set different diffusion coefficient for different layer

As far as I know, only one solute diffusion coefficient can be set for all aqueous species,which is not practical.

Is it possible to set different diffusion coefficient of one solute for different layer in toughreact?

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  • Hi Sue,

    TOUGHREACT V3.32-OMP has the capability to assign different aqueous diffusion coefficients for each primary aqueous species (as a function of temperature). The coefficients for primary species and their sources are given in the modified BRGM thermodynamic database thdem1214tr3hs5i.dat provided with the V3.32-OMP package. There is no sample problem specifically describing it, but it is easy to turn on by setting the aqueous diffusion coefficient in solute.inp to -1.0 and setting MOPR(18) to 1, as described in the documentation. The coefficients or just diffusion coefficients can be added to any database of your choice or read in as a separate file. Note that it does not yet consider charge conservation so it is not an accurate description of charged species diffusion, where charge balance should be maintained.

    Eric

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  • Thanks a lot !

    Another question is, if I divide the model into two layers, both containing copper ions, can we set two different diffusion coefficients (as a constant) of copper ions for these two layers in the new version of TOUGHREACR.

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  • Eric Sonnenthal said:
    TOUGHREACT V3.32-OMP

    Thanks a lot !

    Another question is, if I divide the model into two layers, both containing copper ions, can we set two different diffusion coefficients (as a constant) of copper ions for these two layers in the new version of TOUGHREACR.

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  • Hi Sue,

    It's not clear to me what you are trying to model. Is it different different effective diffusivities/dispersivities in each layer or different molecular diffusion coefficients? For the former, one just needs to modify the tortuosity and or the porosity in each layer, since those are multiplied by the diffusion coefficient. 

    To make 2 different molecular diffusion coefficients for the same component, you will need to modify the thermodynamic database and add another component to mimic a different Cu species. In this case, copy the Cu component and call it something like Cu_Diff2. However, this will need to be carried through all the Cu secondary species and minerals involving Cu. It's tricky, because if you miss anything there will be problems with Cu mass conservation. However each of the Cu components will retain their diffusion coefficient as they cross layers.

    Eric

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  • Thanks Eric!

    Eric Sonnenthal said:

     In the model,  I only consider the diffusion of aqueous species( such as Cu) without chemical reaction. The whole model is completed in Petrasim. In petrasim, I set diffusion coefficients of different phases as Figure 1,  one DIFUN for aqueous species as Figure 2.

    I learned from the  TOUGHREACT User’s Guide that DIFUN (multiplied by the tortuosity , porosity, and liquid saturation)  is the diffusion coefficient as Figure 3.  Is DIFUN the effective diffusivities for Cu?

    I want the effective diffusion coefficient of Cu in each layer to be different.

    I am so confused that

    (1)   DIFUN is a function of  tortuosity , porosity and liquid saturation,  then why we need to give  DIFUN a value instead of letting the software to calculate?

    (2)  Is th DIFUN the aqueous species' diffusion coefficient  in water or in  porous media?

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  • Hi Sue,

    DIFUN is th molecular diffusion coefficient, rather than the diffusivity. DIFUN is not a function of any hydrological property. The diffusivity is calculated in TOUGHREACT as 

    DIFUN*porosity*tortuosity*saturation(liquid)

    So, if you set a different porosity and/or tortuosity, then the diffusivity of the component will be different in each layer, even though the molecular diffusion coefficient is the same. Think of the molecular diffusion coefficient as the value of the component in pure water, without any pores that reduce reduce the diffusive fluxes.

    regards,

    Eric

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  • Thank you so much for your answer! I learn a lot from it.

    regards,

    Sue

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