24.2.8 Including Mass Transfer Effects

As discussed in this section in the separate Theory Guide , mass transfer effects in the framework of ANSYS FLUENT's general multiphase models (i.e., Eulerian multiphase, mixture multiphase, or VOF multiphase) can be modeled in one of three ways:

• Unidirectional constant rate mass transfer (not available for VOF calculations)

• UDF-prescribed mass transfer

• mass transfer through cavitation

Because of the different procedures and limitations involved, defining mass transfer through the Singhal et al. cavitation model is described separately in Section  24.4.2.

To define mass transfer in a multiphase simulation, as unidirectional constant, using a UDF, through population balance, cavitation, or evaporation and condensation, you will need to use the Phase Interaction dialog box (e.g., Figure  24.2.6).

Phases Interaction...

Figure 24.2.6: The Phase Interaction Dialog Box for Mass Transfer

1.   Click the Mass tab in the Phase Interaction dialog box.

2.   Specify the Number of Mass Transfer Mechanisms. You can include any number of mass transfer mechanisms in your simulation. Note also that the same pair of phases can have multiple mass transfer mechanisms and you have the ability to activate and deactivate the mechanisms of your choice.

3.   For each mechanism, specify the phase of the source material under From Phase.

4.   If species transport is part of the simulation, and the source phase is composed of a mixture material, then specify the species of the source phase mixture material in the corresponding Species drop-down list.

5.   For each mechanism, specify the phase of the destination material phase under To Phase.

6.   If species transport is part of the simulation, and the destination phase is composed of a mixture material, then specify the species of the destination phase mixture material in the corresponding Species drop-down list.

7.   For each mass transfer mechanism, select the desired mass transfer correlation under Mechanism. The following choices are available:

constant-rate   enables a constant, unidirectional mass transfer.

user-defined   allows you to implement a correlation reflecting a model of your choice, through a user-defined function.

population-balance   allows you to model flow where a number density function is introduced to account for the particle population. With the aid of particle properties (e.g., particle size, porosity, composition, etc.), different particles in the population can be distinguished and their behavior can be described. For a comprehensive understanding of this option, please refer to the The Population Balance Module Manual.

cavitation   provides you with two model options: Schnerr-Sauer and Zwart-Gerber-Belamri. To open the Cavitation Model dialog box, select cavitation from the Mechanism drop-down list. For information about the cavitation models, refer to this section in the separate Theory Guide.

Figure 24.2.7: The Cavitation Model Dialog Box

• Select Schnerr-Sauer and specify the Bubble Number Density under Model Constants and the Vaporization Pressure under Cavitation Properties.

• Select Zwart-Gerber-Belamri and specify the Bubble Diameter, the Nucleation Site Volume Fraction, the Evaporation Coefficient, and the Condensation Coefficient under Model Constants. Enter the Vaporization Pressure under Cavitation Properties. It is advisable to use the default values for all the model constants in both the Schnerr-Sauer and Zwart-Gerber-Belamri models. for the Vaporization Pressure, you have the choice of constant, polynomial, piecewise-linear, piecewise-polynomial, or user-defined.

If the Mixture multiphase model is enabled, then the Singhal et al. cavitation model can be enabled using the solve/set/expert text command and responding yes to use Singhal-et-al cavitation model?. The Singhal-Et-Al Cavitation Model option will now be visible in the Phase Interaction dialog box, under the Mass tab. Enable this option to include the Singhal et al. cavitation model. Refer to Section  24.4.2 for information about setting the cavitation parameters. Also refer to this section in the separate Theory Guide for information about the Singhal et al. model. To disable this model, first deselect the Singhal-Et-Al Cavitation Model option in the Phase Interaction dialog box, then type the solve/set/expert text command again and enter no when asked if you want to use Singhal-et-al cavitation model?

evaporation-condensation   enables you to apply the evaporation-condensation model as the mass transfer mechanism. This model is available wih the mixture and Eulerian multiphase models. Refer to this section in the separate Theory Guide for a theoretical discussion about this model.

Figure 24.2.8: The Evaporation-Condensation Model Dialog Box

• Enter the Evaporation Frequency and Condensation Frequency model constants. Those values are 0.1 by default. Note that the bubble diameter and accommodation coefficient are usually not very well known, which is why the coefficient $coeff$ ( this equation in the separate Theory Guide) can be fine tuned to match experimental data.

• Specify the Saturation Temperature for your flow regime.

ANSYS FLUENT will automatically include the terms needed to model mass transfer in all relevant conservation equations. Another option to model mass transfer between phases is through the use of user-defined sources and their inclusion in the relevant conservation equations. This approach is a more involved but more powerful, allowing you to split the source terms according to a model of your choice.

Momentum, energy, and turbulence are also transported with the mass that is transferred. ANSYS FLUENT assumes that the reactants are mixed thoroughly before reacting together, thus the heat and momentum transfer is based on this assumption. This assumption can be deactivated using a text command. For more information, contact your ANSYS FLUENT support engineer.

When your model involves the transport of multiphase species, you can define a mass transfer mechanism between species from different phases. If a particular phase does not have a specie associated with it, then the mass transfer throughout the system will be performed by the bulk fluid material.

Including species transport effects in the mass transport of multiphase simulation requires that Species Transport be selected in the Species Model dialog box. Models Species