### 9.3.1. 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.

#### 9.3.1.1. 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. As 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`.

#### 9.3.1.2. Extinction Temperature

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

#### 9.3.1.3. Maximum Flame Temperature

Because of the assumption of complete combustion, the Eddy Dissipation model may over-predict temperature under certain conditions (for example, for hydrocarbon fuels in regions with fuel-rich mixture). When the expected maximum temperature (or an estimated bound for it) is known prior to the simulation, it can be provided to the solver as a constraint. The exact limit is the adiabatic flame temperature for the local mixture. Depending on the application it may be sufficient to use a simple correlation or even a constant limit. CEL expressions may be used in the maximum flame temperature parameter. When the specified maximum flame temperature limit is reached, the reaction rate will be locally stopped, leaving behind fuel and oxidizer. The reaction may continue in other regions of the domain where the fluid temperature is lowered (for example, by mixing with cold fluid or by heat transfer). Note that the maximum flame temperature only bounds temperature increase due to reaction heat release, but it is possible that a different process such as compression or heat transfer can exceed the specified limit.

#### 9.3.1.4. Mixing Rate Limit

The maximum allowed value of the local turbulent mixing rate. Only applicable if the Eddy Dissipation Model is used. The Eddy Dissipation Model sometimes predicts unphysical behavior, such as the flame creeping across walls. This is because the ratio of the turbulence quantities becomes large close to wall boundaries, but the turbulence in these regions is low. This value is used to enforce an upper limit on the value used for computing the reaction rate. For methane/air, a value of approximately 2500[1/s] is reasonable.

#### 9.3.1.5. Eddy Dissipation Model Coefficient A/B

The coefficient A is a factor that appears in both the reactants and the products limiter. For details, see:

- Reactants Limiter in the CFX-Solver Theory Guide

- Products Limiter in the CFX-Solver Theory Guide.

`A` must always be positive. There is special treatment in the solver for negative `B`. If this is the case, the product limiter is not applied and the reaction rate is determined by reactants concentrations and turbulence. When `B` is negative, the magnitude of `B` is not significant.

The following default values apply:

`A = 4.0`, `B = -1`

Variable expressions (CEL) are supported for both parameters.

#### 9.3.1.6. Component Details

This is used to define how the mass fraction (the ratio of the mass of a component to the total mass of the fluid) 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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