Combustion wave stability

In the introduction to this chapter a combustion wave was considered to be propagating in a tube. When the cold premixed gases flow in a direction opposite to the wave propagation and travel at a velocity equal to the propagation velocity (i.e., the laminar flame speed), the wave (flame) becomes stationary with respect to the containing tube. Such a flame would possess only neutral stability, and its actual position would drift [1], If the velocity of the unbumed mixture is increased, the flame will leave the tube and, in most cases, fix itself... 

One of the problems in combustors that utilize premixed flames is the attainment of stable performance over an extended range of operation (turndown ratio). The condition, at which the combustion wave is driven back causing the flame to be extinguished when the flow velocity exceeds the burning velocity everywhere in the flow field, is of particular interest to this study. The physical mechanisms responsible for the blow-out limits and flame stabilization of jet flames is still a topic of extensive research [1, 2]. The flame stabilization technique discussed in this paper is aimed to control the velocity gradient in the region close to... 

Dunkle s Syllabus (1957-1958), pp 99-102 (The Rayleigh-Mikhel son and Fanno lines) 108-10 (Flame stabilization and flameholders) 115-19 (Laminar turbulent burning) 126-27 (Coalescence of shock and combustion wave) ... 

For a more complete discussion of combustion waves, detonations, and flame stability the reader is referred to the very detailed exposition in the text by Lewis and Von Elbe, loc. dt., and also the text by Frank-Kamenetskii, loc. cii. An excellent qualitative description of the initiation of detonations in gases will be found in an article by G. B. Kistiakowsky, Ind. Kng. Cheni., 43, 2794 (1951). 

The stability of combustion wave propagation is an important factor in determining the quality of materials produced by CS. To produce uniform product, a stable combustion regime is desired. Furthermore, it is also important to know the boundaries where combustion, stable or unstable, can propagate. 

Linear stability analysis of gasless combustion wave... 

In order to present here some basic results of the weakly stability analysis, we consider below a general reaction-diffusion system of equations. We assume that the problem has a one-dimensional traveling wave solution that loses stability in the same way as the gasless combustion wave as discussed in greater detail below. A study of a general reaction-diffusion system rather than a specific model is useful, because it allows us to focus on general properties of the solution, independent of a particular model. 

The conditions stated above mean that the basic solution is stable when p <0, and it loses stability when p passes through the critical value /r = 0 via a Hopf bifurcation. (This is exactly the case with the gasless combustion wave as discussed earlier.)... 

In an experimental study of the combustion of a polymer material it is especially important to investigate all the stages in which phase and chemical transformations of the substance are taking place. The complete scheme of the combustion process could be deduced from the study of such characteristics as the morphological structure of the burning surface, the temperature distribution inside the combustion wave, the structure and concentration profiles of each individual-stage product, the heat liberation ant heat losses, the structural properties of the flame and the limitations on combustion stability [16]. 

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. 

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. 

Basevich, V. Ya., and S. M. Kogarko. 1972. Comparison of limiting flame blow-off velocities for different types of stabilizers. Sov. J. Combustion, Explosion, Shock Waves 8(4) 582. 

The flammability and stability limits of Fig. 19.7 were obtained using fuel-air mixtures with the same equivalence ratio in the radial and tangential inlets, and without an axial jet. The lean flammability limit decreased from 0.57 to 0.4 as the swirl number was increased from 0.6 to 3.75, and the region of high-heat release moved closer to the swirler which represented an acoustic pressure antinode for the naturally occurring oscillations associated with a quarter wave in the entire duct, with frequency close to 200 Hz. Thus, swirl led to an increase in the amplitude of oscillations and to an earlier transition from smooth to rough combustion with antinodal RMS pressures up to 10 kPa, and initiated at an equivalence ratio of 0.5 for a swirl number of 3.75... 

Both properties decreased markedly, on the other hand, as the wave traversed the field in the negative-to-positive direction. Application of an axial magnetic field of 22000 gauss had similar but weaker effects. If the gas mixture had been pre-dried, such electrical influences became weaker i.e., the "spin was stabilized . It was inferred that positive ions are involved in the combustion reaction chains. The polar surface of the glass surface may reduce the energy required for ion formation. Eyring et al noted in this connection that unsolvated positive ions always react with low activation energies (Ref 15, pp 130-31)... 

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