Learning Forum

[marius.wollrab]

Dear Ansys-Forum,

I have a 2D simulation of a horizontal pipe with a porous zone acting as a catalyst vertically in it. There, a bunch of exothermic reactions (gas species reacting to gas species) are happening, which are solved using the Stiff Chemistry Solver + ISAT.

The model was initially constructed in Fluent 2019 R2 and is running and converging there without any problems. Now I want to migrate the model into Fluent 2024 R1 and when I do that, I first get the following message:

Warning: Zero number of gas phase reactions.
Please disable the (gas phase) Stiff Chemistry Solver.

 

and after around 5 to 60 iterations, the first messages of:

Integrating chemistry on fluid zone 196 ...

Stabilizing species-1 to enhance linear solver robustness.
Stabilizing species-1 using GMRES to enhance linear solver robustness.

 

appear only to crash the simulation afterward with the message:

 

Divergence detected in AMG solver: species-1

Error at host: floating point exception
Error: floating point exception
Error Object: ()

Error at Node 2: floating point exception

 

Note here: The same message appears for each species, but for simplification, I only copied it of species-1.

The simulations are both run the same way with the same initialization.

 

I played around first with the solution controls relaxation factors for the species, ranging from 1 to 0.1, but this did not help. Additionally, I had a go at the advanced solution controls, but no other cycle-type or stabilization method helped (values of 2019R2 added as pictures; I did not touch the algebraic multigrid controls yet). Furthermore, I tried every species solution method from first order to third with no success. I also switched from ISAT to direct integration with no success.

The case runs without any problems if I turn off the species transport model. I also tried to load the converged 2019 R2 solution and calculate from there, but here the crash appeared nevertheless.

Do you guys have any other Idea what could help this case and/or what changed with the AMG Solver that could lead to such a problem?

Greetings and thank you.

[Ren]

It looks that you have only wall surface reactions on the catalyst and there are no volumetric reactions. If this is true, please disable the stiff chemistry solver as the message suggested (use the default "direct source - No TCI" option).