### 9.9.3. Boundary Settings

When specifying an inlet or opening boundary condition, you will set options for mixture, for reaction progress and for flame surface density. When specifying a wall boundary condition an option will be set only for flame surface density.

#### 9.9.3.1. Mixture

The options available are:

- Fuel: This option is used when the mass fraction of the fuel component is 1.

- Oxidizer: This option is used when the mass fraction of the oxidizer (usually air) is 1.

- Mixture Fraction: Use this parameter when you need to specify the mixture of fuel and oxidizer or the equivalent reacted mixture (for example, in a reacted pilot flame). A value of zero corresponds to 100% oxidizer, and a value of 1 corresponds to 100% fuel.

- Mixture Fraction Mean and Variance: Used in conjunction with the Mixture Fraction parameter, but additionally you can specify the variance for the “unmixedness” of fuel and oxidizer. By default, the three options above imply zero variance.

- Equivalence Ratio: Use this parameter when you need to specify the mixture of fuel and oxidizer, or the equivalent reacted mixture. The difference to the Mixture Fraction option is that the value is given for the equivalence ratio: 1 = stoichiometric, <1 for lean mixtures and >1 for fuel rich mixtures.

#### 9.9.3.2. Reaction Progress

This parameter is used to specify the reaction progress variable . corresponds to fresh (unburned) gases, whereas corresponds to completely burned gases. You can select `Fresh Gases (c=0)`, `Burned Gases (c=1)`, or enter a value or expression for the value of .

#### 9.9.3.3. Flame Surface Density

For inlet or opening boundary conditions a value will be specified for the flame surface density, which is the area of flame surface per volume. A value of `0 [m^-1]` corresponds to no flame existing on the boundary and is appropriate for most inlets or openings.

At wall boundaries an option will be specified controlling how the flame surface interacts with the wall. The following options are available:

- Wall Quenching:Enables the wall quenching model

- Zero Flux: Disables the wall quenching model

- Wall Flux in: Use this parameter in order to implement custom models for flame-wall interaction. Negative values for the Flame Surface Density Flux parameter correspond to destruction of the flame at the wall (quenching).

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