Modeling Natural Convection at Different Altitude

[jlinville]

I have been trying to model a small electronic piece at different altitudes (650 Ft compared to 10000 Ft) with external natural convection. I have changed the environmental air pressure to accommodate for the difference in air pressures at the corresponding elevation. I have also gave the entering boundary condition of the external air volume the temperature of 50 degrees C, but did not change the environmental air temperature. I assumed the boundary condition would vary the air properties to the boundary condition when set on variable. The entrance and exit also have the absolute or zero gage pressure depending on the situation. The other external air surfaces have the film coefficient. I have got plausible results, but I would like to know if this is the correct way to model external natural convection in different altitudes. If there is a better and more accurate way to go about this, please explain to me how to go about setting up the materials and conditions. Thank you in advance. 

[apolo_vanderberg · Accepted solution]

Yes, for a typical natural convection model (where we'd assign a zero pressure gage at the top and bottom) we would adjust the Scenario Environment Pressure to reflect the impact the absolute pressure variance has on the density.

Depending on your assumptions your inlet temperature would be assigned as needed. It is always a good practice for Natural Convection to set the Scenario Environment Reference Temperature to be the same as the Inlet Temperature as this will impact the density taht the domain is initialized at.

[jlinville]

Thank you for the quick reply! Should the inlet and exit pressure boundary conditions for the external air part be assigned as the absolute pressure at elevation, or does the gage pressure change with respect to the environmental pressure variable? 

[apolo_vanderberg · Accepted solution]

Gage pressure are relative to the Reference Pressure

So you could do either  Gage or the actual Absolute for that altitude

Gage tends to be easier as you can always assign the same conditions 0psig and then adjust the Reference to represent the proper altitude.

[jlinville]

Got it! Thank you.