[swtbkim]

Hello!

I am simulating bubble column. The column is filled with water, and CH4 gas is injected from the nozzle submerged in the water tank. Above the water tank is N2.

Brief setup is as follows:
2D volume-of-fluid (VOF) multiphase flow model
Species transport model for secondary phase, that is,
Phase-1: Liquid Water
Phase-2: Gas mixture (CH4, N2)

Left figure is the contour of the volume fraction of Phase-2 (Gas mixture). Bubbling phenomenon is well described, as you can see.
Right figure is the contour of the mass fraction of CH4, which is one component of Phase-2.

Fluent Theory Guide says that the mass fraction is multiplied by the volume fraction in the species transport equation (2021R1 Theory Guide 18.8 Modeling Species Transport in Multiphase Flows).
Then, CH4, which is a component of Phase-2, must not exist in the region occupied by Phase-1.

But, as you can see in the right figure, the contours of the volume fraction of Phase-2 and the mass fraction of CH4 do not match; it seems that CH4 is remained in the liquid region.

Am I missing some steps?
Any comment will be very helpful.
Thank you!