# Mass transfer boundary condition using two UDS (Drying process)

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June 14, 2022, 15:13
Mass transfer boundary condition using two UDS (Drying process)
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Hi everyone,

I'm trying to make a boundary condition in fluent to model the mass transport between a solid medium and the adjacent fluid (moist air) using 2 UDS (drying phenomenon). The first UDS represents the moisture content inside the solid (M -> mass of water / mass of solid) and the second UDS represents the mass fraction of water vapor in the fluid (Y -> mass of water vapor / mass of moist air).

The equation for the boundary condition as well as an illustration are shown in the attached image.

Γ0 and Γ1 are coefficients that are a function of the diffusivities and density of solid and fluid, respectively.

Note that the variables M and Y are defined for different cell zone conditions and the face f delimits the boundary.

Is there any way to adapt the functions BOUNDARY_FACE_GEOMETRY(f,t,A,ds,es,A_by_es,dr0) and BOUNDARY_SECONDARY_GRADIENT_SOURCE(source,n,dphi,d x,A_by_es,k) to solve UDS's Y and M simultaneously in the interface?

Another function I thought to solve this problem was INTERIOR_FACE_GEOMETRY(f,t,A,ds,es,A_by_es,dr0,dr1 ). I don't know if it would fit because M and Y are defined in different cell zones. Also, there is no example in the UDF Manual using this function.

Thank you in advance.
Attached Images
 boundary_condition.png (130.6 KB, 15 views)

 July 23, 2023, 01:49 Mositure trasnfer from solid phase #2 Member   Chhotelal Join Date: Jun 2023 Posts: 31 Rep Power: 2 have you solved this? I have the same query, In a two-phase problem, moisture is present in solid Xp, and other gas phases have some moisture content Xf. I don't understand how UDS scalar terms work, because two scaler terms are not added simultaneously, one is a solid phase and the other is a gas phase.

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