Diffusion flames appearance

Among the areas not covered here is that of intrinsic instabilities associated with chemical-kinetic mechanisms, as exhibited in cool-flame phenomena, for example these subjects are touched briefly in Section B.2.5.3. Intrinsic instabilities of detonations were considered in Section 6.3.1 and will not be revisited. Certain aspects of intrinsic instabilities of diffusion flames were mentioned briefly in Section 3.4.4 diffusion flames appear to exhibit fewer intrinsic instabilities than premixed flames, although under appropriate experimental conditions their effects can be observed, as indicated at the end of Section 9.5.2. Certain chamber instabilities that are not related to acoustic instabilities (such as Coanda effects—oscillatory attachment of flows to different walls) will not be discussed here, but reviews are available [1]. 

The theory of premixed flames essentially consists of an analysis of factors such as mass diffusion, heat diffusion, and the reaction mechanisms as they affect the rate of homogeneous reactions taking place. Inasmuch as the primary mixing processes of fuel and oxidizer appear to dominate the burning processes in diffusion flames, the theories emphasize the rates of mixing (diffusion) in deriving the characteristics of such flames. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

One of the earliest detailed diagnostic efforts on sooting of diffusion flames was that of Wagner et al. [86-88], who made laser scattering and extinction measurements, profile determinations of velocity by LDV, and temperature measurements by thermocouples on a Wolfhard-Parker burner using ethene as the fuel. Their results show quite clearly that soot particles are generated near the reaction zone and are convected farther toward the center of the fuel stream as they travel up the flame. The particle number densities and generation rates decline with distance from the flame zone. The soot formation rate appeared to... 

Sampling in inverse coannular diffusion flames [62] in which propene was the fuel has shown the presence of large quantities of allene. Schalla et al. [57] also have shown that propene is second to butene as the most prolific sooter of the n-olefins. Indeed, this result is consistent with the data for propene and allene in Ref. 72. Allene and its isomer methylacetylene exhibit what at first glance appears to be an unusually high tendency to soot. However, Wu and Kem [111] have shown that both pyrolyze relatively rapidly to form benzene. This pyrolysis step is represented as alternate route C in Fig. 8.23. 

Mention has already been made of beam experiments in which electronic chemiluminescence has been observed from reactions of metal atoms. Palmer s [423-426] and Broida s [427,428] groups have studied reactions of this type in low-pressure diffusion flames. In most cases, the yield of excited species is disappointingly low. The most efficient of the reactions investigated so far (this excludes the reactions of the type X + M2 - MX + M studied by Herschbach s group) appears to be... 

In general, analytical methods employing Rate-Ratio Asymptotics (RRA) can help contribute to understanding of mechanisms of NO.,. production in diffusion flames, and can provide prediction of emission indices within reasonable accuracy. This method appears to hold promise for calculation of contaminant production in the combustion process, and can be extended to those involving novel high-energy hydrocarbon fuels. 

For initially nonpremixed reactants, two limiting cases may be visualized, namely, the limit in which the chemistry is rapid compared with the fluid mechanics and the limit in which it is slow. In the slow-chemistry limit, extensive turbulent mixing may occur prior to chemical reaction, and situations approaching those in well-stirred reactors (see Section 4.1) may develop. There are particular slow-chemistry problems for which the previously identified moment methods and age methods are well suited. These methods are not appropriate for fast-chemistry problems. The primary combustion reactions in ordinary turbulent diffusion flames encountered in the laboratory and in industry appear to lie closer to the fast-chemistry limit. Methods for analyzing turbulent diffusion flames with fast chemistry have been developed recently [15], [20], [27]. These methods, which involve approximations of probability-density functions using moments, will be discussed in this section. 

Equation (42) cannot be used if NO concentrations approach their equilibrium values, since the net production rate then depends on the concentration of NO, thereby bringing bivariate probability-density functions into equation (40). Also, if reactions involving nitrogen in fuel molecules are important, then much more involved considerations of chemical kinetics are needed. Processes of soot production similarly introduce complicated chemical kinetics. However, it may be possible to characterize these complex processes in terms of a small number of rate processes, with rates dependent on concentrations of major species and temperature, in such a way that a function w (Z) can be identified for soot production. Rates of soot-particle production in turbulent diffusion flames would then readily be calculable, but in regions where soot-particle growth or burnup is important as well, it would appear that at least a bivariate probability-density function should be considered in attempting to calculate the net rate of change of soot concentration. 

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