Learning Forum

[Abbasraza20000220]

Hello to all,

I have been using mixture template ( H20+ Licl+ air). While defining multi component diffusion, H20 in air & H20 in Licl and assigning constant values like 2.88e-05 & 3e-09. On running the simulation, I am getting error like,

warning

Mass diffusivity invalid : Zero diffusivity. Please check material properties.

How to fix this error? Any suggestions? Please look at the attached screenshot!

Regards,

Abbas Raza

[DrAmine]

Did you check material property? Is the diffusivity provided via UDF?nAlso please test in one of the latest supported versions.n

[Abbasraza20000220]

No, I didn't use any UDF for diffusion. My Case is Air + water vapors in one fluid & in other fluid mixture is Licl + water vapor. Just I want h20 in air diffusion & h2o in licl solution. I want to trying through built in Multi component diffusion. nPlease what you mean by material property? I have taken built in air & water vapor from Fluent. Latest support versions mean 2020R2? nRegards,nAbbas Razan

[Rob]

Have you set the diffusivity for each species pair? n

[DrAmine]

Are you using modeling multicomponent two-phase flows?n

[Abbasraza20000220]

No, I am using single phase ( Air + LiCl + H20 vapor) in the mixture and having two fluid inlets ( One is for Air + h20) & other is (Licl + h20). nYes by using multi component option, I gave h20 + licl 3e-9 & h20 + air 2.88 e-5 as you can see in the attached screenshots.nRegards,nAbbas Razann

[Rob]

What about air and LiCl ? They don't mix in the model but are defined in the mixture. n

[Abbasraza20000220]

I am taking mass fraction 0.019 & 0.9804 for water vapor & Air in one fluid & while in the other i have taken 0.0055 & 0.9945 for water vapor & licl in the other fluid. So, I will have two components in one fluid & two in the other fluid mixture.nRegards,nAbbas Razan

[DrAmine]

Are you providing for all components a diffusivity? If Not: Check that. nYou can make a quick test by using the default approach which assumes all components uses the same diffusivity.n

[Rob]

Yes, but as you only have one mixture you need to supply diffusion coefficients for all material pairs. The materials panel doesn't know that they're not in the same fluid zone, so flags the error you're seeing. n

[Abbasraza20000220]

Thanks a lot for your suggestion. nI have noted that licl in air or vice versa I have kept 1(One) in the Diffusion coefficient value in the said mixture. Now its not showing error. Is it ok? I mean Is it effecting my simulation results?nRegards.nAbbas Razan

[DrAmine]

Diffusivity will affect the results you are getting. If flow is turbulent It might have only influence in near wall cells or in areas where eddy diffusivity is pretty low.n

[Abbasraza20000220]

No, it s Laminar & single phase flow. nBy doing this diffusion of air in licl & licl in air is keeping one, how results can be affected? As I have to focus only water vapors & mass fraction of air is zero in the other zone.nRegards,nAbbas Razan

[Rob]

As the material pair isn't present it shouldn't have any effect. It's purely to avoid a divide by zero effect in the maths. n

[Abbasraza20000220]

Thanks a lot Mr. Rob!nRegards,nAbbas Razan