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-   -   Cells with t below lower limit (http://www.cfd-online.com/Forums/cd-adapco/76593-cells-t-below-lower-limit.html)

Purushothama May 30, 2010 09:09

Cells with t below lower limit
 
Hi,
I am using Star-CD 3.26 version for fuel cell simulation work. (isothermal condition, operating temperature 353.15 K). Recently, I got a warning message as follows and diverged after few hundreds of iterations. Can anyone tell me what is this warning and how i can resolve this? Thanks in advance.

*** WARNING #007*** SETTING T AT CELL 46460 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46461 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46530 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46531 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46600 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46601 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46670 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46671 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46740 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46741 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46810 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46811 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46880 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46881 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46950 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 46951 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47020 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47021 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47090 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47091 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47160 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47161 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47230 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47231 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47300 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47301 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47370 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47371 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47440 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47441 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47510 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47511 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47580 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47581 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47650 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47651 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47720 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47721 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47790 FROM -NaN TO 5.400E+01
*** WARNING #007*** SETTING T AT CELL 47791 FROM -NaN TO 5.400E+01
40 CELLS WITH T BELOW LOWER LIMIT

Pauli May 31, 2010 10:22

STAR computed those cells should have a very small absolute temperature.

Translated, you have a problem with your model which is causing the solution to diverge. It could be bad cells, solution settings, time step size, etc., etc.

The solution is to do a bit of detective work. Look for clues & other warnings. Does the rest of the output make sense? Simplify the model & run it again (will it run steady state, incompressible, no chemistry). Sorry but their is no easy answer.

Purushothama May 31, 2010 21:58

Thanks Pauli..

I have tried to run the code for simple geometry (simple serpentine flow), it runs successfully and converges well. But when I implemented the same code for large-scale calculation, it becomes unstable and diverges (say after 500 iterations) with the above warning messages. About bad cells, I dont think I have any problem with the cells because I used check tool and moreover I used uniform cells (rectangular cells only) throughout the geometry.

And also simulation runs without any problem when I turn off species calculation. So I tried with keeping it turn off until few hundreds of iterations. But simulation diverges with few runs after species calculation starts. I would appreciate if anyone could help me in this regards. Thanks


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