Fluids

Fluids

Modeling Humidity

    • Bran
      Subscriber

      Hello,


      I wanted to model humid air used for cooling and wanted to know the difference between multiphase and species when assigning fractions of air and h2o.


      From what I've seen, there are two ways to add a mixture of air and h2o into simulations


      1. Multiphase: I have used the VOF model and added air and h2o-liquid phases. In the velocity-inlet boundary condition, I can adjust the mixture by changing h2o liquid to a specific volume fraction.


      From what I know, volume fraction is: fraction of the area of the domain that is allocated for h2o liquid. Example, I set the h2o as 0.1 volume fraction and if the domain is 100mm3, then the volume fraction of the h2o liquid will be 10mm3. 


      2. Species: I selected h2o and air as species mixtures. In the velocity-inlet boundary condition I adjusted the h2o mass fraction.


      From what I know, mass fraction is similar to the volume fraction where whatever mass fraction I specify, it will be relative to the total mass in the system.


      Is my thinking correct for both of these and would species be the best way to model humidity?


      Thanks

    • Rob
      Ansys Employee

      If the materials mix easily (eg water & dye) then it's species. If they don't (gas and liquid) then it's multiphase.  


      In your case if you are not looking at condensation vapour and air are a species mixture. If you use and and water-vapour you'll also find a Relative Humidity item in the post processing under species which makes getting data much easier. 

    • Bran
      Subscriber

      Thanks for your reply. In the case of humid air I do believe humid air is a mixture of liquid water and air, so it does seem like species would be the right case.


      I would like to learn the case of condensation vapour as well. Would I use multiphase in that case, with phase 1 being air and phase 2 being water-vapour, phase 3 being water-liquid and activating evaporation-condensation effect? 


      Also, is there a difference between steam and water vapor in Fluent? Also using species and mixture species I have noticed that among the selected species available there is only h2o. Is there a difference between h2o and water-vapor/water-liquid?


      Thank you.

    • Rob
      Ansys Employee

      Humidity is water vapour & air, and is easy to model as it's just species transport.   Condensation can be handled in the full multiphase models or as a wall film. 


      If you do use multiphase you have one phase that is air & water vapour and a second phase as water liquid. The liquid phase could be droplets (fog) or a film (condensation).  

    • Bran
      Subscriber

      Thank you for that explanation. The multiphase model sounds like what I am looking to model, with 1 phase air and water vapor and the second as water liquid. 


      From what I have seen with working on multiphase models and your explanation, I have added 3 phases from multiphase. Primary phase of air, secondary phase water vapor, and another secondary phase of water liquid.


      I have also added a evaporation-condensation phase interaction as well. What I believe to be occuring in the model I am trying to replicate is humid air (with water droplets) is evaporative cooling on an electronic component. So what I did with the evaporation-condensation phase interaction was added a water-liquid to water-vapor mass transfer interaction, while the electronic component has a volumetric heat source term on it and a specified water-liquid volume fraction. Are those the right steps?


      If I wanted to create a film, would I do the opposite phase interaction (from vapor to liquid) and adding a volume fraction for the vapor instead?

    • Rob
      Ansys Employee

      Two phases:  air + vapour   and   water-liquid.   With 3 phases you'll have more difficultly working out the partial pressure of vapour. 

Viewing 5 reply threads
  • You must be logged in to reply to this topic.