Fluids

Fluids

Modelling wall boiling as UDF in Ansys fluent for flow boiling simulations

    • sachintom1992
      Subscriber

      Can I model wall boiling phenomena ( wall heat flux partitioning) using UDF in ansys fluent?Which UDF macro has to be used for it?

    • DrAmine
      Ansys Employee

      You will require linearized mass transfer UDF and some other UDF's to incorporate subgrid mass transfer and wall partitioning. The latter is very demanding. 

    • sachintom1992
      Subscriber

      Sir,


      What care to be taken with respect to Mesh for Multi fluid VOF scheme run within the two-fluid model?Is it need to be fine mesh against coarse meshes usually followed in flow boiling cases?

    • DrAmine
      Ansys Employee

      Always a challenge to parse a large source term into a small near wall cell. Generally smoothing is required. Perhaps you require some consulancy.

    • sachintom1992
      Subscriber

      Sir,


          Can the bulk bubble diameter be greater than the flowing channel hydraulic diameter? Like in my case the bubble departure diameter is 0.7mm and the bulk bubble diameter is around 8mm whereas the hydraulic diameter of the annulus is 3.2mm.


       


       

    • DrAmine
      Ansys Employee

      Physically It cannot be: if you dealing with microfluidic the well known regimes do not apply: wall boiling usually result in slugging.


      From the Euerlerian Perspective: it can be as it will just affect closure laws.


       


       

    • sachintom1992
      Subscriber
      Sir,n To solve for mass transfer b/w liquid and vapor in all the cells using multifluid VOF model, which define Macro has to be used? nDefine Mass Transfer macro can be applied to only those cells where interface is located i.e. phase volume fraction should be different from 0 or 1. Mine problem initial interface is not known as vapor gets generated once T(liquid) > Tsat.
    • Rob
      Ansys Employee
      For VOF you'll then need to identify the cells with the free surface and then apply mass transfer. nFor Multi-fluid VOF it's a mass transfer, have a look in the UDF Manual in the multiphase section, DEFINE_MASS_TRANSFER looks a good starting place. n
    • sachintom1992
      Subscriber
      Sir,n Thanks for your reply.n how the mesh to be made for using multifluid VOF with mass transfer model? Usually when using wall boiling model coarse mesh is used...it will work for multifluid VOF model? Whether it has be fine like in VOF model?n
    • Rob
      Ansys Employee
      You don't need to be as refined as VOF as the phase interface can smudge due to the droplet size portion of the model, equally it's a source term (effectively) so you don't need as coarse a mesh as for the boiling model. nI'd suggest a reasonable resolution and do the testing in 2d. n
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