A new model is proposed and demonstrated to track the air- (water/ice) interface during solidification of a partially filled water tank. The air-(water/ice) interface is described by a scalar representing the volume fraction of (water/ice) in each cell. The volume fraction was obtained by solving a conservation equation for volume. Transport of excess volume due to expansion was treated using a diffusion model (analogous to the approach used for the energy equation). Sub-time-stepping to steady state was used to solve the nonlinear volume fraction equation. Additionally, for computational efficiency, the solution to the volume transport equation was only calculated once every few time steps of the energy equation. The Finite Volume Method (FVM) was used to discretize the equation and a parallel Conjugate-Gradient Squared (CGS) solver was implemented via User-Defined Functions (UDFs) in the commercial CFD software ANSYS-Fluent^TM to solve the resulting discrete equations. A simulation with a mesh of 1 million cells and 50% fill level was performed and results were validated against experimental data. Temperature data at 6 thermocouple locations exhibited reasonable agreement with experimental data.