Hello!
With volumetric scalar sources, you have flexibility with respect to the units/interpretation of the results. I find that assuming units of concentration for the flux works best.
The idea is simple: whatever unit of concentration you use for the volumetric source, you can interpret the scalar results to be in the same unit of concentration. E.g. if your flux is in ppm (mg/kg or mL/m3), then the results will be in ppm.
The amount of scalar created in a volumetric source will be the concentration (flux) times the volume of the source inside of the computational domain. E.g. a flux of 100 ppm defined to a 1 m3 volume will give you 100 mL/m3 * 1 m3 = 100 mL of scalar being released per second.
If you know how much scalar is being released in g/s, then work out how much that would be in mL/s. Then check the volume of the source that is within the computational domain (‘out of bounds’ doesn’t count). Then divide the volume of scalar being dissipated (in mL) by the volume of the source (m3). This will be your flux in ppm.
Some projects of interest:
Cheers