Difference Between Mass Fractions and Flow Source Fractions Specified on Boundary Conditions
What is the difference between mass fractions and flow source fractions?
This article provides more information on specifying mass fractions or flow source fractions on Boundary Conditions in Flownex® and describes the difference between these options.
DIFFERENCE BETWEEN MASS FRACTIONS AND FLOW SOURCE FRACTIONS
When using fluid mixtures in a network, a user can specify the mass fraction or flow source fraction in the inputs section of a Boundary Condition component.
These options become available when enabling either pressure, mass source, or volumetric flow source on a Boundary Condition component, as seen in Figure 2. These options allow the user to specify the percentage of each fluid in the mixture at the point where the Boundary Condition is connected.
But what is the difference between specifying mass fractions and flow source fractions?
The Specify mass fraction option is used to fix the fluid fraction values at the point where the Boundary Condition is connected. Thus, the different fluid percentages will remain the same and be fixed at the specified mass fraction values.
The Specify flow source fraction is used to define a fluid with fraction values that are added or removed at the point where the Boundary Condition is connected. Thus, the fluid percentages at the point where the Boundary Condition is connected are not fixed and will change as the new mixture is added or removed.
An example of when to specify a mass fraction instead of a flow source fraction is when a user wants to model a container supplying the system with hydrogen, which will mix downstream with air before entering a rocket’s combustion chamber. The container should hold hydrogen, and no air should exist in the container. Using the specify mass fraction option on the container will ensure that only hydrogen is present in the container.
An example of when to specify a flow source fraction is when a user wants to model the mixture of hydrogen and air before combustion without needing to model the process parameters of air. This can be achieved by specifying a mass source at a node where air should be added and specifying a flow source fraction with a value of one for air.
Thus, the fluid mixture percentage will be one for hydrogen up until the point where the new Boundary Condition is connected, adding air into the system. Hereafter, the fluid mixture percentage will change as air is added to the system, as seen in Figure 5.