CHT strange problem
 

Hello,

Please help!! I am having some really strange problems with conjugate heat transfer modelling. I had some unphysical results with certain problem I am trying to solve, so I decided to do a really simple test to try to find the problem:

I created simple rectangular pipe in which I inserted a sphere that represents a solid. I created inlet with inlet temp.300K, outlet with 0 average static pressure, all walls are adiabatic, there aren't any energy sources, and the flow is laminar. Initial value of temperature is 300K in fluid as well as in solid domain.

So, obviously, steady state solution should show 300K everywhere including inside the solid. But this isn't the case!! The solid phase temperature is rising during the solver run and it settles at about 347K. This is really disturbing..It's like there is a heat source inside solid phase, but there isn't! But that is not all..when I enable turbulence model, the results are as expected - 300K everywhere...

It looks like cfx handles interface flux with wall functions correctly, whereas it fails to do so without them.

please help, as I am lost here..

Re: CHT strange problem
 

Hi MatejK,

I suppose that you have answered the question yourself. Thhe near wall mesh resolution for your case should be poor. You need a fine mesh near the conducting solid. What is the Y+ values on the solid surface.

When you switch on the turbulence model, the boundary layer is perdicted properly by the wall functions.

Regards, test

Re: CHT strange problem
 

Hello,

yes, near wall mesh resolution is very poor because I wanted to do a quick test. I will try with better resolution.

But I don't see why this would matter much. The strange thing I encountered - temperature rising in solid domain, even though there aren't any sources - means that energy conservation isn't satisfied. Energy cannot be created from nothing. But isn't the principal advantage of finite volume method that the global conservation is built into it?

I think near wall resolution should only improve prediction of temperature profiles near wall, not the whole conservation issue.

Matej

Re: CHT strange problem
 

well, the message seems to have gone..

here it is again:

i discovered that if I put prismatic cells near solid walls, the problem goes away.. So I guess that it is tetrahedral cells at the interface that are causing problems with energy conservation. It doesn't matter how dense tetrahedral mesh(I tried with extremely dense meshes) is at the interface, the temperature still rises inside the solid.

Anybody has any idea why this is so?