Fluids

Fluids

Simulation does not converge when parameters are changed slightly

    • jFroh
      Subscriber

      Hi everyone,


      I'm simulating a Fischer Tropsch synthesis in a fixed bed reactor using a multiphase species model to simulate the species formation and temperature and pressure.


      I've incorporated my reaction mechanisms with a user defined function because it's a LHHW type kinetic (instead of preinstalled power-law type).


      Now my simulation converges using a tetrahedral mesh and first order upwind solver settings. Most of the rest is at default. The solution gets to steady state that is qualitatively correct (temperature profile and species concentration profile as expected) but the numbers are too low.


      It seems like the reaction rate is too low (not enough heat release). I tried to increase the kinetic parameters or the thermal conductivity of the fixed bed material to try and increase the temperature values to the point where they should be, but either my solution is not converging anymore or the change is very small.


      Do you have any idea what could be my problem and how I can get convergence? I'm not sure whether it's a problem in the model because it works in some cases.


      Thanks in advance!

    • DrAmine
      Ansys Employee

      Recheck your reactions mechanism and your UDF if they are inline with the theory you want to deploy. Espcially the mechanism: that is where Ansys cannot assume any warranty.


      Starting scaling the rates will of course affect the overall behavior. This way is questionable as you are manipulating the rates and so starting deviating from what you are trying to deploy. 


       


      Regarding convergence stiff chemistry solver and smaller time step sizes are always way which might help in this kind of situations.

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