Reaction-sheet regime

The problem to which these methods are applied here is one of turbulent combustion in the reaction-sheet regime. It is known that turbulent reacting flows exhibit different regimes of turbulent combustion, as illustrated in Fig. 25.1. The diagram shown applies to both premixed flames... 

The nonlinear interaction between turbulence and the hydrodynamic instability is an important, difficult, and unresolved problem of turbulent-flame theory for large-scale turbulence in the reaction-sheet regime. 

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. 

In the reaction-sheet regime, the structure of the turbulent flame is determined by the dynamics of wrinkled laminar flames. Thus the thickness of the turbulent flame (if it is large compared with that of the laminar flame) is controlled by the distance to which fluctuations in the laminar-flame position may extend. Statistical aspects of distributions of temperature and of species concentrations in the turbulent flame can be expressed entirely in terms of statistics of the laminar-flame position (through /), orientation (through V //1V / ), and structure (through k). The simplest example is... 

For burning in a reactive atmosphere, conditions may be encountered for which the analysis of the diffusion-flame regime is relevant. Under suitable conditions there are two thin reaction sheets, an inner one at which premixed decomposition of the monopropellant occurs and an outer one having a diffusion-flame character. Categorizations are available for potential limiting behaviors at large Damkohler numbers in reactive environments [206]. There are many flame-structure possibilities, not all of which have been analyzed thoroughly. 

As the premixed-flame temperature decreases, conditions may be calculated for which the second reactant also begins to leak through the reaction zone in appreciable quantities. An asymptotic analysis may be developed for which, in the reaction zone, the factor tp — tf remains effectively constant [184], Departures from equations (82), (83), and (84) then occur even in lowest order on both sides of the flame sheet. This regime has been termed the partial-burning regime [184]. 

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