Defining membrane property through UDF

Hello to all of you.

I have two fluids (One is air & other is Licl solution) in a rectangular ducts (stacking to each other). I am considering sliding face of both fluids as a membrane & here now I want to introduce membrane resistance & its diffusivity of water vapours. I have a confusion where I have to load UDF ? As membrane is a coupled wall it is giving mass fraction H20.

Moreover, I have inlet of Air & outlet of Air. And inlet & outlet of Licl solution.

One more question, As I have given Air as a mixture template (Air + H20 vapours). Now I want this H2O vapours in LICL. For this, What can I do as mixture template is only of (Air + H20). How I can change the licl in to mixture of h20 + Licl?

Seniors, I am looking forward your kind response in this regard.

Thanks in Advance!


Abbas Raza



  • To have a mixture of air and vapour ot any other two materials, Use Volume of fluid model, and you will be able to have any mix at inlets. You will define the velocity of inlet and void fractions of all phases.

    I think you mean a porous media by membrane. Is this right? if so, why do you want it sliding?

  • Thanks for your response.

    First Use volume of fluid model, I did not get it. Secondly, I used mixture template for introducing moisture (H20) by species transport model in which I select Air & H20 in the mixture. While I want in the second duct (the Licl would be changed in to mixture containing Licl + H20 ). As by default, using species mass transport model, we have only one mixture template which I have selected for Air + H20 in first duct.

    Furthermore, Membrane which i have selected is actually a coupled wall the sliding interface between two fluids. By considering as a single part, the sliding interface changes in to a wall then by selecting as a coupled wall I am trying to proceed.

    Last but not the least, In one duct I have (Air + h20 vapors) & in other duct I have (Licl+ H20 vapors ).I am just focusing on the transfer of moisture (water vapours) from Air to Licl & I am considering as a single phase. Looking forward your suggestions in this regard.


    Abbas Raza

  • RobRob UKForum Coordinator

    If it's a membrane I assume you want to govern how much material is passed from one channel to the other? Have a look on here for "membrane" in the "fluids" section as we've covered this on several occasions.

  • Yes, its a coupled wall (membrane) I want to transfer humidity (water vapors) from Air to Licl. Inlet humidity ratio is 0.02kg/kg & want to define membrane resistance which will control the movement of water vapors to a certain limit along with passing water vapors in to other channel which is Licl.

    Furthermore, I have gone through past discussions they are mostly about porous jump etc.

    I need your support in this regard.


    Abbas Raza

  • DrAmineDrAmine GermanyForum Coordinator

    First please do not use any multiphase model and no VOF as you got that as recommendation in the first message: that us wrong.

    You need to use species model to model mixture of air and water phase. If the other component is present as liquid so different aggregate state here using multiphase model is correct.

    To model membrane boundary you require UDF and thus has been discussed several times on this forum. Have you already checked that?

  • Thanks a lot for your response.

    Yes, using mixture (Air + water vapours) is using for the one duct. While the other duct having Licl (fluid) want to change into Licl + water vapors. For this, I have to use multiphase model. This is the answer what I could get from the second line in the above answer.

    Secondly, I do understand the UDF to be used for defining the membrane properties. Here I m asking that particularly in my case, the coupled wall (Which is in between two fluids) & I am considering this as membrane. (A coupled wall). Now there is a option for mass fraction over where I can introduce UDF. But for defining property like diffusivity & resistance of membrane where I have to load UDF.

    Please see the attached picture. Here momentum, temperature & species If i define the diffusivity & resistance of membrane where i have to load UDF.


    Abbas Raza

  • RobRob UKForum Coordinator

    If the phases are separated by the membrane so gas and liquid aren't present in the same volume you don't need multiphase. You define one zone as liquid and the other as the gaseous mixture. As per most of the other membrane models we've seen on here you then use source terms to transfer mass and energy.

  • DrAmineDrAmine GermanyForum Coordinator

    If you want to model a membrane letting one comoonent to go through the wall you require an UDF to put whatever is available on one side of the wall on the other side. Alternative is to work with surface reactions and mimic the transfer through the wall(s) of the memrne by surface deposition and desorption.

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