### 9.4.1. Fluid Models

The models settings described in this section are set when creating a domain in CFX-Pre. For details, see Domains in the CFX-Pre User's Guide.

#### 9.4.1.1. Chemical Time Scale

The reaction rate is set to zero if the turbulent time scale is smaller than this value. When the model for flame extinction at high turbulence is activated, local extinction occurs when the turbulence time scale is smaller than a chemical time scale (quenching time scale) provided by you. Because this is a very simple model for predicting local extinction, the specified chemical time scale may need to be adjusted in order to achieve best results for a specific problem. For methane-air combustion, good starting points are `1.37e-4 [s]` when applying the Kolmogorov time scale, or `5e-4 [s]` when comparing to the mixing time scale. Using the Kolmogorov time scale tends to be more aggressive and may lead to global extinction of the flame, even in situations where this is not physical. It is for this reason that the mixing time scale is recommended. By default the mixing time scale is applied. This may be changed by setting the expert parameter `use kolmogorov ts for extinction` to `T`.

#### 9.4.1.2. Extinction Temperature

This is a simple model that disables the reaction wherever the temperature is less than the specified extinction temperature.

#### 9.4.1.3. Component Details

This is used to define how the mass fraction of the component is to be computed. You must set exactly one component to `Constraint`. The mass fraction of this component will be calculated to be 1 minus the mass fractions of all other components. The constraint could be any of the components, either passive or taking part in the reaction(s). For reasons of accuracy, however, this should be a major component (large mass fraction). For combustion of a fuel in air, the best choice is N2.

For the Eddy Dissipation Model, choose `Automatic` or `Transport Equation` for all other components.

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