Fluids

Fluids

Do I need momentum source term for mass source?

    • Weiqiang Liu
      Subscriber

      Hi all,


      I am modeling a porous soot cake combustion with fluent. In my model, the filtration velocity in porous zone is too small compared to literature results. 


      I spend a lot of efforts and failed to solve this problem. I wonder is it because I did not define momentum source for new generated mass source in the domain. Because the generated mass has to be accelerated by existing momentum. However, the mass source should have momentum by itself.


      I want to know is it necessary to define momentum source for new generated mass source in fluent? If it is necessary, how should I define momentum source for new generated mass?


      Best regards


      Weiqiang

    • DrAmine
      Ansys Employee
      Generated mass will consume mean momentum to get accelerated. One needs to define all secondary fluxes in Fluent: so not only momentum but energy. Thst is why I recommend to use species transport for your case in the past. Is that not mentioned in the literature?
      Is this mass sue to reaction? Or it has been added?
    • Weiqiang Liu
      Subscriber

      Hi Amine,


      Yes, I use species transport model in my case. Mass source is due to reaction. I have defined energy source due to reaction. Do you mean I have to define momentum source for the generated mass source and I also have to define energy source for the generated mass source?


      If I have to define momentum source for generated mass, how should I define momentum source? Should I just define an averaged momentum source for the cell?


      Best regards


      Weiqiang

    • DrAmine
      Ansys Employee
      How are you modelling the reaction? Define vr rate?
    • Weiqiang Liu
      Subscriber

      I define the reaction by giving source term for species, mass and enthalpy with UDF


      Best regards


      Weiqiang

    • DrAmine
      Ansys Employee
      So you are transferring mass from component a to b and accounting for reaction heat? I guess no momentum source term is required as you are not extraction or adding something from external or from another phase.
    • Weiqiang Liu
      Subscriber

      But one of the reactants is carbon which is solid. Therefore, additional mass is indeed adding to the fluid system.


      So I do need a momentum source term, right?


      Best regards


      Weiqiang

    • Weiqiang Liu
      Subscriber

      Hi Amine,


      If I have to define a momentum source for mass source, How should I define that momentum source? My idea is:


      x momentum source=mass source term*cell volume* x velocity


      y momentum source=mass source term* cell volume* y velocity


      I can use macros to access x , y velocity and cell volume.


      Am I right?


      Best regards


      Weiqiang

    • DrAmine
      Ansys Employee
      1/If you are adding something not formed by reaction then yes.

      2/Could be a way yes if you do not now the momentum of injection of the specie. Check docu too.
    • Weiqiang Liu
      Subscriber

      Hi Amine,


      Yes, external carbon mass is added to fluid. However, I don't know momentum of added mass, I just assume it has same velocity with surrounding fluid.


      Best


      Weiqiang

    • DrAmine
      Ansys Employee
      Ok
    • Weiqiang Liu
      Subscriber

      Hi Amine,


      Thanks very much. I learned a lot from you.


      Best regards


      Weiqiang

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