When using fluid mixtures in Flownex, Boundary Condition components can be configured to use either Mass Fractions or Flow Source Fractions. What is the difference between these two options, and when should each be used?
Both Mass Fractions and Flow Source Fractions are used to define the composition of fluids in a mixture, but they serve different purposes.
The Specify Mass Fraction option fixes the fluid composition at the location where the Boundary Condition is connected. This means that the specified fluid percentages remain constant at that boundary. This option should be used when the composition of the fluid entering the network is known and must remain fixed. For example, if a storage vessel contains only hydrogen, specifying a hydrogen mass fraction of 100% ensures that no other fluids are present at that boundary.
The Specify Flow Source Fraction option defines the composition of a fluid stream that is added to or removed from the system at the Boundary Condition. In this case, the fluid composition at the connection point is not fixed. Instead, the mixture composition changes as the specified fluid source is added to or removed from the network. This option is useful when modelling the injection or extraction of a fluid without explicitly modelling its upstream source. For example, air can be added to a hydrogen flow stream by specifying a flow source fraction of 100% air at a mass source boundary condition.
As a result, the fluid composition can vary throughout the network. Upstream of the flow source, the mixture may consist entirely of one fluid, while downstream the composition changes as additional fluids are introduced.