16.3.5 Using the Unsteady Laminar Flamelet Model

The unsteady laminar flamelet model can only be enabled from a valid steady-state, steady laminar flamelet solution. When enabled, the unsteady laminar flamelet model will change this case to unsteady and post-process a marker probability equation on the frozen flow field. You should hence ensure that the starting steady-state, steady laminar flamelet solution is fully converged.

When the Unsteady Flamelet is enabled in the Chemistry tab, the Import Flamelet File for Restart... button appears in the dialog box, allowing you to run the simulation from a previously saved case, data and unsteady flamelet file.

The Unsteady Laminar Flamelet Model requires four user inputs in the Flamelet tab:

• The Number of Grid Points in Flamelet.

• The Mixture Fraction Lower Limit for Initial Probability. The initial condition of the marker probability field is unity for all mean mixture fractions above the Mixture Fraction Lower Limit for Initial Probability, and zero for mean mixture fractions below it. Note that this should be specified to be greater than the stoichiometric mixture fraction.

• Maximum Scalar Dissipation. Laminar flamelets may extinguish at high scalar dissipations because diffusion in the flamelet overwhelms reaction. It is possible to have unrealistically high modeled scalar dissipation in the 2D or 3D ANSYS FLUENT simulations, which gets transferred to the 1D unsteady flamelet. In order to avoid excessive diffusion in the 1D unsteady flamelet, the instantaneous scalar dissipation in the 1D flamelet is limited to the specified Maximum Scalar Dissipation.

• Courant Number. The time step for the unsteady probability marker equation is calculated automatically by ANSYS FLUENT based on the Courant Number. Larger values imply fewer time steps to convect/diffuse the marker probability out of the domain, but also results in a larger numerical error. The Courant Number should be small enough so that the unsteady flamelet mean mass fractions are unchanged with any smaller Courant Number. The default value of 1 should be sufficient for most applications.

When these inputs have been set, clicking the Initialize Unsteady Flamelet Probability button initializes the marker probability equation, automatically enabling the Unsteady solver, while disabling all equations except the Unsteady Flamelet Probability equation in the Solution Controls dialog box. This initialization in the Flamelet tab also sets the Time Step Size in the Iterate dialog box. (Clicking the Apply button in the Chemistry tab will also automatically change the solver settings).

Do not initialize your solution using the Solve/Initialize/Initialize... menu path. Note that you are postprocessing a probability field on the frozen steady-state flow field, and by clicking the Initialize Unsteady Flamelet Probability button, you have already initialized the probability marker field.

If you disable the Unsteady Laminar Flamelet model and you want to revert to solving a steady laminar flamelet simulation, make sure you enable Steady in the General task page and enable all the equations in the Solution Controls task page.