diffusion in general
 

Dear all,

I have got a general question about diffusion of a species of small mass fraction inside a "carrier" fluid of high concentration which I am simulating with Fluent.
Is it enough in this case to define the diffusion coefficient of the dilute species inside the carrier fluid or do I have to define a diffusion coefficient for the carrier fluid as well? It's probably a stupid question, but I was just thinking about that the diffusion of the carrier fluid (inside the dilute fluid) would also influence the diffusion of the dilute species at which I am interested.
Does anybody have an idea about this?

Thank you in advance!
Lilly

mass diffusivity at fluent
 

Dear all,

I am still thinking about the same problem: I have a small amount of species in a large amount of carrier fluid.
When I use the "constant-dilute" method at Fluent, Fluent uses the same diffusion coefficient for both species. Do I have to use the "dilute-approx." method and set the diffusion coefficient of the carrier fluid to zero? On the other hand, the diffusion coefficient of the carrier fluid should not be included at the mass transport equation for my species anyway, so does it matter?
I also found this citation at the fluent user guide: "Coefficient:
sets the diffusion coefficient for Species Di in Species Dj (which is equivalent to the diffusion coefficient for Species Dj in Species Di)." Does this means everything is fine, when I am using the "constant-dilute" method and have the same diffusion coefficient for both species?
It would be really nice, if somebody could help me!
Lilly

Modelling transport of additional variables in Fluent
 

Hi everyone,


I am trying to model the transport of two species, A and B, in blood (carrier fluid). In ANSYS CFX we can do this by defining the carrier fluid properties at first and then setting the diffusivity coefficients of each additional variables/species. Then we need to define the mass concentration of each variable at boundaries. The summation of total mass concentration needs not to be equal to one in CFX. Also there is no need to define the mass diffusivity coefficient of the carrier fluid.


However, in Fluent, to model the mass transport of species, we need to define a mixture at first, which contains the carrier fluid (blood) and both species (A and B). The input mass concentration of these components needs to be equal to one. Also, to define the diffusivity of the components, we have to define the their diffusivity coefficients in the mixture one by one as "dilute approx.". This means that we need to define the diffusivity coefficient of species A in the mixture, B in the mixture and the also the diffusivity of the carrier fluid(blood) in the mixture.



My question are:

-why we need to set the diffusivity of the carrier fluid?

-why the summation of the input concentration should be equal to one?


-Is there any way to model this transport problem in Fluent like the way we can do it in CFX?


Many thanks in advance.

That's how the species transport model works for a fluid mixture. A mixture is the sum of its parts, so the mass fractions must add to 1. Every species has a diffusivity (even if it is 0). There is no carrier fluid in a species transport model. CFX is the same way.

You are using the wrong model. What you are looking for is the user defined scalar.

Dear LuckyTran,


I really didn't know I should proceed that way. Thanks a lot for letting me know about this. :):):)

I've got the same problem, someone could be more specific about how to use this function and how to set it? I have to present this project this week and I don't really know how to solve this problem