CHEMKIN reactor ‘floating point exception’

- January 28, 2020 at 2:20 pm
  
  juniorbeibei
  Subscriber
  
  Hello,
  
  I'm currently trying to do some simple simulations in CHEMKIN pro 19.0 with the 1-D plug flow reactor model, and my goal is to learn about the reactant consumption and the detailed kinetic pathways. The pre-processing step is successful, however, the calculation keeps failing. As I look at the output file it says 'A floating point exception has occurred' and the CKRUN_ERROR file says 'exit_status=12'. Based on my understanding, the floating point exception is caused by 0 appearing at the denominator, but I really don't know why this would happen in CHEMKIN since I have never seen this before. I was wondering if it is the problem with the chemical mechanism because it is a big one compared with others that I used before. Another weird thing is I have no problem at all running this chemical mechanism with the steady 0-D PSR reactor model, while the same error pops up if the PSR model is transient.
  
  I'm really grateful if anyone could help me with this issue. Thanks a lot!
- January 28, 2020 at 4:18 pm
  
  Rob
  Forum Moderator
  
  When you use transient are there any additional settings? Ie boundary profiles etc.
- January 29, 2020 at 4:01 am
  
  juniorbeibei
  Subscriber
  
  Hello rwoolhou,
  
  I don't think I have any additional settings except the necessary ones. For example, in the simulation with 1-D PFR model, I set the problem type as 'Fix Gas Temperature' with constant T, P and axial velocity at the inlet, nothing more. Please let me know if any more information should be provided.
  
  Thanks a lot for your kind help!
- February 4, 2020 at 3:49 pm
  
  pjstopfo
  Ansys Employee
  
  A floating point exception is not usually a 'divide by zero' error but simply that a real number has gone out of range. As Chemkin runs in double precision, it is often caused by divergence. I would first check the out file in the run directory as this should contain more information about what was happening just before the error occurred. In my experience, most problems like this are caused by the user's mechanism or properties. Running the mechanism through 'Mechanism Inspector' (a utility in Reaction Workbench) can help identify such issues, e.g. unphysical rates, negative values, discontinuities, etc.
- February 4, 2020 at 5:19 pm
  
  juniorbeibei
  Subscriber
  
  Hi pjstopfo,
  
  Thank you for your attention. I just tried the mechanism inspector as you mentioned in your post and it looks like there is no obvious error in the current mechanism, but just too big reaction rate, negative activation energy, etc., which are all actually physical. I inspected another mechanism that runs the PFR model perfectly, and yet the same 'too big reaction rate' kind of problem while the mechanism itself surely could work. Besides, If there is really something wrong with the mechanism, then I guess the PSR model could not work as well, but the fact is only PFR is not working with that specific mechanism.
  
  Any more suggestions would be appreciated. Thanks!
- February 6, 2020 at 3:00 pm
  
  juniorbeibei
  Subscriber
  
  Hey guys,
  
  I just solved this issue by removing a problematic reaction from the INP file. It appears that this reaction has a too high TROE parameter (1E8) and I guess this parameter would cause the floating point exception error during the calculation. Once the reaction is removed, the whole kinetic model could run perfectly. Thank you all for your kind help and suggestions!