### 1.7.1. CFX-Pre Set-up

To set up a case, you require a variable composition mixture, the condensable component of which must be part of a homogeneous binary mixture. The latter is not required to be a fluid in the domain, but must be part of the case definition. The variable composition mixture must consist of one condensable component and one or more non-condensable components.

Note: There is a template model file (wall_condensation.ccl) to aid the set up of cases that use the wall condensation model.

The following topics are discussed:

- Domain Fluid Model Specification
- Boundary Condition Specification
- Restrictions
- Convergence Tip

#### 1.7.1.1. Domain Fluid Model Specification

To use the wall condensation model, visit the Fluid Models tab of the domain, and set Component Models > Wall Condensation Model > Option to `Concentration Boundary Layer Model`.

Note that the wall condensation model is allowed only for multi-component fluids defined on fluid domains on which the Thermal Energy or Total Energy model is active. Also, there must exist a unique condensable component. It is necessary to indicate which component is condensable, and which are non-condensable. For the condensable component, it is necessary to name an associated Homogeneous Binary Mixture that will be used to compute the required saturation material properties. The named Homogeneous Binary Mixture (HBM) must exist in the materials library, and must contain the condensable component as one of its components, and the condensate as the other component.

Note that the condensation model settings can be applied to only components that have the `Transport Equation` or `Algebraic Slip` option. In most applications, the condensable component will be vapor and the condensate will be liquid. However, the model is valid in other contexts, such as sublimation, so all thermodynamic states are permissible for the condensable component and the condensate.

#### 1.7.1.2. Boundary Condition Specification

Wall condensation must be explicitly activated on those wall or fluid-solid boundaries for which it is required. The default condition on these boundaries, for all components, is Zero Flux.

Note that there are slight differences in the reporting of heat flux imbalances at a condensing fluid-solid interface, depending on whether the interface is one-to-one periodic or GGI. For a one-to-one periodic interface, condensation heat fluxes are computed internally in the same way that user boundary sources are computed, so the condensation heat fluxes are reported as boundary sources associated with the boundary condition. For a GGI interface, condensation heat fluxes are added to the volumetric heat sources, so the condensation heat fluxes appear as part of the heat source contribution from the domain.

#### 1.7.1.3. Restrictions

The wall condensation model has the following restrictions:

- The wall condensation model is allowed only for multi-component fluids defined on fluid domains on which the Thermal Energy or Total Energy model is active.

- Only one condensable component is permitted. There is no restriction on the number of non-condensable components. The condensable component should either have Option = Transport Equation or Option = Algebraic Slip.

- Condensation boundary conditions are restricted to walls with specified temperature, and fluid-solid interfaces. The following further restrictions are imposed:

  - You cannot specify wall condensation on non-overlap boundaries.

  - Free slip and specified stress walls are allowed for laminar flows but are disallowed for turbulent flows.

- It is not possible to specify both wall condensation boundary conditions and additional user boundary mass sources on a given boundary surface.

#### 1.7.1.4. Convergence Tip

At CHT boundaries adjacent to a low conductivity solid, convergence can be very slow or difficult to obtain. This can be helped significantly by applying explicit under-relaxation to both the condensation mass flux and condensation heat flux at the interface. This can be achieved by the following CCL:

    LIBRARY:
      VARIABLE: condmflux
         Option = Definition
         Under Relaxation Factor = <real>
      END
      VARIABLE: condhflux
         Option = Definition
         Under Relaxation Factor = <real>
      END
    END

Under-relaxation values between 0.01 and 0.1 are recommended.

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