[me.po]

Hi
I've tried to simulate a classical problem of flow and heat transfer in a circular tube with constant heat flux, radius of 10mm and 60mm length. I've used the periodic boundary condition, K-epsilon model with st. wall function and the Reynolds number between 10000 to 40000. When I use the finer mesh, the friction factor and Nusselt increase drastically and get distance from the analytical results . This problem is more serious for low Reynolds (10000<Re<20000). I've checked several grids from 22000 to 1 billion cells, both fine and coarse, with boundary layer mesh (first layer at 10-6 m and I've also used adaption. I've even solved the problem with non-periodic condition and fully developed profile but the problem still remains. I checked the mesh with laminar velocities (and laminar model) and the results were consistent with analytical results (less than 3% error).
Please guide me to solve this problem.

[LuckyTran]

This is exactly why validation & mesh sensitivity studies are always necessary.

Most likely a mesh quality problem. If your grid was automatically generated, it's probably related to increased grid skewness when you went to the finer grid.

[me.po]

Thanks,
What's your suggestion???

[buidu]

The turbulence intensity may have influence on the convective coefficient. The empirical equations are on the assumption that TI is very low. You can find it on textbook. From you simulation, you can also notice it.
Perhaps you can try to run your case on lower TI.

[me.po]

Originally Posted by buidu

The turbulence intensity may have influence on the convective coefficient. The empirical equations are on the assumption that TI is very low. You can find it on textbook. From you simulation, you can also notice it.
Perhaps you can try to run your case on lower TI.

Thank you for your advice,
Yes, it's too high. My turbulence intensity is Min 50% (at core region) and Max 91% (at wall). How can I control TI in periodic domain?

[buidu]

Originally Posted by me.po

Thank you for your advice,
Yes, it's too high. My turbulence intensity is Min 50% (at core region) and Max 91% (at wall). How can I control TI in periodic domain?

You can control your TI in the setting of inlet.

Regards

[me.po]

But that wasn't the cause of the error. when you use periodic conditions, there is no longer any inlet or outlet and flow is fully developed. I checked the problem with finer mesh with very good quality and the problem still remains!!!!!!!!!!!!!!!!!!

[vaibhavhiwale]

Did this problem is solved? im also facing the same problem