hi Feflow-forum,

I have problem regarding the mass transfer rate, in the Feflow 6.2 help there is no correct explanation on that:

Regarding the Mass-tranfer BC it's written in the help that

Q[sub]mass[/sub]    =    A      *      transfer rate      *    (c[sub]ref[/sub] - c)

the units in 3d are described as follows

[g/m³]    =    [m²]    *        [m/d]        *  [g/m³]    ? ? ?

How is this possible? Am I blind?

What is the definition of the mass transfer rate (compared to the definition of Transfer Rate (fluid), where in the Feflow-help its written that "The transfer rate is a conductance term describing the properties of a clogging layer. It is defined as  ? = K/d"?

Can somebody please provide some help? Thank u so much!

Hi Heraklit,

The resulting unit is not entirely correct: [m²] * [m/d] * [g/m³] = [g/d]

Björn
The in-transfer rate and out-transfer rate associated with the Mass-Transfer BC (Cauchy) can be considered as a leaching parameter, which constrain the mass flux through the boundary.

If the transfer-rate = 0, the boundary is impervious. In contrast, if the transfer-rate is very large, the Cauchy BC is reduced to a mass-concentration BC (Dirichlet).

If you consider the thickness d for the leaching body and apply Ficks’s law the transfer-rate for the mass Phi=D/d, where D is the molecular diffusion.

Essentially, that’s the same principle like the transfer for groundwater Phi(fluid)=k/d.

Classical examples are flow over a salt dome with a diffusive input component of the mass (e.g. HYDROCOIN)
thank u bjoern, makes sense now :)

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