A Caution message in AFT Impulse.

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.

This warning is shown when the user has selected variable fluid properties in the Variable Fluids panel, and the density of the fluid is changing between pipes. AFT Impulse performs a mass balance at junctions, and fluids of different densities on either side of the junction may violate the mass conservation at a junction.

If the change in density is due to a change in temperature across the junction, the caution message can likely be ignored. A density change at a junction due to temperature means the real system likely still has a mass balance at the junction.

If the change in density is due to a change in fluid type or moving interface, the caution message likely should not be ignored. An example of a change in fluid or moving interface is a a pipeline carrying fluids of different properties such as oil of different density. In this example, the volumetric flowrate is likely conserved across a junction in the real system even if mass is not conserved. The Volume Balance junction can be used at the fluid interface to capture this situation. However, the Volume Balance is limited in that it cannot capture the interface location changing with time.