Combustion instability flame

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which... 

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

ICF flame motion during cyclic modulation of the flow <3> = 0.8, v = 1.87 = m/s,/= 150Hz, and v = 0.15m/s. (Adapted from Candel, S., Durox, D., and Schuller, T., Flame interactions as a source of noise and combustion instabilities, AIAA paper 2004—2928,10th AlAAl CEAS Aeroacoustics Conference, Manchester, U.K., May 2004. With permission.)... 

N. Noiray, D. Durox, T. Schuller, and S. Candel. A unified framework for nonlinear combustion instability analysis based on the flame describing function. /. Fluid Mech., 2008 (In press). 

S. Candel, D. Durox, T. Schuller. Flame interactions as a source of noise and combustion instabilities, AIAA paper 2004—2928, 10th ALAA/CEAS Aeroacoustics Conference, Manchester, U.K., May 2004. 

The fundamentals of combustion instability are presented by G. Searby in Chapter 5.1 and phenomena examined by him fall into two categories instability of flame fronts and thermo-acoustic instabilities. Each category can be subdivided further, and these are discussed. 

In Chapter 5.2, S. Candel, D. Durox, and T. Schuller consider certain aspects of perturbed flame dynamics. The relation between combustion instability and noise generation is described by reference to systematic experiments. The data indicate that acoustic emission is determined by flame dynamics. On this basis, combustion noise can be linked with combustion instability. 

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

Heitor, M., A.M. K.P. Taylor, and J.H. Whitelaw. 1984. Influence of confinement on combustion instabilities of premixed flames stabilized on axisymmetric baflfles. Combustion Flame 57 109-21. 

Lee, J. G., and D. A. Santavicca. 1998. Application of flame evolution imaging to the optimization of an active control system for suppressing combustion instability. 27th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 

Lang, W., T. Poinsot, and S. Candel. 1987. Active control of combustion instability. Combustion Flame 70 281-89. 

This criterion is the so-called T combustion instability. The stability criterion expressed by < 1 is not sufficient to obtain stable combustion when the flame temperature is dependent on pressure.lO In general, m is approximately zero in the high-pressure region for most propellants. However, l/of nitropolymer propellants such as single-base and double-base propellants decreases with decreasing pressure below about 5 MPa. Since direct determination of m is difficult, the heat of explosion, is evaluated as a function of... 

The combustion tests conducted for a rocket motor show that the combustion becomes unstable below 1.7 MPa and that the burning acquires a chuffing mode in the case of the uncatalyzed propellant. However, as expected, the combustion is stable even below 0.5 MPa for the nickel-catalyzed NC-NG propellant, as shown in Fig. 13.13. Propellants for which the flame temperature decreases with decreasing pressure tend to exhibit T combustion instability. 

This criterion is the so-called T combustion instability. The stability criterion expressed by n < 1 is not sufficient to obtain stable combustion when the flame temperature is dependent on pressure.ti]... 

C. Angelberger, D. Veynante, F. Egolfopoulos, and T. Poinsot. Large eddy simulations of combustion instabilities in premixed flames. In Proc. of the Summer Program, pages 61-82. Center for Turbulence Research, NASA Ames/Stanford Univ., 1998. 

Chapter 9 represents an effort to provide a unified and tutorial presentation of the broad field of the theory of combustion instabilities. The length of the chapter attests to the vastness of the field and to the progress that has been made therein in recent years. The final section of this chapter, on the theory of instabilities of premixed flames, is basic to analyses of premixed turbulent flame propagation and also has a bearing on aspects of flammability limits. 

Experimentally it has been shown that the threshold pressure at which combustion instability can be induced artificially in composite proplnts by pulsing is a function of the burning rate of the proplnt (in a motor size of 5-inch diameter and 40-inch length) (Ref 45). This relationship is shown in Figs 17 and 18 for both aluminized and non-aluminized composite proplnts. It was also found that potassium perchlorate, lithium perchlorate and AN proplnts were resistant to this induced instability. Since AP composites were the only proplnts, other than double-base, which were driven unstable, the rate controlling reactions and response function are those related to AP decompn and perhaps the diffusion flame between oxidizer and binder... 

HIM) Hatami, R. Calculation of Combustion Instabilities of Enclosed Diffusion Flames. 

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