Natural convection flow, chimney effect in a box

[TexasHank]

Does anyone have the equations for the "chimney effect"? I am trying to solve, with equations, the heat loss through natural convection, where the different air temperatures create a boyancy effect driving the air through vents in a box.

More specifically, I am considering a electronic outdoor box with 25C internally and 45C external ambient. I assume there is a vent slot at the bottom and one at the top, say about 4 sqin each and 50" vertically apart.

Incropera or my other thermals book does not cover this very well. Thanks!

 

[MintJulep]

The general equation for stack draft is:

D = L(ra - rh)/5.2

where

D is draft in inches of water
L is height of the stack in feet
ra is density of the ambient air in pounds per cubic foot
rh is density of the hot air in the stack in pounds per cubic foot
5.2 makes all the units fit together

 

[TexasHank]

This is a pretty good link I just found, cheers!

http://www.coolingzone.com/Guest/News/NL_OCT_2002/Tony/Tony_Oct2002.html