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23.11.1 Defining the Phases for the Mixture Model

Instructions for specifying the necessary information for the primary and secondary phases and their interaction for a mixture model calculation are provided below.

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Recall that only one of the phases can be a compressible ideal gas. Be sure that you do not select a compressible ideal gas material (i.e., a material that uses the compressible ideal gas law for density) for more than one of the phases. See Section  23.11.3 for details.



Defining the Primary Phase


The procedure for defining the primary phase in a mixture model calculation is the same as for a VOF calculation. See Section  23.10.3 for details.



Defining a Nongranular Secondary Phase


To define a nongranular (i.e., liquid or vapor) secondary phase in a mixture multiphase calculation, perform the following steps:

1.   Select the phase (e.g., phase-2) in the Phase list.

2.   Click Set... to open the Secondary Phase panel (Figure  23.11.1).

Figure 23.11.1: The Secondary Phase Panel for the Mixture Model
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3.   In the Secondary Phase panel, enter a Name for the phase.

4.   Specify which material the phase contains by choosing the appropriate material in the Phase Material drop-down list.

5.   Define the material properties for the Phase Material, following the same procedure you used to set the material properties for the primary phase (see Section  23.10.3). For a particulate phase (which must be placed in the fluid materials category, as mentioned in Section  23.9), you need to specify only the density; you can ignore the values for the other properties, since they will not be used.

6.   In the Secondary Phase panel, specify the Diameter of the bubbles, droplets, or particles of this phase ( $d_p$ in Equation  23.4-12). You can specify a constant value, or use a user-defined function. See the separate UDF Manual for details about user-defined functions. Note that when you are using the mixture model without slip velocity, this input is not necessary, and it will not be available to you.

7.   Click OK in the Secondary Phase panel.



Defining a Granular Secondary Phase


To define a granular (i.e., particulate) secondary phase in a mixture model multiphase calculation, perform the following steps:

1.   Select the phase (e.g., phase-2) in the Phase list.

2.   Click Set... to open the Secondary Phase panel (Figure  23.11.2) .

Figure 23.11.2: The Secondary Phase Panel for a Granular Phase Using the Mixture Model
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3.   In the Secondary Phase panel, enter a Name for the phase.

4.   Specify which material the phase contains by choosing the appropriate material in the Phase Material drop-down list.

5.   Define the material properties for the Phase Material, following the same procedure you used to set the material properties for the primary phase (see Section  23.10.3). For a granular phase (which must be placed in the fluid materials category, as mentioned in Section  23.9), you need to specify only the density; you can ignore the values for the other properties, since they will not be used.

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Note that all properties for granular flows can utilize user-defined functions (UDFs).

See the separate UDF Manual for details about user-defined functions.

6.   Turn on the Granular option.

7.   In the Secondary Phase panel, specify the following properties of the particles of this phase:

Diameter   specifies the diameter of the particles. You can select constant in the drop-down list and specify a constant value, or select user-defined to use a user-defined function. See the separate UDF Manual for details about user-defined functions.

Granular Viscosity    specifies the kinetic part of the granular viscosity of the particles ( $\mu_{s,{\rm kin}}$ in Equation  23.4-18). You can select constant (the default) in the drop-down list and specify a constant value, select syamlal-obrien to compute the value using Equation  23.4-20, select gidaspow to compute the value using Equation  23.4-21, or select user-defined to use a user-defined function.

Granular Temperature    specifies temperature for the solids phase and is proportional to the kinetic energy of the random motion of the particles. Choose either the algebraic, the constant, or user-defined option.

Solids Pressure    specifies the pressure gradient term, $\nabla p_s$, in the granular-phase momentum equation. Choose either the lun-et-al, the syamlal-obrien, the ma-ahmadi, or the user-defined option.

Radial Distribution    specifies a correction factor that modifies the probability of collisions between grains when the solid granular phase becomes dense. Choose either the lun-et-al, the syamlal-obrien, the ma-ahmadi, the arastoopour, or a user-defined option.

Elasticity Modulus    is defined as
 G = \frac{\partial P_s}{\partial \alpha_s} (23.11-1)

with $G \ge 0$.

Choose either the derived or user-defined options.

Packing Limit    specifies the maximum volume fraction for the granular phase ( $\alpha_{s,{\rm max}}$). For monodispersed spheres, the packing limit is about 0.63, which is the default value in FLUENT. In polydispersed cases, however, smaller spheres can fill the small gaps between larger spheres, so you may need to increase the maximum packing limit.

8.   Click OK in the Secondary Phase panel.



Defining Drag Between Phases


For mixture multiphase flows with slip velocity, you can specify the drag function to be used in the calculation. The functions available here are a subset of those discussed in Section  23.12.2. See Section  23.4.5 for more information.

To specify drag laws, click Interaction... to open the Phase Interaction panel (Figure  23.11.3) , and then click the Drag tab.

Figure 23.11.3: The Phase Interaction Panel for the Mixture Model ( Drag Tab)
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Defining the Slip Velocity


If you are solving for slip velocities during the mixture calculation, and you want to modify the slip velocity definition, click Interaction... to open the Phase Interaction panel (Figure  23.11.4) , and then click the Slip tab.

Figure 23.11.4: The Phase Interaction Panel for the Mixture Model ( Slip Tab)
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Under Slip Velocity, you can specify the slip velocity function for each secondary phase with respect to the primary phase by choosing the appropriate item in the adjacent drop-down list.


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