Use this node to define the temperature and the position of the interface between two phases of the same material, using the Stefan condition. It is applicable on boundaries between Fluid and Solid domains, and on interior boundaries of Fluid, Solid, and Porous Medium domains, in 2D, 2D axisymmetric, and 3D components.

For an equivalent condition applicable on exterior boundaries, use Phase Change Interface, Exterior instead.

In addition, in time-dependent studies, the Stefan condition defines the phase change interface velocity vn from the conductive heat flux jump across the interface, q, the latent heat of phase change from solid to fluid, Ls → f, and the solid density, ρs:

The fluid velocity at the interface, vf, is defined as:

The phase change velocity vn appearing in Stefan condition is relative to the solid position. In case of translation of the solid (using Translational Motion subfeature under Solid with Translational Motion feature for instance), the solid translation velocity us contributes to vn to describe the interface velocity relative to the spatial frame:

In order to simulate the deformation of the fluid and solid domains, a Deforming Domain node (under the Deformed Geometry branch in Definitions) should be active on the adjacent domains on each side of the phase change interface, the velocity vn is set on the boundaries where the Phase Change Interface node is active, in the same way as if you apply the Prescribed Normal Mesh Velocity node. Note that for large deformations of the interface, you may need to add an Automatic Remeshing node in the solver sequence to preserve the mesh quality. See Prescribed Normal Mesh Velocity and Automatic Remeshing in the COMSOL Multiphysics Reference Manual for details.

First, set the Phase change temperature, Tpc (SI unit: K), and the Latent heat, Ls → f (SI unit: J/kg), associated to the transition from solid to fluid phase. A positive value should be set for the latent heat.

Then, choose which side of the phase change interface is the Solid side, either Upside, or Downside.

Finally, choose the method of Heat flux jump evaluation. By default the Lagrange multiplier option is selected and introduces a variable on the boundary, the Lagrange multiplier, to prescribe the temperature and calculate the heat flux jump accurately.

Alternatively, select Temperature gradient to prescribe the temperature condition using a strong constraint and evaluate the heat flux jump from the temperature field gradient on each side of the interface.

The Temperature gradient option leads to a formulation that is easier to handle from a numerical point of view, especially for the iterative solvers, however the accuracy of the heat flux jump is strongly dependent on the size of the mesh next to the boundary. In this case a very fine mesh may be needed to reach the same degree of accuracy as the default option with a default mesh. See Weak Constraints in the COMSOL Multiphysics Reference Manual for more details on the use of Lagrange multipliers.

Numerical oscillations may appear in the interface velocity, with both methods of Heat flux jump evaluation.

To overcome this difficulty, select the Enable moving boundary smoothing check box to stabilize the boundary displacement. This setting smooths the normal mesh velocity by adding the following smoothing velocity contribution, proportional to the mean curvature of the interface, H (SI unit: 1/m), and the mesh element size, h (SI unit: m):

Note that adding this smoothing term modifies the physical modeling, and may alter mass conservation. Set the Moving boundary smoothing tuning parameter, δmbs, a dimensionless number to allow a tradeoff between numerical stability and precision of the physical modeling.

Physics tab with interface as Heat Transfer in Solids and Fluids, Heat Transfer in Solids, Heat Transfer in Fluids, or Heat Transfer in Porous Media selected: