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[FAQ]

Mass fraction of phase-i can be evaluated using the following expression via custom-field-function:
mass fraction of phase-i = (phase-i volume fraction)*(phase-i density)/(mixture-density) … Eq. (1)
where mixture-density is another predefined custom-field-function as mixture-density = sum of [(phase-i volume fraction)*(phase-i density)] for all the phases “i” in the domain
For example, in a two-phase simulation:
mixture-density = (phase-1 volume fraction * phase-1 density) + (phase-2 volume fraction * phase-2 density) mass fraction of phase-2 = (phase-2 volume fraction)*(phase-2 density)/(mixture-density)
Note: To evaluate the average mass fraction of a phase across a flow boundary, it can be obtained using: mass fraction of phase-i = (mass flow rate of phase-i/mixture mass flow rate) … Eq. (2)
At a flow boundary, the mass-weighted average of the mass fraction values obtained using Eq.(1) will be nearly equal to that obtained using Eq.(2) if the interphase slip is negligible.