# Help: "Ions-Transport"

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 February 3, 2012, 14:55 Help: "Ions-Transport" #1 New Member   Join Date: Feb 2012 Posts: 10 Rep Power: 7 Hello guys; first, sorry for my bad english . I have to simulate a single-phase system, with anions and cations diffuse in an aqueous solution. Ions and cations are obviously scalars. How can I insert the effects due to electrical interactions? How can I insert the condition: C = A in each element (electroneutrality)? where A and C are the concentrations of anions and cations, respectively thanks Last edited by CrisCFD; February 4, 2012 at 07:32.

 February 4, 2012, 11:34 #2 New Member   Join Date: Feb 2012 Posts: 10 Rep Power: 7 Maybe I didn't explain well...

 February 5, 2012, 18:18 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,993 Rep Power: 107 If A=C then why model two concentrations? Just model one and the other will be the same. This is easily done with a additional variable. But I do not know what electrical interactions you wish to include.

February 6, 2012, 04:47
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 Originally Posted by ghorrocks If A=C then why model two concentrations? Just model one and the other will be the same. This is easily done with a additional variable. But I do not know what electrical interactions you wish to include.
I'm modelling a reverse electrodialysis system...so I have Na+ (cations) and Cl- (anions).
I have to consider the electrical interactions in the transport equation as an additional term, as a source term.
I just found an expression like this: S+ = - σ / ε * (C-A)
and
S- = σ / ε * (C-A)

But the very low value of the costant "ε " (dielectric constant = 2,21 E-10) is a problem for the solver, a numerical problem, I think.

 February 6, 2012, 20:06 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,993 Rep Power: 107 But again, you said A=C so the expression evaluates to zero.

February 7, 2012, 04:27
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Quote:
 Originally Posted by ghorrocks But again, you said A=C so the expression evaluates to zero.
I have deleted this condition (A = C) from the model and I have considered the source term S

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