## 5.13. Interphase Mass Transfer

Interphase mass transfer occurs when mass is carried from one phase into another. It is applicable to both the inhomogeneous and homogeneous multiphase models. For details, see Interphase Mass Transfer in the CFX-Solver Modeling Guide.

Mass transfer is represented by sources in the phasic continuity equations:

| (5–220)

- describes user specified mass sources. For details, see Sources.

- is the mass source per unit volume into phase due to interphase mass transfer. This is expressed as follows:

  | (5–221)

- is the mass flow rate per unit volume from phase to phase . You must have:

  | (5–222)

As it is important to keep track of the direction of mass transfer processes, it is convenient to express as follows:

| (5–223)

- The term: represents the positive mass flow rate per unit volume from phase to phase .

For mass transfer processes across a phase interphase, it is useful to express the volumetric mass sources in terms of mass fluxes:

| (5–224)

- is the mass flow rate per unit interfacial area from phase to phase , and is the interfacial area density between the phases.

As interfacial area is commonly proportional to volume fraction, this permits automatic linearization of mass transfer terms relative to volume fraction.

By default, is the clipped interfacial area density, which does not reach zero as volume fraction goes to zero, being limited to a small clipping value. Using the clipped interfacial area density can be useful because it serves as a crude nucleation model for some kinds of mass transfer. Note that the droplet condensation model does not use the clipped interfacial area density for nucleation; instead, it uses an explicit nucleation model.

In some cases, using the clipped interfacial area density can prevent obtaining a mass-conserving solution. If so, using the unclipped interfacial area density may give better results. To use the unclipped interfacial area density, set expert parameter `ipmt area density clip = f`.

The clipping value is obtained by using the specified minimum volume fraction (see Minimum Volume Fraction in the CFX-Solver Modeling Guide) in the calculation of interfacial area density.

Release 18.2 - © ANSYS, Inc. All rights reserved.