.......the bottom of the chamber is open as well as the top and you can get a slight flow of air rising up the tube.......
Ahhh,
Now here's one of my favorite discussion points regards air flow, often gets heated discussion, and at times I get others to agree with my lecturer from the past.
Hot Air Does Not Rise!What can happen though is colder more dense air displaces the warm lighter air upwards.
Just like dropping a brick in a bucket of water; the water doesn't really rise, it is displaced by something denser.
When you apply that philosophy to air movement it takes a different turn.
Swap hot air for just air, & cold air for water & then visualise how everything flows & it paints a bit of a different picture.
Got my flame suit on... Accustom to the Blow Torches coming out on this topic.
Phil.