, should it exist, this kind of flame does constitute a clearly and simply well-posed problem, eventually free from any boundary conditions when the flame has been initiated in one point far from the walls. 

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. 

Chemiluminescence images of a turbulent partially premixed CH4/ air jet flame stabilized by premixed pilot flames. 

In this burner configuration, fuel is injected directly into the combustion chamber and hence, one would initially categorize it as a nonpremixed burner. However, the overall combustion process is quite complex and involves features of nonpremixed, partially premixed, and stratified combustion, as well as the possibility that the autoignition of hot mixtures of fuel, air, and recirculated combushon products may play a role in stabilizing the flame. Thus, while one may start from simple concepts of nonpremixed turbulent flames, the inclusion of local exhnchon or flame lift-off quickly increases the physical and computational complexity of flames that begin with nonpremixed streams of fuel and oxidizer. 

Although a laminar flame speed. S L is a physicochemical and chemical kinetic property of the unbumed gas mixture that can be assigned, a turbulent flame speed. S T is, in reality, a mass consumption rate per unit area divided by the unbumed gas mixture density. Thus,. S r must depend on the properties of the turbulent field in which it exists and the method by which the flame is stabilized. Of course, difficulty arises with this definition of. S T because the time-averaged turbulent flame is bushy (thick) and there is a large difference between the area on the unbumed gas side of the flame and that on the burned gas side. Nevertheless, many experimental data points are reported as. S T. 

In the discussion of premixed turbulent flames, the case of infinitely fast mixing of reactants and products was introduced. Generally this concept is referred to as a stirred reactor. Many investigators have employed stirred reactor theory not only to describe turbulent flame phenomena, but also to determine overall reaction kinetic rates [23] and to understand stabilization in high-velocity streams [62], Stirred reactor theory is also important from a practical point of view because it predicts the maximum energy release rate possible in a fixed volume at a particular pressure. 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

Recirculation of combustion products can be obtained by several means (1) by inserting solid obstacles in the stream, as in ramjet technology (bluff-body stabilization) (2) by directing part of the flow or one of the flow constituents, usually air, opposed or normal to the main stream, as in gas turbine combustion chambers (aerodynamic stabilization), or (3) by using a step in the wall enclosure (step stabilization), as in the so-called dump combustors. These modes of stabilization are depicted in Fig. 4.52. Complete reviews of flame stabilization of premixed turbulent gases appear in Refs. [66, 67],... 

New computational approaches are developed to explore flame stabilization techniques in subsonic ramjets. The primary focus is statistical modeling of turbulent combustion and derivation of the adequate boundary conditions at open boundaries. The mechanism of flame stabilization and blow-off in ramjet burners is discussed. The criterion of flame stability based on the clearly defined characteristic residence and reaction times is suggested and validated by numerical simulations. 

Early theoretical treatments of bluff-body stabilized flame spreading have been based, in general, on the assumption that the flame is a discontinuous surface separating gas streams of different densities and temperatures [1, 15-17]. These theories neglect the finite thickness of turbulent flame zone and predict the increase of the spreading rate both with the density ratio across the flame, and with the increase in the laminar flame velocity of fuel-air mixture. This does not correspond to experimental observations (e.g., [8, 10]). 

Currently, computing the structure of bluff-body stabilized flames has become a subject of intense activity. The general objective of numerical studies is to describe the phenomenon by solving the fundamental differential equations coupled with turbulence and combustion closures. Since there are many possible approaches, more or less substantiated, the reported results are often contradictory. Apparently, this is caused by the lack of basic understanding of the physico-chemical phenomena accompanying flame stabilization and spreading. 

Thus, theoretical modeling of bluff-body flame stabilization cannot yet compete with the experimental approaches. Partially, it is due to the fundamental problems relevant to the turbulent combustion theory. The underestimation of the role of theory is also due to the lack of systematic studies based on existing models. 

The present study is to elaborate on the computational approaches to explore flame stabilization techniques in subsonic ramjets, and to control combustion both passively and actively. The primary focus is on statistical models of turbulent combustion, in particular, the Presumed Probability Density Function (PPDF) method and the Pressure-Coupled Joint Velocity-Scalar Probability Density Function (PC JVS PDF) method [23, 24]. 

The behavior of confined flames differs considerably from that of unconfined flames. Acceleration of the gases, caused by confinement, results in the generation of shear stresses and turbulent motions, which decrease the influence of approach stream turbulence and the effect of chemical kinetic factors. How the implementation of the ABC and the PPDF method helps to obtain the experimentally observed flow patterns and to understand the mechanism of flame stabilization and blow-off is demonstrated in this section. 

LongweU, J. P. 1953. Flame stabilization by bluff bodies and turbulent flames in ducts. 4th Symposium (International) on Combustion, Combustion and Detonation Waves Proceedings. Baltimore The Wilhams and Wilkins Co. 90. 

Solntsev, V. P. 1961. Influence of turbulence parameters on the combustion process of homogeneous gasoline-air mixture behind a stabilizer under conditions of confined flow. In Flame stabilization and the development of combustion in turbulent How. Ed. G. N. Gorbunov. Moscow Oborongiz. 75. 

In Fig. 15.4, the measured turbulent flame speeds, normalized with mixture-specific laminar flame velocities obtained recently by Vagelopoulos et al. [14], are compared with experimental and theoretical results obtained in earlier studies. Also shown in the figure are the measurements made by Abdel-Gayed et al. [3] for methane-air mixtures with = 0.9 and = 1 a correlation of measured turbulent flame speeds with intensity obtained by Cheng and Shepherd [1] for rod-stabilized v-flames, tube-stabilized conical flames, and stagnation-flow stabilized flames, Ut/Ul = l + i.2 u /U ) a correlation of measured turbulent flame... 

Gore et al. [8] reported, for the first time, the existence of an optimum level of partial premixing for minimum NO emissions from turbulent jet flames. The optimum equivalence ratio ( 5) for a minimum emission index was found to be f.5, which is less than that found for the laminar flames discussed above. Lyle et al. [f5] confirmed the existence of an optimum level of partial premixing for both confined and unconfined turbulent flames. Lyle et al. [15] established that changes in thermal NO production do not control the emission behavior of partially premixed turbulent flames. More recently, Kemal et al. [10] have shown that a minimum in NO emissions can also be obtained for sudden dump-stabilized turbulent partially premixed flames. 

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