16.6.2 Unsteady Flamelet

In the Flamelet tab of the Species Model dialog box (Figure  16.6.2), you will enter values for parameters of the flamelet.

Figure 16.6.2: The Flamelet Tab for the Unsteady Laminar Flamelet Model

The Unsteady Flamelet Parameters are as follows:

Number of Grid Points in Flamelet   specifies the number of mixture fraction grid points distributed between the oxidizer ( $f=0$) and the fuel ( $f=1$). Increased resolution will provide greater accuracy, but since the flamelet species and temperature are solved coupled and implicit in $f$ space, the solution time and memory requirements increase with the number of $f$ grid points.

Mixture Fraction Lower Limit for Initial Probability   is the mixture fraction above which the marker probability will be initialized to 1, and below which the marker probability will be initialized to 0. In general, it should be set greater than the stoichiometric mixture fraction.

Maximum Scalar Dissipation   is where flamelets extinguish at large scalar dissipation (mixing) rates. To prevent excessive mixing in the flamelet, ANSYS FLUENT allows you to specify a Maximum Scalar Dissipation rate for the 1D flamelet equations. A reasonable value for this is the steady flamelet extinction scalar dissipation. The default value of 30/s is near the steady extinction scalar dissipation of a methane-air flame at standard temperature and pressure.

Courant Number   is the number at which ANSYS FLUENT automatically selects the time step for the probability equation based on this convective Courant number.

Click Initialize Unsteady Flamelet Probability to initialize the unsteady flamelet and its probability marker equation. ANSYS FLUENT is now ready for postprocessing the 1D unsteady flamelet and the 2D/3D unsteady marker probabilty equation.