Fluids

Fluids

UDF for energy & mass transfer source terms in multiphase VOF model

    • cbliss1
      Subscriber

      Hello, I have a model of a wick-less heat pipe with which I am trying to simulate the evaporation and condensation processes. Using a UDF I am hooking the mass source terms and energy source term to the respective governing equations for VOF (continuity) and energy equation. The model consists of a water-vapor region and a water-liquid region. When implementing the source terms for mass and energy using DEFINE_SOURCE, the energy source is hooked to the mixture level cell zone, and the respective mass sources (one for vapor and one for liquid) are hooked to the sub level (phase) zones of the mixture thread. When this is done, does Fluent subtract the source contributions from each of the phases? I ask this because it seems to be rather vague as to how fluent deals with multi-phase mass source contributions. The theory guide does explain that the mass source in the VOF equation is treated as "m_vl - m_lv" which is why I am asking if this is how it is implemented when defining the source terms with a UDF.


      Thank you,


      CB

    • DrAmine
      Ansys Employee

      Please use DEFINE_MASS_TRANSFER or linearized version of it to avoid any headaches.

    • cbliss1
      Subscriber
      Thank you Amine!

      When using the DEFINE_MASS_TRANSFER macro, does fluent implement the correct energy equation as it would when using the built in Lee model? Or does a DEFINE_SOURCE still need to be used for the energy source?

      Also, in order to capture both evaporation & condensation does one need to use 2 mass transfer mechanisms and therefore have two DEFINE_MASS_TRANSFER macros being equal and opposite in the UDF script?
    • DrAmine
      Ansys Employee

      Yes: all are accounted as it should. Only one UDF is required. Have a look into the examples from the manual.

    • cbliss1
      Subscriber

      The example that is in the manual for DEFINE_MASS_TRANSFER seems to be the correct implementation of the lee model.


      When one mass transfer mechanism is implemented and the provided UDF snippet in the manual is used. Under "phase interactions", lets say the from-phase is vapor and the to-phase is liquid. Based on the temperature conditions in the UDF: the something is being reduced when the temperature of the cell is greater than the saturation temperature and something is being increased when the temperature of the cell is less than the saturation temperature. I am confused as to exactly what is being increased or decreased is it the vapor or the liquid? primary or secondary? 


       


      Further to this, Wouldn't it make sense to use two mass transfer mechanisms to account for the following: 


         --When the cell temperature is greater than the saturation temperature - an amount of gas is created and a corresponding amount of liquid is destroyed


         --When the cell temperature is less than the saturation temperature - the reverse happens to account for liquid condensing. 


       


      The reason I am laying this all out is because I simply can not figure out how fluent is treating this process. Does fluent automatically account for the mass of liquid changed to gas (or the reverse) AS WELL AS the mass of the corresponding "from" phases to make way for the incoming mass transfer when only one mass transfer mechanism is implemented?

    • Nitin_Bansal
      Subscriber
      nhellonare you able to overcome your above mentioned problem?nI am trying to do same for liquid nitrogen case and unable to get how to write UDF of source term for continuity and energy equation in VOF. please help me out and suggest any way.nthanksn
    • DrAmine
      Ansys Employee

      If not receiving a timely answer please create your own thread with your question.

    • Nitin_Bansal
      Subscriber
      nsure sirnthanks for responsenn
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