T h e M i x t u r e M o d e l I n t e r f a c e s

In this section:

•The Mixture Model, Laminar Flow Interface

•The Mixture Model, Turbulent Flow Interface

•Mixture Properties

•Mass Transfer

•Initial Values

•Volume Force

•Gravity

•Boundary Conditions for the Mixture Model Interfaces

The Mixture Model, Laminar Flow Interface

The Mixture Model, Laminar Flow interface (), found under the Multiphase Flow>Mixture Model branch () in the Model Wizard, has the equations, boundary conditions, and volume forces for modeling laminar two-phase flow of mixtures consisting of solid particles or liquid droplets immersed in a liquid, solving for the mixture velocity and pressure and for the dispersed phase volume fraction.

•Two-Phase Flow Modeling of a Dense Suspension: Model Library path

CFD_Module>Multiphase_Benchmarks>dense_suspension

Model

The main feature is the Mixture Properties feature, which adds the equations for the mixture and provides an interface for defining the fluid materials for the continuous phase and the dispersed phase and the mixture viscosity model to use.

When this interface is added, these default nodes are also added to the Model Builder— Mixture Model, Laminar Flow, Mixture Properties, Wall (the default boundary type, using No slip and No dispersed phase flux as the default boundary conditions), and Initial Values.

Right-click the Mixture Model, Laminar Flow node to add other features that implement, for example, boundary conditions and volume forces.

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I N T E R F A C E I D E N T I F I E R

The interface identifier is a text string that can be used to reference the respective physics interface if appropriate. Such situations could occur when coupling this interface to another physics interface, or when trying to identify and use variables defined by this physics interface, which is used to reach the fields and variables in expressions, for example. It can be changed to any unique string in the Identifier field.

The default identifier (for the first interface in the model) is mm.

D O M A I N S E L E C T I O N

The default setting is to include All domains in the model to define the mixture’s velocity and pressure and the dispersed phase volume fraction, along with the equations that describe these fields. To choose specific domains, select Manual from the

Selection list.

P H Y S I C A L M O D E L

Specify the characteristics of the dispersed phase, the model for the slip velocity, and whether to solve for the interfacial area or not.

Dispersed Phase

To characterize the Dispersed phase, select Solid particles or Liquid droplets/bubbles from the list.

	The selection from the Dispersed Phase list changes the features available
	on the Mixture Properties Settings window, under the section Mixture
Note	Model.

Slip Model

The Mixture Model interface includes several models for the slip velocity, uslip

(SI unit: m/s). Use the Slip model list to select one of the following:

•The Homogeneous flow model assumes that the velocity of the two phases are equal, that is, uslip = 0. In most cases however there is a significant difference in the velocity fields, mainly due to gravity, must be accounted for.

•Select Hadamard-Rybczynski to compute the slip velocities using the Hadamard-Rybczynski model.

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•Select Schiller-Naumann to compute the slip velocities using the Schiller-Naumann model.

•Select User defined to specify an arbitrary expression for the relative velocity. For example, give a constant velocity based on experimental data.

Turbulence Model Type

The default selection with this version of the Mixture Model interface is None, which means that no turbulence model is active and the equations are suited for laminar flow.

This interface changes to a Mixture Model, Turbulent Flow interface when the Turbulence model type selected is RANS (k ). See The Mixture Model,

Tip Turbulent Flow Interface for details.

Mass Transfer and Interfacial Area

For the Mass Transfer rate, use a two-film theory model, which includes the interfacial area between the two phases per unit volume. It is possible to compute the interfacial area per volume if the number density n (that is, the number of dispersed particles per volume) is known. Select the Solve for interfacial area check box to add the following equation for the number density n:

nt + nud = 0

This equation states that a dispersed phase particle cannot disappear, appear, or merge with other particles, although it may expand or shrink.

The Mixture Model interface calculates the interfacial area a (SI unit: m2/m3) from

a = 4n 1 3 3 d 2 3

D E P E N D E N T V A R I A B L E S

The dependent variables (field variables) are for the Velocity field, Mixture; Pressure;

Volume fraction, dispersed phase, Squared slip velocity; and Number density, dispersed phase. The names can be changed but the names of fields and dependent variables must be unique within a model.

S T A B I L I Z A T I O N

To display this section, click the Show button () and select Stabilization. This section contains the settings for stabilization of the momentum transport (the fluid flow)

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