5.8.1 Definition
- Component: An element or compound with its own chemical composition which mixes with the other components, i.e. there is no distinct interface in between. Examples are salt in water; nitrogen, oxygen, and carbon dioxide in air, etc.
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Average Concentration: The average concentration of the multicomponent integrated over a volume or area, based on a volume or area weighted average.
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Concentration: The mass fraction of the component relative to the fluid mass.
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Mass. Avg. Concentration: The average concentration of the component integrated over a volume, based on a mass weighted average.
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Vol. Avg. Concentration: The average concentration of the component integrated over a volume, based on a volume weighted average.
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Species Mass: The total mass (kg) of a component in a mixture volume.
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Diffusivity: Diffusivity is a transport property, relating the flux of a component to the concentration gradient of the component (
) according to Fick’s law:
5.440
where
Multicomponent flux (kg/m2-s)
Diffusivity (m2/s) 
Fluid density (kg/m3) 
Concentration (mass fraction) of component “ 
” -
Schmidt Number: Ratio of momentum diffusivity (kinematic viscosity) to species (mass) diffusivity.
5.441
where
is the dynamic viscosity of the fluid (Pa-s). -
Flux per Area: The average flux (kg/m2) of a component crossing a selected boundary or interface.
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Total Flux: The total amount of a component (kg) crossing a selected boundary or interface.
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Volumetric Flux: The volumetric flux of a component crossing a selected boundary or interface. The unit for volumetric flux is m3/s.