The mass source terms affecting the continuity equations, are referred to as primary mass sources. Clearly, if mass transfer occurs between phases, then this also induces additional sources and sinks between all other transport equations. These are referred to as secondary sources, or secondary fluxes, due to mass transfer.

The default form of the secondary source terms for a transported variable is:

(5–228)

That is, mass transfer from a phase into a phase carries the bulk conserved quantity into phase . This is referred to as an upwind formulation, as the upwinded value is carried out of the outgoing phase, and into the incoming phase.

This default formulation is modified in certain circumstances, in order to take account of discontinuities in the transported variable at the phase interface, for example, enthalpy in the case of phase change. For details, see The Thermal Phase Change Model.