The energy equation, also shared among the phases, is shown below.

(14–16)

where is the effective conductivity (, where is the turbulent thermal conductivity, defined according to the turbulence model being used), is the diffusion flux of species , is enthalpy of species in phase , and is diffusive flux of species in phase . The first three terms on the right-hand side of Equation 14–16 represent energy transfer due to conduction, species diffusion, and viscous dissipation, respectively. includes volumetric heat sources that you have defined but not the heat sources generated by finite-rate volumetric or surface reactions since species formation enthalpy is already included in the total enthalpy calculation as described in Energy Sources Due to Reaction.

The VOF model treats energy, , as a mass-averaged variable:

(14–17)

In Equation 14–16,

(14–18)

where for each phase is based on the specific heat of that phase and the shared temperature.

The properties , (effective thermal conductivity) and (effective viscosity) are calculated by volumetric averaging over the phases. The source term, , contains contributions from radiation, as well as any other volumetric heat sources.

As with the velocity field, the accuracy of the temperature near the interface is limited in cases where large temperature differences exist between the phases. Such problems also arise in cases where the properties vary by several orders of magnitude. For example, if a model includes liquid metal in combination with air, the conductivities of the materials can differ by as much as four orders of magnitude. Such large discrepancies in properties lead to equation sets with anisotropic coefficients, which in turn can lead to convergence and precision limitations.