I have defined a fluid specific source term which includes the specific interfacial area per unit volume. I note that
the documentation says that they are not automatically multiplied by volume fraction. Do I need to multiply my
source term by the fluid volume fraction or not?
Volumetric fluid specific sources are multiplied by the volume of the cell, not by the volume of the phase. As the documentation notes, you`d have to multiply by the volume fraction if the sources were defined per unit volume of phase. If your expression for interfacial transfer is similar to kLa (C*  C) where a is the specific interfacial area per unit volume of the twophase mixture, then you do not need to multiply it by the fluid volume fraction since the correct volume to multiply this by is the volume of the cell. 

