Junction X has a density change across it

A Caution message in AFT Impulse

Full Message Text

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.

Explanation

This warning is shown when the user has selected Variable Fluid properties on the Variable Fluids panel, and the density of the fluid is changing between pipes. The Solver 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.

Required Action

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 change 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 pipeline carrying fluids of different properties such as oils of different density, or a system is being flushed with water. 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, and is only applicable to steady-state or a very short transient in a long pipeline.