Numerical problem
Hello! I have inserted a constant in my model (dielectric constant), which is very low (2,21 E10) and the simulation does not converge.
What can I do? 
You question is meaningless. What does the constant do? How is it coupled to the simulation? What are you modelling?

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I'm modelling a reverse electrodialysis system...so I have Na+ (cations) and Cl (anions). So I have two transport equation (cations and 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: 1) S+ =  σ / ε * (CA) (cations) and 2) S = σ / ε * (CA) (anions) Where A and C are the concentrations of anions and cations, respectively But the very low value of the costant "ε " (dielectric constant = 2,21 E10) is a problem for the solver, a numerical problem, I think. Also theese source terns which are dependent on each other could be a problem...I know it is a particular issue... What do you think about it ? 
Just because a constant term is small does not tell you it has a big effect or small. It is how it interacts with the flow which is important. There will be some form of non dimensional number to describe these effects, pesumably the ratio of electrical forces to fluid flow inertial forces or something like that. That sort of number starts telling you how significant your constant term is.

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Starting simulation without knowing which effects are important is a reciepe for disaster. You need to do some back of the envelope calculations to work out what is significant and what is not before starting modelling.

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Do you know how many times it has come back to "my supervisor told me to"...
Regardless of what your supervisor says, do some back of the envelope calculations to check it is significant. And the degree of significance will tell you what sort of approach you need to take. 
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My previous post explains what you must do. If the electrical terms are relevant, then what about the viscous terms? The inertial terms? If any of these are irrelevant then you can select the correct options to proceed. Otherwise you are just guessing.
But what is the physics of what you are modelling anyway? I seem to remember you asking about imposing a constant drift velocity on top the fluid velocity  is this the same issue? 
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So I have to consider the electric interations as source terms in the transport equations (cations and anions). We have a laminar flow in the channel 
My question is what is the physics of what you are modelling. To model it you need to know exactly how each important force works because you need to describe an equation which accurately describes it.
Questions include: * How is charge balance maintained? Does it have a conservation equation? * How is motion of the charges coupled to the fluid flow? * Is the charge motion independant of the fluid flow? But my point about working out the significant forces needs to be considered first. It will tell you what approach you need to take and what things can be ignored. It is quite likely that CFX is not appropriate for what you are modelling at all  you will only know once you know what forces are significant. 
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