24.2.9 Defining Multiphase Cell Zone and Boundary Conditions

The procedure for setting multiphase boundary conditions is slightly different than for single-phase models. You will need to set some conditions separately for individual phases, while other conditions are shared by all phases (i.e., the mixture), as described in detail below (Figure  24.2.9).

Boundary Conditions

Figure 24.2.9: The Boundary Conditions Task Page

Boundary Conditions for the Mixture and the Individual Phases

The conditions you need to specify for the mixture and those you need to specify for the individual phases will depend on which of the three multiphase models you are using. Details for each model are provided below.

VOF Model

If you are using the VOF model, the conditions you need to specify for each type of zone are listed below and summarized in Table  24.2.1.

• For an exhaust fan, inlet vent, intake fan, outlet vent, pressure inlet, pressure outlet, or velocity inlet, there are no conditions to be specified for the primary phase. For each secondary phase, you will need to set the backflow volume fraction as a constant, a profile (see Section  7.6), or a user-defined function (see the separate UDF Manual). All other conditions are specified for the mixture.

• For a mass flow inlet, you will need to set the mass flow rate, mass flux, or average mass flux for each individual phase. All other conditions are specified for the mixture.

  Note that if you read a VOF case that was set up in a version of ANSYS FLUENT prior to 6.1, you will need to redefine the conditions at the mass flow inlets.

• For an axis, fan, outflow, periodic, porous jump, radiator, solid, symmetry, or wall zone, all conditions are specified for the mixture. There are no conditions to be set for the individual phases.

• For a wall zone, you can specify the contact angle for the mixture if the wall adhesion option is enabled.

• For a fluid zone, mass sources are specified for the individual phases, and all other sources are specified for the mixture.

  If the fluid zone is not porous, all other conditions are specified for the mixture.

  If the fluid zone is porous, you will enable the Porous Zone option in the Fluid dialog box for the mixture. The porosity inputs (if relevant) are also specified for the mixture. The resistance coefficients and direction vectors, however, are specified separately for each phase. See Section  7.2.3 for details about these inputs. All other conditions are specified for the mixture.

See Chapter  7 for details about the relevant conditions for each type of boundary. Note that the pressure far-field boundary is not available with the VOF model.

Table 24.2.1: Phase-Specific and Mixture Conditions for the VOF Model

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; inlet vent; intake fan; outlet vent; pressure inlet; pressure outlet; velocity inlet	nothing	volume fraction	all others
mass flow inlet	mass flow/flux	mass flow/flux	all others
axis; fan; outflow; periodic; porous jump; radiator; solid; symmetry; wall	nothing	nothing	all others
pressure far-field	not available	not available	not available
fluid	mass source; other porous inputs	mass source; other porous inputs	porous zone; porosity; all others

Mixture Model

If you are using the mixture model, the conditions you need to specify for each type of zone are listed below and summarized in Table  24.2.2.

• For an exhaust fan, outlet vent, or pressure outlet, there are no conditions to be specified for the primary phase. For each secondary phase, you will need to set the volume fraction as a constant, a profile (see Section  7.6), or a user-defined function (see the separate UDF Manual) and if applicable, the backflow granular temperature. All other conditions are specified for the mixture.

• For an inlet vent, intake fan, or pressure inlet, you will specify for the mixture which direction specification method will be used at this boundary ( Normal to Boundary or Direction Vector). If you select the Direction Vector specification method, you will specify the coordinate system (3D only) and flow-direction components for the individual phases. For each secondary phase, you will need to set the volume fraction (as described above). All other conditions are specified for the mixture.

• For a mass flow inlet, you will need to set the mass flow rate, mass flux, or average mass flux for each individual phase. All other conditions are specified for the mixture.

  Note that if you read a mixture multiphase case that was set up in a version of ANSYS FLUENT previous to 6.1, you will need to redefine the conditions at the mass flow inlets.

• For a velocity inlet, you will specify the velocity for the individual phases. For each secondary phase, you will need to set the volume fraction (as described above). All other conditions are specified for the mixture.

• For an axis, fan, outflow, periodic, porous jump, radiator, solid, symmetry, or wall zone, all conditions are specified for the mixture. There are no conditions to be set for the individual phases. Outflow boundary conditions are not available for the cavitation model.

• For a fluid zone, mass sources are specified for the individual phases, and all other sources are specified for the mixture.

  If the fluid zone is not porous, all other conditions are specified for the mixture.

  If the fluid zone is porous, you will enable the Porous Zone option in the Fluid dialog box for the mixture. The porosity inputs (if relevant) are also specified for the mixture. The resistance coefficients and direction vectors, however, are specified separately for each phase. See Section  7.2.3 for details about these inputs. All other conditions are specified for the mixture.

See Chapter  7 for details about the relevant conditions for each type of boundary. Note that the pressure far-field boundary is not available with the mixture model.

Table 24.2.2: Phase-Specific and Mixture Conditions for the Mixture Model

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; outlet vent; pressure outlet	nothing	volume fraction	all others
inlet vent; intake fan; pressure inlet	coord. system; flow direction	coord. system; flow direction; volume fraction	dir. spec. method; all others
mass flow inlet	mass flow/flux	mass flow/flux	all others
velocity inlet	velocity	velocity; volume fraction	all others
axis; fan; outflow (n/a for cavitation model); periodic; porous jump; radiator; solid; symmetry; wall	nothing	nothing	all others
pressure far-field	not available	not available	not available
fluid	mass source; other porous inputs	mass source; other porous inputs	porous zone; porosity; all others

Eulerian Model

If you are using the Eulerian model, the conditions you need to specify for each type of zone are listed below and summarized in Tables  24.2.3, 24.2.4, 24.2.5, and 24.2.6. Note that the specification of turbulence parameters will depend on which of the three multiphase turbulence models you are using, as indicated in Tables  24.2.4- 24.2.6. See this section in the separate Theory Guide and Section  24.5.4 for more information about multiphase turbulence models.

• For an exhaust fan, outlet vent, or pressure outlet, there are no conditions to be specified for the primary phase if you are modeling laminar flow or using the mixture turbulence model (the default multiphase turbulence model), except for backflow total temperature if heat transfer is on.

  For each secondary phase, you will need to set the backflow volume fraction as a constant, a profile (see Section  7.6), or a user-defined function (see the separate UDF Manual). If the phase is granular, you will also need to set its backflowmgranular temperature. If heat transfer is on, you will also need to set the backflow total temperature.

  If you are using the mixture turbulence model, you will need to specify the turbulence boundary conditions for the mixture. If you are using the dispersed turbulence model, you will need to specify them for the primary phase. If you are using the per-phase turbulence model, you will need to specify them for the primary phase and for each secondary phase.

  All other conditions are specified for the mixture.

• For an inlet vent, intake fan, or pressure inlet, you will specify for the mixture which direction specification method will be used at this boundary ( Normal to Boundary or Direction Vector). If you select the Direction Vector specification method, you will specify the coordinate system (3D only) and flow-direction components for the individual phases. If heat transfer is on, you will also need to set the total temperature for the individual phases.

  For each secondary phase, you will need to set the volume fraction (as described above). If the phase is granular, you will also need to set its granular temperature.

  If you are using the mixture turbulence model, you will need to specify the turbulence boundary conditions for the mixture. If you are using the dispersed turbulence model, you will need to specify them for the primary phase. If you are using the per-phase turbulence model, you will need to specify them for the primary phase and for each secondary phase.

  All other conditions are specified for the mixture.

• For a mass flow inlet, you will need to set the mass flow rate, mass flux, or average mass flux for each individual phase. You will also need to specify the temperature of each phase, since the energy equations are solved for each phase.

  For mass flow inlet boundary conditions, you can specify the slip velocity between phases. When you select a mass flow inlet boundary for the secondary phase, two options will be available for the Slip Velocity Specification Method, as shown in Figure  24.2.10:

  • Velocity Ratio

    The value for the Phase Velocity Ratio is the secondary phase to primary phase velocity ratio. By default, it is 1.0, which means velocities are the same (no slip). By entering a ratio that is greater than 1.0, you are indicating a larger secondary phase velocity. Otherwise, you can enter a ratio that is less than 1.0 to indicate a smaller secondary phase velocity.

  • Volume Fraction

    If you specify the volume fraction at an inlet, ANSYS FLUENT will calculate the phase velocities.

    If a secondary phase has zero mass flux (i.e., the Eulerian model is used to run a single phase case), neither Phase Velocity Ratio nor Volume Fraction will affect the solution.

  Figure 24.2.10: Mass-Flow Inlet Boundary Condition Dialog Box

  

• For a velocity inlet, you will specify the velocity for the individual phases. If heat transfer is on, you will also need to set the total temperature for the individual phases.

  For each secondary phase, you will need to set the volume fraction (as described above). If the phase is granular, you will also need to set its granular temperature.

  If you are using the mixture turbulence model, you will need to specify the turbulence boundary conditions for the mixture. If you are using the dispersed turbulence model, you will need to specify them for the primary phase. If you are using the per-phase turbulence model, you will need to specify them for the primary phase and for each secondary phase.

  All other conditions are specified for the mixture.

• For an axis, outflow, periodic, solid, or symmetry zone, all conditions are specified for the mixture. There are no conditions to be set for the individual phases.

• For a wall zone, shear conditions are specified for the individual phases. All other conditions are specified for the mixture, including thermal boundary conditions, if heat transfer is on.

• For a fluid zone, all source terms and fixed values are specified for the individual phases, unless you are using the mixture turbulence model or the dispersed turbulence model. If you are using the mixture turbulence model, source terms and fixed values for turbulence are specified instead for the mixture. If you are using the dispersed turbulence model, they are specified only for the primary phase.

  If the fluid zone is not porous, all other conditions are specified for the mixture.

  If the fluid zone is porous, you will enable the Porous Zone option in the Fluid dialog box for the mixture. The porosity inputs (if relevant) are also specified for the mixture. The resistance coefficients and direction vectors, however, are specified separately for each phase. See Section  7.2.3 for details about these inputs. All other conditions are specified for the mixture.

See Chapter  7 for details about the relevant conditions for each type of boundary. Note that the pressure far-field, fan, porous jump, radiator, and mass flow inlet boundaries are not available with the Eulerian model.

Table 24.2.3: Phase-Specific and Mixture Conditions for the Eulerian Model (for Laminar Flow)

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; outlet vent; pressure outlet	(tot. temperature)	volume fraction; gran. temperature (tot. temperature)	all others
inlet vent; intake fan; pressure inlet	coord. system; flow direction (tot. temperature)	coord. system; flow direction; volume fraction; gran. temperature (tot. temperature)	dir. spec. method; all others
velocity inlet	velocity (tot. temperature)	velocity; volume fraction; gran. temperature (tot. temperature)	all others
axis; outflow; periodic; solid; symmetry	nothing	nothing	all others
wall	shear condition	shear condition	all others
pressure far-field; fan; porous jump; radiator; mass flow inlet	not available	not available	not available
fluid	all source terms; all fixed values; other porous inputs	all source terms; all fixed values; other porous inputs	porous zone; porosity; all others

Table 24.2.4: Phase-Specific and Mixture Conditions for the Eulerian Model (with the Mixture Turbulence Model)

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; outlet vent; pressure outlet	(tot. temperature)	volume fraction; gran. temperature (tot. temperature)	all others
inlet vent; intake fan; pressure inlet	coord. system; flow direction (tot. temperature)	coord. system; flow direction; volume fraction; gran. temperature (tot. temperature)	dir. spec. method; all others
velocity inlet	velocity (tot. temperature)	velocity; volume fraction; gran. temperature (tot. temperature)	all others
axis; outflow; periodic; solid; symmetry	nothing	nothing	all others
wall	shear condition	shear condition	all others
pressure far-field; fan; porous jump; radiator; mass flow inlet	not available	not available	not available
fluid	other source terms; other fixed values; other porous inputs	other source terms; other fixed values; other porous inputs	source terms for turbulence; fixed values for turbulence; porous zone; porosity; all others

Table 24.2.5: Phase-Specific and Mixture Conditions for the Eulerian Model (with the Dispersed Turbulence Model)

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; outlet vent; pressure outlet	turb. parameters (tot. temperature)	volume fraction; gran. temperature (tot. temperature)	all others
inlet vent; intake fan; pressure inlet	coord. system; flow direction; turb. parameters; (tot. temperature)	coord. system; flow direction; volume fraction; gran. temperature (tot. temperature)	dir. spec. method; all others
velocity inlet	velocity; turb. parameters (tot. temperature)	velocity; volume fraction; gran. temperature (tot. temperature)	all others
axis; outflow; periodic; solid; symmetry	nothing	nothing	all others
wall	shear condition	shear condition	all others
pressure far-field; fan; porous jump; radiator; mass flow inlet	not available	not available	not available
fluid	momentum, mass, turb. sources; momentum, mass, turb. fixed values; other porous inputs	momentum and mass sources; momentum and mass fixed values; other porous inputs	porous zone; porosity; all others

Table 24.2.6: Phase-Specific and Mixture Conditions for the Eulerian Model (with the Per-Phase Turbulence Model)

Type	Primary Phase	Secondary Phase	Mixture
exhaust fan; outlet vent; pressure outlet	turb. parameters (tot. temperature)	volume fraction; turb. parameters; gran. temperature (tot. temperature)	all others
inlet vent; intake fan; pressure inlet	coord. system; flow direction; turb. parameters (tot. temperature)	coord. system; flow direction; volume fraction; turb. parameters; gran. temperature (tot. temperature)	dir. spec. method; all others
velocity inlet	velocity; turb. parameters (tot. temperature)	velocity; volume fraction; turb. parameters; gran. temperature (tot. temperature)	all others
axis; outflow; periodic; solid; symmetry	nothing	nothing	all others
wall	shear condition	shear condition	all others
pressure far-field; fan; porous jump; radiator; mass flow inlet	not available	not available	not available
fluid	momentum, mass, turb. sources; momentum, mass, turb. fixed values; other porous inputs	momentum, mass, turb. sources; momentum, mass, turb. fixed values; other porous inputs	porous zone; porosity; all others

Steps for Setting Boundary Conditions

The steps you need to perform for each boundary are as follows:

1.   Select the boundary in the Zone list in the Boundary Conditions task page.

2.   Set the conditions for the mixture at this boundary, if necessary. (See above for information about which conditions need to be set for the mixture.)

(a)   In the Phase drop-down list, select mixture.

(b)   If the current Type for this zone is correct, click Edit... to open the corresponding dialog box (e.g., the Pressure Inlet dialog box); otherwise, choose the correct zone type in the Type drop-down list, confirm the change (when prompted), and the corresponding dialog box will open automatically.

(c)   In the corresponding dialog box for the zone type you have selected (e.g., the Pressure Inlet dialog box for the Eulerian model, shown in Figure  24.2.11), specify the mixture boundary conditions.

Figure 24.2.11: The Pressure Inlet Dialog Box for a Mixture

Note that only those conditions that apply to all phases, as described above, will appear in this dialog box.

For a VOF and Eulerian multiphase calculation, if you enabled the Wall Adhesion option in the Phase Interaction dialog box, you can specify the contact angle at the wall for each pair of phases as a constant (as shown in Figure  24.2.12) or a UDF (see the UDF manual for more information).

The contact angle ( $\theta_w$ in Figure  24.3.9) is the angle between the wall and the tangent to the interface at the wall, measured inside the phase listed in the left column under Wall Adhesion in the Momentum tab of the Wall dialog box. For example, if you are setting the contact angle between the oil and air phases in the Wall dialog box shown in Figure  24.2.12, $\theta_w$ is measured inside the oil phase.

Figure 24.2.12: The Wall Dialog Box for a Mixture in a VOF or Eulerian Multiphase Calculation with Wall Adhesion

The default value for all pairs is 90 degrees, which is equivalent to no wall adhesion effects (i.e., the interface is normal to the adjacent wall). A contact angle of 45 $^\circ$, for example, corresponds to water creeping up the side of a container, as is common with water in a glass.

(d)   Click OK when you are done setting the mixture boundary conditions.

3.   Set the conditions for each phase at this boundary, if necessary. (See above for information about which conditions need to be set for the individual phases.)

(a)   In the Phase drop-down list, select the phase (e.g., water).

Note that, when you select one of the individual phases (rather than the mixture), only one type of zone appears in the Type drop-down list. It is not possible to assign phase-specific zone types at a given boundary; the zone type is specified for the mixture, and it applies to all of the individual phases.

(b)   Click Edit... to open the dialog box for this phase's conditions (e.g., the Pressure Inlet dialog box, shown in Figure  24.2.13).

Figure 24.2.13: The Pressure Inlet Dialog Box for a Phase

(c)   Specify the conditions for the phase. Note that only those conditions that apply to the individual phase, as described above, will appear in this dialog box.

(d)   Click OK when you are done setting the phase-specific boundary conditions.

Steps for Copying Cell Zone and Boundary Conditions

The steps for copying cell zone and boundary conditions for a multiphase flow are slightly different from those described in Section  7.1.5 for a single-phase flow. The modified steps are listed below:

1.   In the Cell Zone Conditions or Boundary Conditions task page, click the Copy... button. This will open the Copy Conditions dialog box.

2.   In the From Cell Zone or From Boundary Zone list, select the zone that has the conditions you want to copy.

3.   In the To Cell Zones or To Boundary Zones list, select the zone or zones to which you want to copy the conditions.

4.   In the Phase drop-down list, select the phase for which you want to copy the conditions (either mixture or one of the individual phases).

Note that copying the boundary conditions for one phase does not automatically result in the boundary conditions for the other phases and the mixture being copied as well. You need to copy the conditions for each phase on each boundary of interest.

5.   Click Copy. ANSYS FLUENT will set all of the selected phase's (or mixture's) boundary conditions on the zones selected in the To Cell Zones or To Boundary Zones list to be the same as that phase's conditions on the zone selected in the From Cell Zone or From Boundary Zone list. (You cannot copy a subset of the conditions, such as only the thermal conditions.)

See Section  7.1.5 for additional information about copying boundary conditions, including limitations.