Combustion flame stabilization

In summary, the bad features of partial combustion processes are the cost of oxygen and the dilution of the cracked gases with combustion products. Flame stability is always a potential problem. These features are more than offset by the inherent simplicity of the operation, which is the reason that partial combustion is the predominant process for manufacturing acetylene from hydrocarbons. 

Length. Combustor length must be sufficient to provide for flame stabilization, combustion, and mixing with dilution air. The typical value of the length-to-diameter ratio for liners ranges from three to six. Ratios for casing range from two to four. 

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. 

Y. -C. Chen and R. W. Bilger, Stabilization mechanism of lifted laminar flames in axisymmetric jet flows. Combust. Flame 122 377-399, 2000. 

S. Ghosal and L. Vervisch, Stability diagram for lift-off and blowout of a round jet laminar diffusion flame. Combust. Flame 124 646-655,2001. 

T. Echekki, J. -Y. Ghen, and U. Hedge, Numerical investigation of buoyancy effects on triple flame stability. Combust. Sci. Technol. 176(3) 381-407, 2004. 

M. S. Mansour, Stability characteristics of lifted turbulent partially premixed jets. Combust. Flame 133 411 20, 2003. 

T. Schuller, D. Durox, and S. Candel. Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners. Combustion and Flame, 135 525-537, 2003. 

Y. Huang and V. Yang. Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor Transition from stable to unstable flame. Combust. Flame, 136(3) 383-389, 2004. 

Bechtold, J.K. and Matalon, M., Hydrodynamic and diffusion effects on the stability of spherically expanding flames. Combust. Flame, 67, 77,1987. 

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.)... 

Muniz, L. and Mungal, M. G., Instantaneous flame-stabilization velocities in Ufted-jet diffusion flames. Combust. Flame, 111, 16,1997. 

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. 

Robson, K. and Wilson, M.J.G., The stability of laminar diffusion flames of methane. Combust. Flame, 13, 626, 1969. 

Alsairafi, A., Lee, S.T., and T ien, J.S., Modeling gravity effects on diffusion flames stabilized around a cylindrical wick saturated with liquid fuel. Combust. Sci. Technol., 176, 2165, 2004. 

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

Lewis, B. und Elbe, G. von Combustion, Flames and Explosives of Gases, 3. Aufl., Academic Press, Orlando, Florida 1987 Energetic Materials New Synthesis Routes, Ignition, Propagation and Stability of Detonation, Hrsg. Field, J.E. und Gray, R, The Royal Society, London 1992... 

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. 

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. 

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]. 

For attaining higher combustion efficiencies with shorter tailpipes, a provision should be made for several flame holders in the combustion chamber. The optimum arrangement of the flame holders in the combustor in terms of the combustion efficiency, flame stability, and pressure loss should be found. The methodology suggested in sections 12.2 and 12.3 helps to solve this problem. 

As mentioned in section 12.1, Dunskii [12] was the first who put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence time, L, and combustion time, tc, in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone behind the bluff body. Dunskii s condition for flame blow-off is U/tc = Mi, where Mi is the Mikhelson number close to unity (for example, for cone flame holder the measurements give Mi = 0.45 [36]). Residence time L is taken proportional to the flame holder size, H, and inversely proportional to the approach flow velocity, U, i.e., L = H/U. Combustion time is estimated as tc = at/Si, where... 

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. 

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