What Is A Phase Fraction

The Phase Fraction

The phase fraction is a measure of how much of a fluid is in each phase, measured as a fraction of the total fluid. It is usually expressed on either a mass or volume basis, often described as the phase mass fraction or phase volume fraction (or variations of this). The concept is really useful, because it describes relative quantities and is not dependent on the geometry (specifically, the volume) of the system of interest, so it is often featured in models of multiphase fluids.

Volumetric Basis

It’s pretty easy to visualise volumetric phase fractions. Imagine you have an empty 1L bottle, to which you add 750mL of water (leaving 250mL of air). The volumetric fraction of the water phase (i.e. liquid fraction on a volume basis) is the volume of liquid divided by the total volume (i.e. 750mL / 1000 mL), which is 0.75. The volumetric gas fraction is therefore 0.25.

Mass Basis

The phase fraction on a mass basis is conceptually similar, but numerically very different because it is influenced by the relative densities of the different phases. The 750mL of water we added has a mass near enough to 750g, because it’s density is approximately 1kg/L. The air however has a much lower density (1.225g/L), so it has a mass of only mass of only 0.3g. As the total mass is only 750.3g, the mass fraction of the liquid phase is 0.9996 and the gas phase only 0.0004!

Visualising phase fractions.



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