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:
 | (5–223) |
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:
(5–224)
is the mass flow rate per unit volume from
phase 
to phase 
. You must
have:
(5–225)
As it is important to keep track of the direction of mass transfer
processes, it is convenient to express 
as follows:
 | (5–226) |
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:
 | (5–227) |
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.