A Caution message in AFT Fathom

Full message is:

"Junction X has a density change across it. If this is due to a moving interface and not heat input or output then a Volume Balance junction is recommended.

AFT Fathom performs mass balances at junctions. Moving interfaces require volume balances. Otherwise results will be incorrect."

This warning only displays when the user has selected variable fluid properties in Analysis Setup on the Heat Transfer/Variable Fluids panel. The fluid property of interest is the density of the fluid. Density can vary in a system for a couple reasons. One is that there is a temperature change due to heat input or output. If this is the case then this warning can be ignored. A density change at a junction due to temperature means the fluid still has a mass balance at the junction.

The other possibility is that the density can change due to a moving interface. Typically this moving interface is a different fluid. A good example is a pipeline carrying fluids of different properties such as oil of different density. This situation is really not a steady-state solution because the mass in the system changes with time.

To model this in a steady-state solver like AFT Fathom it is best to use a Volume Balance junction at the interface.