How can I use CFX to model the solidification of molten aluminum ... on the perimeter of a pipe?


You should be able to do this with an Equilibrium Phase Change Model, with the limiting assumption that the solidification is an equilibrium process.

1. Create a material that is a binary mixture of solid/liquid phases of your substance and set saturation properties, such as a saturation temperature.

2. Make this binary mixture your working fluid and set one of the components (solid) as the equilibrium constraint.

3. The use of a binary mixture will enforce solidification where local conditions favor a phase change. Make sure that the reference enthalpy difference between your solid and liquid components is equal to the heat of fusion at the saturation temperature

4. Make a sub-domain for the volume in which solidification is to occur. This will allow you to apply momentum sinks to the solid, which you want to remain stationary after it has formed.

5. Tie the magnitude of the momentum sink to the mass fraction of solid, so that momentum sinks are highest at the walls of the pipe where solid forms. You can do this by creating an expression:

Momentumsource =-1E15*( [kg m^-3 s^-1]

and using it as the momentum source coefficient in a General Momentum source (isotropic).

6. The buildup of the high density phase at the perimeter of the pipe should cause the moving core of fluid to speed up. The pressure drop will be taken care of by hydrodynamics.

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