Combustion instabilities

If the combustion products of a propellant attain a state of thermal equilibrium, the combustion temperature may be determined theoretically, as described in Chapter 2. However, the combustion in a rocket motor is incomplete and so the flame temperature remains below the adiabatic flame temperature.If one assumes that the flame temperature, varies with pressure, p, in a rocket motor, is expressed by[5] 

Equation (13.29) is derived in Section 14.1.3. Using the burning rate of a propellant given by Eqs. (3.68) and (13.29), the criterion for stable burning isit ] 

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] 

In general, m is approximately zero in the high-pressure region for most propellants. However, T 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 

Combustion tests carried out for a rocket motor demonstrate a typical V combustion instability. Double-base propellants composed of NC-NG propellants with and without a catalyst (1 % nickel powder) were burned. Detailed chemical compositions of both propellants are given in Section 6.4.6 and the burning rate characteristics are shown in Fig. 6.29. The addition of nickel is seen to have no effect on burning rate and the pressure exponent is n = 0.70 for both propellants. 

In order to obtain higher burning rate, various sizes and lengths of wire are added to make propellant grains. Chopped fine wires or very fine metal sheets are useful materials cast within propellant grains. 

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Combustion instability only used to be a problem with eonventional eombustors at very low engine powers. The phenomenon was ealled rumble. It was assoeiated with the fuel-lean zones of a eombustor, where the eonditions for burning are less attraetive. The eomplex 3D-flow strueture that exists in a eombustor will always have some zones that are suseeptible to the oseillatory burning. In a eonventional eombustor, the heat release from these oseillating zones was only a signifieant pereentage of the total eombustor heat release at low power eonditions. 

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... 

R.D. Gould, Combustion Instability of Solid Propellants Effect of Oxidizer Particle Size, Oxidizer/Fuel Ratio and Addition of Titanium Dioxide to Plastic Pro pell ants , Rept No RPE-TR-68/1, Westcott (Engl)... 

It can be concluded that if axial combustion instability is observed it can be eliminated by simple ballistic design changes such as propint burning rate characteristics, motor operating pressure decrease, or a reduction in grain length... 

R.S. Brown et al, AdvanChemEng 7, 1—69 (1968) CA 72, 11368 (1970) The topics reviewed include types of solid proplnts, sohd-proplnt rocket motors, ignition, steady-state combustion, and combustion instability and termination... 

CA 68,61103 (1968) Nonsteady burning of solid propints is being investigated both theoretically and exptly with attention to combustion instability, transient burning during motor ignition, and extinction by depressurization. 

A discussion with 10 refs of combustion instability in solid-proplnt rocket engines. Two different types of instability are considered acoustic and non-acoustic instability... 

The most important parameter in the analysis of pressure-coupled combustion instability is the acoustic admittance Y, which is the ratio of the amplitude of the acoustic velocity V to the amplitude of the acoustic pressure amplitude of the acoustic velocity V to the amplitude of the acoustic pressure P ... 

A6. Anderson, R., and Brown, R. S., paper presented at ICRPG (Interagency Chem. Rocket Propulsion Group) Combust. Instability Con/., 1st, Orlando, Florida, 1964. 

Church s equations, 176-177 Combustion instability, 52-57 bulk-coupled, 56-57 pressure-coupled, 52-55 velocity-coupled, 55-56 steady-state, 29-51 prediction, 30 pressure plateaus, 34 propellants, 31-50 termination, 57-64 depressurization, 58-62 fluid-injection, 63-64 L, 62-63... 

Combustion features a wide range of instabilities, which have received considerable attention in recent years. The subject is of fundamental interest and it also has many practical implications. Combustion instability... 

L. Crocco and S. Cheng. Theory of Combustion Instability in Liquid Propellant Rocket Motors. Butterworths, London, 1956. 

D.T. Harrje and F.H. Reardon. Liquid propellant rocket combustion instability. Technical Report SP-194, NASA, Washington, DC, 1972. 

F.E.C. Culick. Combustion instabilities in liquid-fueled propulsion systems, an overview. AGARD Conference Proceedings Combustion Instabilities in Liquid Fuelled Propulsion Systems, 450, pp. 1.1-1.73. NATO, 1988. 

V. Yang and A. Anderson. Liquid rocket engine combustion instability, volume 169 of Progress in Astronautics and Aeronautics. AIAA, Washington DC, 1995. 

T. Poinsot, A. Trouve, D. Veynante, S. Candel, and E. Esposito. Vortex driven acoustically coupled combustion instabilities. Journal of Fluid Mechanics, 177 265-292, 1987. 

T.C. Lieuwen, Y. Neumeier, and B.T. Zinn. The role of unmixedness and chemical kinetics in driving combustion instabilities in lean premixed combustors. Combustion Science and Technology, 135 193-211,1998. 

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. 

Many items in this coupled process have been extensively explored in recent years [13-23]. The munerical simulation of combustion instabilities has also progressed quite remarkably and this topic is covered in Ref. [12] and... 

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.)... 

L. Crocco. Aspects of combustion instability in liquid propellant rocket motors, part 1. /. Am. Rocket Soc., 21 163-178, 1951. 

M. Barrere and F.A. Williams. Comparison of combustion instabilities found in various types of combustion chambers. Proc. Combust. Inst., 12 169-181, 1969. 

T.C. Lieuwen and V. Yang, eds. Combustion Instabilities in Gas Turbine Engines Operational Experience, Fundamental Mechanisms, and Modeling. Progress in Astronautics and Aeronautics, Vol. 210, AlAA, 2005. 

K. McManus, T. Poinsot, and S. Candel. A review of active control of combustion instabilities. Prog. Energ. Combust. Sci., 19 1-29,1993. 

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. Gandel, D. Durox, and T. Schuller. Elame interactions as a source of noise and combustion instabilities. AIAA Raper 2004-2928, 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. 

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