Approximation Options Properties

Relax to Chemical Equilibrium: When activated, an Approximation Options > Relax to Chemical Equilibrium node appears.; The Relax to Chemical Equilibrium (RTCE) option relaxes the species composition in each cell to the local equilibrium composition.; Use the RTCE option when a chemical kinetic mechanism is not available, or for a fast solution with a few major species where only the flame position is required.; You are advised to use this option if NH₃ is the only fuel.

Relax to Chemical Equilibrium

When the Complex Chemistry > Approximation Options property Relax to Chemical Equilibrium (RTCE) is activated, the species field is relaxed to its chemical equilibrium compositions over a set turbulent time-scale $τ_{c h a r}$ .

When using the Relax to Chemical Equilibrium option with the Complex Chemistry model—but without the Turbulent Flame Speed Closure model— $τ_{c h a r}$ is determined by considering the timescales for the flow and chemistry as in Eqn. (3416).
When using the Relax to Chemical Equilibrium option with the Complex Chemistry model and the Turbulent Flame Speed Closure model, $τ_{c h a r}$ is determined by the Kolmogorov timescale—which you can scale using a timescale constant as in Eqn. (3417).

Since no chemical kinetics are calculated, the RTCE option is an approximation that is usually used as an initial condition for finite-rate chemistry simulations. The RTCE method cannot predict ignition or slowly forming pollutants such as NOx.

The chemistry time scale is computed based on the specified fuel species.

See Complex Chemistry.

Relax to Chemical Equilibrium Properties

Timescale Constant: $c_{t}$ in Eqn. (3417) which allows you to control the relaxation time of the reaction behind the flame front by scaling the Kolmogorov timescale $τ_{k}$ .

The following properties are unavailable when using the Turbulent Flame Speed Closure (TFC) model:

Flow Time Constant: Scales the flow timescale $τ_{f l o w}$ that is used to calculate $τ_{c h a r}$ in Eqn. (3416).
Chemistry Time Constant: Scales the chemical timescale $τ_{c h e m}$ of the fuel in Eqn. (3416).
Include Chemistry Time: Only available when reactions are specified.; When activated, the chemical timescales of the selected Fuel Components are considered alongside the turbulent flow timescale to determine the characteristic time scale.; Including the chemical timescales can prevent auto-ignition ahead of the flame front.
Fuel Components: The fuel component species that are considered when determining the chemistry time step when Include Chemistry Time is activated.