Because there is no universal model for mass transfer, Ansys Fluent provides a UDF that you can use to input models for different types of mass transfer, for example, evaporation, condensation, boiling, and so on. Note that when using this UDF, Ansys Fluent will automatically add the source contribution to all relevant momentum and scalar equations. This contribution is based on the assumption that the mass "created" or "destroyed" will have the same momentum and energy of the phase from which it was created or destroyed. If you would like to input your source terms directly into momentum, energy, or scalar equations, then the appropriate path is to use UDFs for user-defined sources for all equations, rather than the UDF for mass transfer. See the Fluent Customization Manual for more information about UDF-based mass transfer in multiphase.