### 7.16.1. Wall Boundaries in Multiphase

For multiphase flows, it is convenient to enable two distinct methods for you to specify wall boundary conditions:

- Bulk wall boundary conditions are defined as boundary condition attributes of the wall, and subsequently shared among the phases in a well-defined manner. The algorithm for sharing wall fluxes among fluids depends on the concept of a wall contact model (which will be explained shortly).

- Alternatively, wall boundary conditions may be defined on a per fluid basis. This is a more advanced option, giving you complete flexibility in the definition of multiphase wall boundary conditions.

#### 7.16.1.1. Bulk Wall Boundary Conditions

This is the simplest and most useful option. In most applications, wall boundary conditions are known as attributes of the wall. It is then part of the modeling procedure to decide how the consequent wall fluxes are allocated among the phases in contact with the wall.

For example, for bulk heat transfer, you may specify either:

- A bulk heat flux to the wall, Q wall. The heat fluxes to each phase are determined as follows:

  | (7–10)

  Where is the contact area fraction of phase at the wall, calculated from the Area Contact Model as described below.

- A bulk heat transfer coefficient h wall and outer temperature T o. The heat fluxes to each phase are determined as follows:

  | (7–11)

  The difference between the specified wall temperature and the calculated phase temperature together with the heat transfer coefficient determines the heat flux to the phase. The contact area fraction is also taken into account.

- A bulk wall temperature T wall. The specified wall temperature is allocated to all phases in contact with the wall, and resulting wall heat fluxes are partitioned among phases using the contact area fraction.

##### 7.16.1.1.1. Area Contact Model

By default, the contact area fraction of phase at the wall is identical to the volume fraction in the control volume adjacent to the wall:

| (7–12)

However, there are situations in which an alternative model may be required, in which case it can be overridden by providing a value or an expression for each phase. For example, for annular gas-liquid flow, the liquid forms a thin film at the wall and is therefore sensible to assign:

| (7–13)

Similarly, in dispersed gas-solid flows with spherical solid particles, the wall contact area of the dispersed phase may be considered to be negligible relative to the continuous phase.

Hence, ANSYS CFX includes a facility for you to override the default wall contact area model by providing a value or an expression for the contact area fraction for each phase. Note:

- Contact area fractions must sum to unity over all phases.

- Specified contact area fractions are applied to all transport processes at the wall, that is, wall stresses, heat fluxes, Additional Variable fluxes, and so on.

- You should only use the simple model:

  | (7–14)

  in situations where phase does not vanish at the wall. Otherwise, it is possible that you will be applying a non-zero wall flux to a region of zero volume of phase , which could cause the calculation to overflow. For the same reason, your initial guess should not set the fluid volume fraction to be zero at a wall when that phase uses a non-zero area fraction.

- In general it, is desirable, though not essential, to express contact area fractions as functions of volume fraction such that:

  | (7–15)

This ensures that finite wall fluxes do not enter regions of zero volume of phase .

#### 7.16.1.2. Fluid Dependent Wall Boundary Conditions

This option is included to give the advanced user complete flexibility in assigning wall boundary conditions to individual phases. For example, for heat transfer, you may specify either:

- The temperature, of phase , or

- The heat flux to phase , or

- Phase specific heat transfer coefficient outer temperature , in which case:

  | (7–16)

Note:

- These options may be mixed arbitrarily among the phases.

- Heat fluxes arising from phase specific isothermal boundary conditions are automatically multiplied by contact area fraction.

- However, specified heat fluxes are not automatically multiplied by contact area fractions . You should include these as multiplicative factors in your expressions for or if this is the effect that you require.

- Care should be exercised to ensure:

  | (7–17)

to prevent overflow errors due to finite sources being applied to zero fluid volumes.

#### 7.16.1.3. Wall Deposition

This option is only available for the Algebraic Slip Model. For details, see Wall Deposition.

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