Fluids

Fluids

temperature limited in reactive modeling

    • maliuzair
      Subscriber



      I am simulating reactions using eulerian eulerian multiphase approach, and species transport.
      When I turn on the reactions after getting the cold flow stable, I get the problem "temperature limited to 1.000000e+00 in 7 cells on zone 2 in domain 3" (Pic attached). I get this problem with the granular phase which is a mixture, not the other one as described below. I reduced the diameter of mixture granular phase from 2 mm to 0.5 mm, but of no use. rest of the details are as follows


      2 granular phases


         a) 0.5 mm, mixture


         b) 0.5 mm (not mixture) 


       


      1 continuous phase (mixture)


       


      Geometry:


      cell size: 3mm


      bottom: velocity inlet (continuous phase)


      side: velocity inlet (granular phase a (mixture))


      top: pressure outlet


      walls: temperature: 1020 K, no slip for continuous phase, free slip for granular phases


       


      initial conditions:


      sand initial volume fraction: 0.6, Temperature: 1020 K


      Air:  temperature: 673 K


      Biomass (introduced later when the cold flow is stabilized): 423 K


       


      Drag: air-solid: Huilin Gidaspow


               solid-solid: Syamlal Obrein symmetric


       


      Evaporation-condensation model: Lee


      Heterogeneous Reactions: UDF


      Homogeneous reactions


       


       


       

    • maliuzair
      Subscriber

    • Rob
      Ansys Employee

      Turn the reactions off and see how it behaves. If the reaction rate is UDF controlled what stops it pulling heat from a cell if (for example) activation energy is reached? 

    • maliuzair
      Subscriber

      I turned off all the reactions, and it shows strange behavior. the sand is patched at 1020 K, and the wall T is 1020 K. Air enters the system at 673 K, and biomass at 423 K. but the static temperature of biomass reaches 1700 K. 

    • Rob
      Ansys Employee

      Steady or transient? What do the residuals look like?

    • maliuzair
      Subscriber

      It's a Transient case

    • Rob
      Ansys Employee

      It looks like a rogue hot spot, so may be convergence/gradient related. How's the volume fraction of solids looking?  You will probably need to turn off one of the granular phases to see how that behaves then gradually make the model more complex: that's our approach when we work through models from commercial clients. 

    • maliuzair
      Subscriber

      simulations without reactions converge easily.


      the pics of volume fractions are attached. one is volume fraction of sand with bubbles. and the other is the species of the phase for which I am having issues.


       


      My approach for simulations was.:


      first I patched sand. NO biomass. 


      solve for flow and volume fraction.


      turn on energy and species once flow is stable. and after some time introduce 2nd secondary granular phase.

    • Rob
      Ansys Employee

      Looks sensible, so we (you) need to focus on the phase change & reaction rate. 

    • maliuzair
      Subscriber

      Is the temperature limited problem the analogous to reverse flow?


      I am asking because the temperature limited problem emerges in some cells and then vanishes, and then reappears just like reversible flow appears and vanishes in cold flows (and doesn't affect the solution in general)

    • Rob
      Ansys Employee

      Yes and no. It's when something causes the solver to increase/decrease the temperature too far. With phase change it's usually that too much material changes phase in one cell and there's not enough energy available to do that. Ideally once the model is running the warnings disappear. 

    • maliuzair
      Subscriber

      Another question related to my problem but not related to the topic.


      Can we use velocity inlet and mass flow inlet simultaneously? are they compatible?

    • Rob
      Ansys Employee

      Not on the same boundary, but yes you can in the same model. 

    • Hesam062
      Subscriber
      Hi Maliuzair,nDid you solve the problem of Temperature limitation?nI've got the same issue during simulation of biomass gasification in fluidized bed
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