Modeling Natural Convection Heat Transfer with presence of Relative Humidity

- June 17, 2020 at 3:23 pm
  
  kfouladi
  Subscriber
  
  Hello All,
  
  I would like to model natural convection with some moisture in the air. I know to turn on "Species" and then switch mixture with water vapor (first) and air in the species list. What I don't know is how to set density for the mixture in Fluent for natural convection and the mixture. I can use Boussinesq model for air in Material_Fluid. However, the air in Mixture does not give me that option. Any help would be appreciated.
  
  -Kamran
- June 18, 2020 at 12:46 pm
  
  Karthik Remella
  Administrator
  
  Hello,
  
  The Boussinesq model cannot be used with Species. Here are other limitations of the Boussinesq model.
  
  https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/flu_ug/flu_ug_sec_hxfer_buoy.html?q=boussinesq
  
  Having said that, if you are modeling an incompressible flow, you could use the incompressible ideal gas law to estimate the density. Density becomes a function of temperature. Please note that you might have to change the operating density accordingly.
  
  Please let me know if this helps.
  
  Thanks.
  
  Karthik
- June 18, 2020 at 4:55 pm
  
  kfouladi
  Subscriber
  
  Hello,
  
  Thanks for the quick response and your help. Where should I set the density as the incompressible ideal gas?
  
  I can set the density of air in "Fluid" as Incompressible-ideal gas. However, that option is not available for air in the mixture-template under "Mixture". Any help?
- June 18, 2020 at 6:46 pm
  
  Karthik Remella
  Administrator
  
  Are you able to see this?
  
  Thank you!
  
  Karthik
- June 18, 2020 at 7:46 pm
  
  kfouladi
  Subscriber
  
  Hi Karthik,
  
  Thanks and I see that. I thought for moist air I need to use "Volume weighted mixing law". Is that not true?
- June 19, 2020 at 1:38 am
  
  Karthik Remella
  Administrator
  
  Volume weighted mixing law is generally used for mixtures with compressible liquids. However, let me quickly check internally and get back.
  
  Thanks.
  
  Karthik
- June 19, 2020 at 8:18 am
  
  Rob
  Forum Moderator
  
  I tend to set the species density as incompressible and then the mixture as volume weighted. It doesn't really matter much given the vapour concentrations and similar density.
  
  As an aside, make sure you use air & water vapour as that switches on the RH calculation in Fluent post processing.
  
  EDIT. Forgot to add, you need to be very careful with the operating density if you have openings in the box. If you don't you need the ideal gas model.
- June 19, 2020 at 12:45 pm
  
  kfouladi
  Subscriber
  
  Thank you again.
  
  (1) I don't have any option to choose density as incompressible. Please see the attached picture.
  
  (2) "make sure you use air & water vapour...." Do you mean air first and water vapour 2nd? I thought it was the other way around.
  
  (3) can I set my operating density as a constant value?
- June 19, 2020 at 4:30 pm
  
  Rob
  Forum Moderator
  
  Check the mixture density, and make sure it's set as volume mixed. The mixture setting can override the available species options
  
  Water-vapour first, air second: most common species is always last.
  
  Yes, it's in the operating conditions panel.