Modeling of Turbulent Combustion

Abstract

In turbulent combustion, spatial and temporal fluctuations always play a predominant role. Along with the strong nonlinearity in combustion physics and chemistry, for example, in expressions for reaction rates in terms of species concentrations and temperature (and pressure), this results in a nontrivial closure problem. In this chapter, turbulence modeling and turbulent scales are first briefly discussed, after which details of the two extreme combustion configurations – premixed combustion and nonpremixed combustion – are outlined. Finally, some issues are noted on partially premixed combustion. The text is restricted to single‐phase combustion and, for turbulent premixed combustion, the relevant time and length scales are discussed in typical turbulent flame structures. Based on this, the combustion regime diagram is introduced, turbulent flame speed is discussed, and progress variable formalism described. Fast chemistry and finite rate chemistry effects, with associated closure issues, are also considered. For nonpremixed combustion, the eddy break‐up model and the eddy dissipation concept are introduced, after which the mixture fraction approach is explained. The preassumed probability density function (PDF) method is introduced, the laminar flamelet concept discussed, and the importance of the scalar dissipation rate highlighted. Finite rate chemistry effects are also discussed. Finally, the transported PDF and conditional moment closure (CMC) frameworks are introduced.