[wassli]

Hi everyone
I want to know about energy and wall prantle number options available for Realizible k epsilon model.
I have another question that are energy prantle number and turbulent prantle number the same? or these are two different things because when i manipulate with energy prantle number then i get reasonable results.

Thank you

[wassli]

Does any Body know the answer of my question because i am really desperate to know and could not found the answer that i am looking for.
Atleast i should know the range for Energy Pr No for the case of freon r134a.

Any suggestions will be highly appreciated..


Thanks

[agustinvo]

Hello,

I suppose that you talk about the Prandtl number. Normally, you have the molecular one, inherent to your fluid, and then the turbulent Prandtl number, related with your turbulent flow. The second one is the one you can tune, since it's kind of a thermal model.

What do you mean by energy Prandtl number? If you specify better your problem, maybe more people could help you.

[wassli]

Actually i am Modelling Supercritical freon r134a in Fluent but the results to get match with experimental data i have to change energy Pr No, i know that energy prantle number is the turbulent Pr No for energy and noting else more, people have suggested corelations for water and CO2 but not for Freon r134a. So i have varied it B/w 0.6 to 2.5. i am just not satisfied that what i am doing is right or not ?
I have tried Different Turbulence models, among them Realizable k epsilon is giving better results but i have to vary Energy Pr No at every Inlet Temperature.
To Capture Deteriorated Heat Transfer, i had to choose 2.5 and in enhanced H.T i choosed 0.6 and also varied within this range.

SO my Question is that what i am doing is right or not ?

[agustinvo]

Ok, so the problem is that you don't know what value of the turbulent Prandtl number you have to use. Normally, a value of 0.8-0.9 is used for fluids whose molecular Prandtl is around 1. You have to check of the Prandtl number of the fluid in order to set a good value.

After that, take a look if the mesh and turbulence model are coherent.

[wassli]

Thank you so much for reply,
actually molecular Pr No is varying from 1 to 14 as you can see in the figure, And i am using Realizible K epsilon with enhance wall treatment and keeping y+ below 1,
i had to choose Turbulent Pr No upto 2.5, could this be right?
plz see the attachements

pr_no.png

mesh.png

[LuckyTran]

Actually wassli is using the correct verbiage, it's just that most people are not aware these things even exist.

Be careful because often the energy Prandtl number can be what we normally call the turbulent Prandtl number. It's called energy Prandtl number because it is the turbulent Prandtl number appearing in the energy equation. You can also have a turbulent Prandtl number appearing in the epsilon equation or k equation (where you call these TDR Prandtl number and TKE Prandtl number, etc). Some codes don't use Prandtl numbers but turbulent Schmidt numbers.

The wall Prandtl number is also a turbulent Prandtl number, but it's specific to the wall model that you use. To understand what this means, you really need to dig up the specific wall model that you are using. Turbulent viscosity is very similar. Far from walls you calculate turbulent viscosity according to your primary turbulence models, but close to walls you usually switch to another formulation and you have a wall turbulent viscosity.