Bluff-body stabilization

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.)... 

Dally, B. B., Masri, A. R., Barlow, R. S., and Fiechtner, G. J., Instantaneous and mean compositional structure of bluff-body stabilized nonpremixed flames. Combust. Flame, 114, 119, 1998. 

Raman, V. and Pitsch, H., Large-eddy simulation of a bluff-body-stabilized non-premixed flame using a recursive filter-refinement procedure. Combust. Flame, 142, 329, 2005. 

Correa, S. M., A. Gulati, and S. B. Pope (1994). Raman measurements and joint pdf modeling of a nonpremixed bluff-body-stabilized methane flame. In Twenty-fifth International Symposium on Combustion, pp. 1167-1173. Pittsburgh, PA The Combustion Institute. Corrsin, S. (1951a). The decay of isotropic temperature fluctuations in an isotropic turbulence. Journal of Aeronautical Science 18,417 -23. 

Dally, B. B., D. F. Fletcher, and A. R. Masri (1998). Measurements and modeling of a bluff-body stabilized flame. Combustion Theory and Modelling 2, 193-219. 

Jenny, P., M. Muradoglu, S. B. Pope, and D. A. Caughey (2001a). PDF simulations of a bluff-body stabilized flow. Journal of Computational Physics 169, 1-23. 

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],... 

There were many early experimental investigations of bluff-body stabilization. Most of this work [69] used premixed gaseous fuel-air systems and typically plotted the blowoff velocity as a function of the air-fuel ratio for various stabilized sizes, as shown in Fig. 4.56. Early attempts to correlate the data appeared to indicate that the dimensional dependence of blowoff velocity was different for different bluff-body shapes. Later, it was shown that the Reynolds number range was different for different experiments and that a simple independent dimensional dependence did not exist. Furthermore, the state of turbulence, the temperature of the stabilizer, incoming mixture temperature, etc., also had secondary effects. All these facts suggest that fluid mechanics plays a significant role in the process. 

Stirred reactor theory was initially applied to stabilization in gas turbine combustor cans in which the primary zone was treated as a completely stirred region. This theory has sometimes been extended to bluff-body stabilization, even though aspects of the theory appear inconsistent with experimental measurements made in the wake of a flame holder. Nevertheless, it would appear that stirred reactor theory gives the same functional dependence as the other correlations developed. In the previous section, it was found from stirred reactor considerations that... 

It is interesting to note that Eq. (7.47) is essentially the condition used in bluff-body stabilization conditions in Chapter 4, Section F. This result gives the intuitively expected answer that the higher the ambient temperature, the shorter is the ignition time. Hydrocarbon droplet and gas fuel injection ignition data correlate well with the dependences as shown in Eq. (7.47) [8,9],... 

Fureby, C., and C. Lofstrom. 1994. Large-eddy simulations of bluff body stabilized flames. 25th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 1257-64. 

Available experimental studies of premixed flame stabilization focus on the effect of the bluff-body (stabilizer) configuration, combustion chamber geometry... 

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. 

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. 

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. 

H. El-Asrag and S. Menon. Large eddy simulation of bluff-body stabilized swirling non-premixed flames. Proc. Combust. /nsL, 31 1747-1754, 2007. 

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... 

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... 

The types of obstacles used in stabilization of flames in high-speed flows could be rods, vee gutters, toroids, disks, strips, etc. But in choosing the bluff-body stabilizer, the designer must consider not only the maximum blowoff velocity... 

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