Flame instability

Laminar flame instabilities are dominated by diffusional effects that can only be of importance in flows with a low turbulence intensity, where molecular transport is of the same order of magnitude as turbulent transport (28). Flame instabilities do not appear to be capable of generating turbulence. They result in the growth of certain disturbances, leading to orderly three-dimensional stmctures which, though complex, are steady (1,2,8,9). 

Combustion analysis This ineludes the use of pyrometers to deteet metal temperatures of both stationary and rotating eomponents sueh as turbine blades. The use of dynamie pressure transdueers to deteet flame instabilities in the eombustor espeeially in the new dry low NO applieations. 

In all of these tests, flame acceleration was minimal or absent. Acceleration, when it occurred, was entirely due to intrinsic flame instability, for example, hydrodynamic instability (Istratov and Librovich 1969) or instability due to selective diffusion (Markstein 1964). To investigate whether the flame would accelerate when allowed to propagate over greater distances, tests were carried out in an open-sided test apparatus 45 m long (Harris and Wickens 1989). Flame acceleration was found to be no greater than in the balloon experiments (Table 4.1a). 

Real-life premixed flame fronts are rarely planar. Of course, if the flow is turbulent, gas motion will continuously deform and modify the geometry of the flame front, see Chapter 7. However, even when a flame propagates in a quiescent mixture, the front rapidly becomes structured. In this chapter, we will discuss hydrodynamic flame instability, thermo-diffusive instability, and thermo-acoushc instability. 

G. Searby, J.M. Truffaut, and G. Joulin. Comparison of experiments and a non-linear model for spatially developing flame instability. Physics of Fluids, 13 3270-3276, 2001. 

The dramatic dynamics of the flame-shape change shown in Figure 5.3.1, along with its proposed relationship to the flame instability and flame-generated flow, has periodically sparked an interest in its study. Before reviewing this flame-shape transition phenomenon, it will be useful to trace the history of the "tulip" name and distinguish this particular flame shape from the myriad of others with which it is often equated or confused. 

N Konga,B., Fernandez, G.,Guillard,H.,and Larrouturou, B., Numerical investigations of the tulip flame instability—comparisons with experimental results, Combustion Science and Technology, 87, 69-89,1992. 

Kadowaki, S. and Hasegawa, T, Numerical simulation of d)mamics of premixed flames Flame instability and vortex-flame interaction. Progress in Energy and Combustion Science, 31, 193-241, 2005. 

Since the concern here is with the destruction of a contiguous laminar flame in a turbulent field, consideration must also be given to certain inherent instabilities in laminar flames themselves. There is a fundamental hydro-dynamic instability as well as an instability arising from the fact that mass and heat can diffuse at different rates that is, the Lewis number (Le) is nonunity. In the latter mechanism, a flame instability can occur when the Le number (oJD) is less than 1. 

Due to strict environmental regulations, turbine engine manufacturers and users are facing great challenges in reducing NO, SO , CO, and hydrocarbons to meet the compliance limits. To date, several innovative methods have been developed and introduced into the market to combat these problems. However, often, while reducing NOj, or other pollutants, new problems arise. These problems are lower turn-down ratio, poor thermal efficiency, flame instability, uneven temperature distribution in the combustor, and noise. These problems are even more pronounced in a liquid fuel fired turbine combustor. Therefore, the present research... 

Catalytic combustion has been commercially demonstrated to reduce NO.. emissions to below 3 ppm while keeping CO and UHC emissions below 10 ppm without the need for expensive exhaust clean-up systems. In addition, a catalytic combustor reduces typical DLN problems such as risk of blow-out and flame instability. Also, the economic advantage of primary methods including catalytic combustion as opposed to secondary clean-up measures (SCR and SCONOx) has recently been assessed [1]. 

Thermal expansion of a gas in a curved flame front leads to the formation of gasdynamic vorticity in the combustion products and is the cause of a flame instability discovered by L. D. Landau, and also by G. Darriet (France), in 1944. It turned out, however, that this instability was very reluctant to exhibit itself in experiments The first explanation of such a phenomenon—using the example of a spherical flame—was given by A. G. Istratov and V. B. Librovich. Ya.B. and his coauthors [34] proposed a method for calculating rapid combustion in a tube containing an elongated flame... 

However, it should be emphasized here that the complete stability question is complex. Flame stability is the subject of the following chapter. For Lewis numbers different from unity, there are diffusive-thermal instabilities that are influenced by heat loss and that have led to various statements concerning the dependence of the stability in Figure 8.2 on the Lewis number [46]. We shall postpone any further discussion of these stability questions until the following chapter. It seems sufficient here to reemphasize that if special types of flame instabilities are not observed in an experiment, then the upper branch of Figure 8.2 may be expected to apply, with I = 1/e providing a correct extinction criterion (within about 10% accuracy since corrections of order 1/P may be anticipated). 

The titles of the first six chapters remain essentially the same as in the first edition. Chapter 7 of the first edition, which was devoted mainly to a discussion of experiments on turbulent flames, has been deleted and replaced by a considerably more extensive chapter (Chapter 10) that develops turbulent-flame theory from first principles noteworthy advances in the theory during the past 20 years have made this possible. Presentation of material on turbulent combustion, formerly in Chapter 7, is postponed until Chapter 10 because the new knowledge relies in part on various results that have been obtained in studies of laminar-flame instabilities (now... 

Air and fuel are intimately mixed prior to entenng the combustion zone, and hence the local air-fiiel ratios can be controlled and variations minimized. The homogeneous air-fuel mixture is combusted at a very high air-fuel ratio, and hence involves only a small adiabatic temperature nse. Therefore, the maximum temperature in the combustor is kept at levels at which the thermal NOx concentration is low. Lean-premix systems suffer from stability problems at ultralean conditions, i.e, low temperatures, and may therefore require stabilization by a diffusion flame [28] It was already mentioned previously that optimizing both CO/UHC and NO is difficult. This can be deduced from Fig. 3, showing the NO and CO emissions versus adiabatic flame temperature for a hypothetical lean-premix combustor. At low temperatures, the CO emissions increase rapidly, due to flame instability (the comparable increase in NOx levels at high temperatures has already been discussed in the previous section). It is clear that a temperature zone exists where low levels of both CO and NOx niay be obtained. 

Why is atomic emission more sensitive to flame instability than atomic absorption or fluorescence ... 

NO c emissions can also be reduced by using a dilute fuel/air mixture in so-called lean premixed combustion. Air and fuel are premixed at a low fuel air ratio before entering the combustor, resulting in a lower flame temperature and hence lower NOj emissions. Lean premix combustors (sometimes called dry low NO c combustors) based on this concept have generally achieved NO c levels of 25 ppm in commercial operation. To achieve lower NO levels, however, this technology must overcome some significant hurdles. As the fuel air mixture is increasingly diluted, the flame temperature approaches the flammability limit and the flame becomes unstable. The flame instability produces noise and vibration that can reduce the combustor life, increase maintenance costs and adversely impact the operational reliability of the turbine [3]. 

Vaezi, V., and R. C. Aldredge. 2000. Laminar-flame instabilities in a Taylor-Couette combustor. Combustion Flame 121 356. 

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