[Hamda]

Hi,


My problem is that the value of solid temperature is higher than the expected value. Even I increased mesh density in solid domain but nothing has changed. The maximum value of solid temperature must not exceed 1800 K but I got 2000 k.


Thanks in advance.

[ghorrocks]

To artificially impose a limit on the temperature is very, very bad practice. If the temperature has gone that high then there is something in your simulation which made it - your boundary conditions could be wrong, your materials could be wrong, your physics could be wrong, or you have numerical problems. This is why it is bad practice to impose a limit - you will have the actual problem still present, and then an artificial temperature limit which is a second non-physical factor. Two wrongs don't make a right.

So fix the underlying problem, do not "fix" it by artificially imposing a limit.

[Hamda]

Thanks for your response.
Well, I got exact results for fluid domain and my problem is just solid temperature that must not exceed 1873 but after several simulations with denser mesh I got about 1900K for maximum solid temperature, of course I did this simulation using other fluid too and didn't have any problem in solid domain and the maximum value of solid temperature was very less than 1873 but for nitrogen the solid temperature is higher.

[ghorrocks]

Please post an image of what you are modelling, something showing the high temperature you are getting and your output file. Also an image showing how you get a good result when using a different fluid would be good. Attach them to the forum post, do not use third party download sites.

[Hamda]

Originally Posted by ghorrocks

Please post an image of what you are modelling, something showing the high temperature you are getting and your output file. Also an image showing how you get a good result when using a different fluid would be good. Attach them to the forum post, do not use third party download sites.

Thank your for your response.

The first image is my geometry. each Sphere is fuel element and generates heat and the fluid enters from the top of the geometry and passes through the spheres and collects heat from them and exists from the bottom of the geometry.


The second figures shows solid temperature along the axial position (from top to bottom of the geometry) for two fluids and as can be seen, T_s is small for helium but for nitrogen...
and third file is my output file.

Thanks

[ghorrocks]

Why would you expect the N2 and He results to be the same? They are different gasses with different material properties, so you would expect the results to be different.

Looking at your output file:
* You have the same viscosity and conductivity for He and N2. This does not sound right.
* You have the buoyancy reference temperature at 36 kg/m3. Are you sure this is correct?
* You have viscous heating on. Unless you expect it to do something you should turn it off.
* You are setting the body force averaging, high speed numerics, Rhie Chow and total pressure option settings. Unless you know you need these you should leave these at defaults.
* Your convergence is quite poor. I would not trust the results from loose residuals like this. You need to converge tighter: see FAQ https://www.cfd-online.com/Wiki/Ansy...gence_criteria
* Likewise you have the

[Hamda]

Originally Posted by ghorrocks

Why would you expect the N2 and He results to be the same? They are different gasses with different material properties, so you would expect the results to be different.

Looking at your output file:
* You have the same viscosity and conductivity for He and N2. This does not sound right.
* You have the buoyancy reference temperature at 36 kg/m3. Are you sure this is correct?
* You have viscous heating on. Unless you expect it to do something you should turn it off.
* You are setting the body force averaging, high speed numerics, Rhie Chow and total pressure option settings. Unless you know you need these you should leave these at defaults.
* Your convergence is quite poor. I would not trust the results from loose residuals like this. You need to converge tighter: see FAQ https://www.cfd-online.com/Wiki/Ansy...gence_criteria
* Likewise you have the

well I have never ever expected to get the same results for two different fluids. I said before that I didn't have any problem with other gas i.e. helium since I calculated some results with hand to check CFD results and there was a good agreement between the calculation and CFD results. but for Nitrogen, solid temperature was a problem. I got 1900 K but indeed it is expected to be under 1873.

Also, the output file is just for nitrogen and not helium, I already know that they are different and I wrote some expressions for their properties separately.

About your other points I must check them and thanks for mentioning them.

About convergence, yes I already know that. But I have worked hard on meshing stage even with 2.5 million element cells (quadratic)I still can't get a desirable convergence. One reason maybe because of defining many expressions for properties of the gas (e.g. conductivity, viscosity) in which the properties have been defined as a function of temperature also I assume nitrogen as an ideal gas.