Turbulent flames Subject

The basic problem in turbulent flames is thus to determine the structure of the flame. It is unfortunate, in view of the great technical importance of turbulent burning, that this has not yet been done. There are two schools of thought on the subject ... 

In the first edition of this book the comment was made that, in a sense, the topic of premixed turbulent flames did not belong in a book on combustion theory because the theoretical basis of the subject was practically undeveloped. This statement is no longer true. In recent years remarkable progress has been made in the theory of turbulent flames, and a number of new approaches to the subject are available for both premixed and non-premixed systems. Moreover, a few of these approaches are sufficiently firmly based to enable analyses to be pursued with high confidence in limiting cases. Therefore, it is now entirely appropriate to discuss theories of turbulent combustion here. In fact, the subject is so broad that we shall be able to cover only a portion of it and must refer to recent reviews [13]-[40] for more extensive presentations. In particular, [27] is a 250-page book devoted to the subject. 

Chapter 10 on turbulent-flame theory also is long, as it must be because so many different viewpoints and approaches to the subject now are available. Included in this chapter are discussions of analyses of effects of strain on laminar flame sheets, topics of interest in themselves as well as in connection with turbulent combustion. Evolution equations for laminar flames in non-uniform flows also are given. The results outlined for turbulent burning velocities emphasize those aspects that have the strongest basic theoretical justifications. Since turbulent-flame theory is a subject of continuing development, improvements of results presented herein might be anticipated to be available in the not-too-distant future. 

Some characteristics of edge-flames are identified by S.H. Chung in Chapter 4.3. Flames with edges occur in many forms. A thorough understanding of this subject is essential for turbulent combustion modeling. 

Currently, computing the structure of bluff-body stabilized flames has become a subject of intense activity. The general objective of numerical studies is to describe the phenomenon by solving the fundamental differential equations coupled with turbulence and combustion closures. Since there are many possible approaches, more or less substantiated, the reported results are often contradictory. Apparently, this is caused by the lack of basic understanding of the physico-chemical phenomena accompanying flame stabilization and spreading. 

Laminar flames in turbulent flows are subjected to strain and develop curvature as consequences of the velocity fluctuations. These influences modify the internal structure of the flame and thereby affect its response to the turbulence. The resulting changes are expected to be of negligible consequence at sufficiently large values of Jb in Figure 10.5, but as turbulence scales approach laminar-flame thicknesses, they become important. Therefore, at least in part of the reaction-sheet regime, consideration of these effects is warranted. The effects of curvature were discussed in Section 9.5.2.3. Here we shall focus our attention mainly on influences of strain. 

A similar flat flame technique—one that does not require a heat loss correction—is the so-called opposed jet system. This approach to measuring flame speeds was introduced to determine the effect of flame stretch on the measured laminar flame velocity. The concept of stretch was introduced in attempts to understand the effects of turbulence on the mass burning rate of premixed systems. (This subject is considered in more detail in Section 4.E.) The technique uses two... 

Early explosion tests with gas clouds were used to derive simple empirical correlations concerning flame extension or lifetime as a function of fuel mass released, e.g., WHAZAN, a software package of the World Bank [115]. Other approaches are subjected to a turbulent jet flame [74] or the dynamics of a rising and expanding fire ball or the impact by the heat radiation flux [31]. Common to all models is the fact that they are based on a more or less empirical approach with a contentious capability of predicting the consequences of an explosion. 

For a non-premixed flame, the relatively thin reaction zone is viewed as a collection of laminar flamelets subject to turbulence fluctuations. The chemical reactions within a flamelet can be treated as a locally one-dimensional phenomenon that depends on the fuel-oxidizer mixture. This mixture is expressed in terms of the mixture fraction... 

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