The Ansys Fluent multiphase mass transfer model accommodates mass transfer between species belonging to different phases. Instead of a matrix-type input, multiple mass transfer mechanisms need to be input. Each mass transfer mechanism defines the mass transfer phenomenon from one entity to another entity. An entity is either a particular species in a phase, or the bulk phase itself if the phase does not have a mixture material associated with it. The mass transfer phenomenon could be specified either through the built-in unidirectional "constant-rate" mass transfer (Unidirectional Constant Rate Mass Transfer) or through user-defined functions.
Ansys Fluent loops through all the mass transfer mechanisms to compute the net mass source/sink of each species in each phase. The net mass source/sink of a species is used to compute species and mass source terms. Ansys Fluent will also automatically add the source contribution to all relevant momentum and energy equations based on that assumption that the momentum and energy carried along with the transferred mass. For other equations, the transport due to mass transfer must be explicitly modeled by you.
Consider the following reaction:
 | (14–666) |
Let as assume that 
and 
belong to phase 
and 
and 
to phase 
.
Mass source for the phases are given by:
 | (14–667) |
 | (14–668) |
where 
is the mass source, 
is the molecular weight, and 
is the reaction rate.
The general expression for the mass source for the 
phase is
 | (14–669) |
 | (14–670) |
 | (14–671) |
where 
is the stoichiometric coefficient, 
represents the product,
and 
represents the reactant.
Momentum transfer is more complicated, but we can assume that the reactants mix (conserving momentum) and the products take momentum in the ratio of the rate of their formation.
The net velocity, 
, of the reactants is given by:
 | (14–672) |
The general expression for the net velocity of the reactants is given by:
 | (14–673) |
where 
represents the 
item
(either a reactant or a product).
Momentum transfer for the phases is then given by:
 | (14–674) |
 | (14–675) |
The general expression is
 | (14–676) |
If we assume that there is no momentum transfer, then the above term will be zero.
For heat transfer, we need to consider the formation enthalpies of the reactants and products as well:
The net enthalpy of the reactants is given by:
 | (14–680) |
where 
represents the formation enthalpy,
and 
represents the enthalpy.
The general expression for 
is:
 | (14–681) |
If we assume that this enthalpy gets distributed to the products in the ratio of their mass production rates, heat transfer for the phases are given by:
 | (14–682) |
 | (14–683) |
The last term in the above equations appears because our enthalpy is with reference to the formation enthalpy.
The general expression for the heat source is:
 | (14–684) |
If we assume that there is no heat transfer, we can assume that
the different species only carry their formation enthalpies with them.
Therefore the expression for 
will
be:
 | (14–685) |
The expression 
will be
 | (14–686) |