# defining mixture & defining mass fractions + AMG SOlver

**59**

Hello to all of you!

I am here to make a mixture of AIr + Licl solution + H20 vapor. These are 3 components & I want to add mass fraction in the boundary condition. In the screenshot, You can see its giving only two components H20 & licl but not giving Air.

What I am thinking , its gives only 2 components in this section & 3rd one (Air) is not showing. By default, Fluent will take the mass fraction of Air = 1- (MassFraction Licl+ Mass fraction H20). is it like this? Please clarify it.

Secondly, I have two ducts (I want to define Air + H20 in one duct while in the second duct Licl +H20. In this regard, I can define only one mixture. I am confuse how I can give two different mixtures? What I am thinking that I have to select all components (Air +Licl sol. +H20) in the mixture & then defining mass fraction, I have to kept Licl zero in one duct & air zero in the other duct. Is the right way to define it? Need suggestions.

Last but not the least, I am getting an error AMG Solver in the x momentum & AMG solver divergence in species-o. after interpreting the UDF , on running the simulations. One more thing I am here to add, model is working (simulations ) without UDF.

Regards,

Abbas Raza

## Comments

8,371You are correct, set the largest volume material as the last in the list, so in your case whatever isn't H2O or LiCl is air.

Are you using two phases? Otherwise you only have one mixture, and you set the boundary at whatever species fraction you want. Once the material is in the domain it's all one species mixture unless you have two phases.

59Thanks a lot for your response.

Yes, I am working on a Single phase.

I want to keep in the same way as you are mentioning in the above comments. For example, I have set H20,LICL & Air in the mixture. Its giving mass fractions of H20 & LICl in the mass fraction while Air is missing. If i want to define the mixture of H20 & Licl in the other duct where we have to mention mass fractions in the respective fields. If I am not wrong, here air also exists If we take the above line as a reference. As by default fluent gives only one mixture. How I can remove air from that duct in which I want H20 & LICl only.

Need your valuable suggestion in this matter.

Regards,

Abbas Raza

6,642Air is the ballast component. So if you put o for the two components air will have mass fraction equal to one.

8,371And if you put H2O+LiCl = 1.0 there is no air on that inlet.

59Thanks for your answer.

I have a situation like I have 0.65 mass fraction Licl, 0.15 H20 mass fraction & wanted air mass fraction should be zero. As 0.65 + 0.15 =0.8 knowing the fact sum of mass fractions should be 1. By default it will take 0.2 mass fraction of Air which I don't want. How I can overcome this situation?

& Respected DrAmine, what is meant by ballast? Sorry I didn't get it.

Regards,

Abbas Raza

8,371Erm, the sum of the mass (or volume) fraction has to be 1. That's the whole point of it being a mass (or volume) fraction.

6,642Ballast component is the abundant component and is the default last component in the mixture in Fluent which is not solved for.

You should scale your 0.65 and 0.15 to have 1 at the end: 0.8125 and 0.1875 as mass fractions!

59Thanks a lot for your response.

I agreed that sum of whole mass fractions should be equal to 1. In a given situation, I want to observe the moisture(mass fraction H20) is transferred across the membrane. (As I want to model mass fraction will move from one mixture to the other mixture in between there is a membrane (Coupled wall).

How It will be transferred? knowing the fact adding more mass fraction will lead to more than 1 ( sum of whole mass fractions of components) .

Regards,

Abbas Raza

6,642You need an UDF which transfer the flux across the membrane: Source terms in one side and sink terms in the other side.

59Thanks once again for your answer.

Regards,

Abbas Raza

6,642Welcome!