Fluids

Fluids

Evaporation under natural conditions

    • poimsm
      Subscriber
      Hello, I am trying to simulate the mud drying process with a biphasic Eulerian model.nPhases:nLiquid phase with species: mud + liquid waternGaseous phase with species: air + water vapornNatural evaporation occurs at any temperature below the boiling point. It seems to me that evaporation should be modeled as a diffusion of water at the interface (and not as a boiling process or Lee model).nMy question is:nHow could I model a diffusion driven mass transfer mechanism between liquid water and vapor at the interface without using UDF?nMy case is not a flat surface that evaporates water, but a 3D model of a horizontal mud mixer blown by a dry air stream.n
    • Surya Deb
      Ansys Employee
      Hello, nLee Model is sort of Mechanistic where the evaporation/condensation coefficients might need to be adjusted based on experimental data.nA more physical model that can be used with E-E multiphase is the Thermal Phase Change. The mass transfer in this model is totally based on the heat transfer across the phase interface. So essentially, this does not require any coefficient calibration like Lee Model. Also use the two-resistance heat transfer along with this model.nYou might want to take a look at the theory for this in the link : https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/flu_th/flu_th_sec_mp_evap_cond.html?q=thermal%20phase%20changenHope this helps.nRegards,nSDnn
    • poimsm
      Subscriber
      Do you mean Phase Change model in Fluent or Thermal Phase Change in CFX? In the case of Fluent, this model seems to me to be designed for flash boiling, while for the CFX model, this is a boiling model and it is not for natural evaporation phenomena. If I were to use the boiling model and set a low saturation temperature to force the liquid to vaporize, the mud would remain at that constant saturation temperature during the phase change process, and the effect of relative humidity of the air would not be taken into account.nFlash boiling (flashing) is a process of phase change from liquid to vapor. It distinguishes itself from traditional boiling by the way the liquid gets superheated. It is caused by depressurization or pressure drop instead of heating.nIdeally I would like to implement a model in Fluent like the one described in this image with a mass transfer driven by diffusion in a biphasic case...nn
    • Surya Deb
      Ansys Employee
      Species diffusion driven mass transfer can be implemented in Fluent using Interphase species mass transfer option. In that case, the mass transfer will solely occur due to concentration gradients. nI was mentioning Thermal Phase Change in Fluent which can be used as an alternative to Lee Model when using evaporation/condensation. But yes, the mass transfer mechanism is dependent on heat exchange across the interface for this case.nI think you will need Species Mass Transfer instead. Please have a look at this link : https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/flu_th/InterfaceSpeciesMassTransfer-3A72B96D.htmlnRegards,nSDn
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