The steady laminar flamelet approach models a turbulent flame brush as an ensemble of discrete, steady laminar flames, called flamelets. The individual flamelets are assumed to have the same structure as laminar flames in simple configurations, and are obtained by experiments or calculations. Using detailed chemical mechanisms, FLUENT can calculate laminar opposed-flow diffusion flamelets for non-premixed combustion. The laminar flamelets are then embedded in a turbulent flame using statistical PDF methods.
The advantage of the laminar flamelet approach is that realistic chemical kinetic effects can be incorporated into turbulent flames. The chemistry can then be preprocessed and tabulated, offering tremendous computational savings. However, the steady laminar flamelet model is limited to modeling combustion with relatively fast chemistry. The flame is assumed to respond instantaneously to the aerodynamic strain, and thus the model cannot capture deep non-equilibrium effects such as ignition, extinction, and slow chemistry (like NOx).
For general information about the mixture fraction model, see Section 15.1.