Flame stabilization by bluff body

The technique of flame stabilization by bluff bodies has been used for a long time however, the mechanism by which the fresh gases are ignited, the flame spreads and blows off, is not well defined. Even for the simplest case of premixed flame stabilization there are still a number of uncertainties that are reflected in inconsistencies as indicated in available literature. As for the flames of fuel sprays, a topic of significant practical importance, the understanding of relevant phenomena is far from satisfactory. 

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. 

Longwell, J. P., Flame Stabilization by Bluff Bodies and Turbulent Flames in Ducts, ... 

Equation (62) can be applied to derive an approximate criterion for flame stabilization by bluff-body stabilizers. Empirically, when the approach flow velocity Uj in the combustor exceeds a critical blowoff velocity Wi niax Ihe flame is blown downstream and can no longer be stabilized by the bluff body. A knowledge of is essential in the design of ramjet... 

Figure 19.7 also shows that the amplitude of oscillations decreased with equivalence ratios greater than around 0.8 for swirl numbers up to 1.35, and at smaller values of equivalence ratio for larger swirl numbers. This is in contrast with results for ducted flames behind steps and bluff bodies, where the amplitude is nearly always a maximum near stoichiometry. This appears to be due to a shift in the location of flame stabilization by up to 50 mm, from close to the exit of the swirler to the end of the expansion section, since the amplitude of oscillations depends strongly on the intensity of heat release near the acoustic pressure antinode. This shift in flame location may have been related to the movement of the flame attachment with pressure oscillations. 

Fluid streams may be used either to augment the performance of bluff-body stabilizers or to produce flame stabilization by themselves [2]. A practical example in which extensive use is made of fluid streams is the... 

A bluff-body stabilized flame of CH4/H2 in air (designated HMl by Dally et al. [22]) (a) time-averaged photograph of flame luminosity, (b) time-averaged streamlines from LES, (c) instantaneous visualization of OH "luminosity" from LES, and (d) instantaneous temperature field from LES. (b and d are adapted from Raman, V. and Pitch, H., Combust. Flame, 142,329,2005. With permission.)... 

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],... 

Bluff-body flame stabilization in nonpremixed and partially premixed gaseous systems is complicated by the mixing of fuel and oxidizer. In addition to the aspects considered above, it is necessary to control the fuel distribution in the burner. 

Theoretical studies are primarily concentrated on the treatment of flame blow-off phenomenon and the prediction of flame spreading rates. Dunskii [12] is apparently the first to put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence and combustion times in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone. It was shown in [13] that the criteria of [1, 2, 5] reduce to Dunskii s criterion. Longwell et al. [14] suggested the theory of bluff-body stabilized flames assuming that the recirculation zone in the wake of the baffle is so intensely mixed that it becomes homogeneous. The combustion is described by a second-order rate equation for the reaction of fuel and air. 

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. 

The results of numerical simulation of bluff-body stabilized premixed flames by the PPDF method are presented in section 12.2. This method was developed to conduct parametric studies before applying a more sophisticated and CPU time consuming PC JVS PDF method. The adequate boundary conditions (ABC) at open boundaries derived in section 12.3 play an essential role in the analysis. Section 12.4 deals with testing and validating the computational method and discussing the mechanism of flame stabilization and blow-off. 

Figures 12.3 and 12.3c show mean velocity (Fig. 12.36) and mean temperature (Fig. 12.3c) fields under bluff-body stabilized combustion of stoichiometric methane-air mixture at inlet velocity 10 m/s, and ABC of Eq. (12.19) at the combustor outlet. Functions Wj, Wij, and W2j in Eq. (12.1) were obtained by solving the problem of laminar flame propagation with the detailed reaction mechanism [31] of Ci-C2-hydrocarbon oxidation (35 species, 280 reactions) including CH4 oxidation chemistry. The PDF of Eq. (12.4) was used in this calculation.

Analyzing Fig. 12.3, it is noticed that the flame width in the bluff-body stabilized flame increases almost linearly with the distance from the baffle with the spreading angle of about 3° to 5°. Since the flame spreading angle directly affects the ramjet combustion efficiency, it is important to check the performance of the ABC by applying it to combustors with different tailpipes. 

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. 

CARS measurements were made in a bluff-body stabilized flame with turbulent and recirculating flow characteristics similar to those found in many practical combustors. The combustor was operated at atmospheric pressure with inlet air temperatures between 280 and 300K, an air flow rate of 0.5 kg/ sec, and an upstream Reynolds number 1.5 x 105. Gaseous propane was injected from a hollow-cone nozzle located at the center of the bluff-body combustor at a flow rate of 7.06 kg/hr. The flame consisted of a blue cone originating at the nozzle followed by a yellow-luminous tail. 

Limitations on temperatures of solid materials often cause the methods of stabilization by solid elements, discussed so far, to be impractical. In most applications of stabilization by solid elements the flame is attached in the wake behind the element, so that the solid is not fully exposed to the flame temperature. Representative examples are bluff-body flame stabilizers, such as the stabilizing rods or plates placed normal to the flow in ramjets and afterburners, which were mentioned in Sections 5.1.1 and 10.3.5. A distinctive feature of bluff-body flame stabilization is the presence of a recirculation zone behind the body. Unlike the alternate vortices shed from bluff bodies in cold flow over the Reynolds-number range of practical interest, a well-defined vortex, steady in the mean, is observed to exist just downstream from the stabilizer when combustion occurs. This is a toroidal vortex for an axisymmetric stabilizer or a pair of identical counterrotating line vortices for rodlike stabilizers. The reason for the drastic change in the... 

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