An enthalpy-porosity technique  [676],  [678],  [679] is used in Ansys Fluent for modeling the solidification/melting process. In this technique, the melt interface is not tracked explicitly. Instead, a quantity called the liquid fraction, which indicates the fraction of the cell volume that is in liquid form, is associated with each cell in the domain. The liquid fraction is computed at each iteration, based on an enthalpy balance.

The mushy zone is a region in which the liquid fraction lies between 0 and 1. The mushy zone is modeled as a "pseudo" porous medium in which the porosity decreases from 1 to 0 as the material solidifies. When the material has fully solidified in a cell, the porosity becomes zero and hence the velocities also drop to zero.

In this section, an overview of the solidification/melting theory is given. Refer to Voller and Prakash  [679] for details on the enthalpy-porosity method.