...
The exit has the same principle as the rear of a rocket engine - for a moment let's forget the fans and think only in passive air circulation. Since the hot air expands, it will hit the walls.
But those walls angle will make it go out, instead of returning.
Since the air exits, it will create a very low pressure in the middle, pushing the air from the back.
Oh, now I get it. I should do more testing
. Btw, I ordered some temperature sensors (two), only need something to heat in the middle section. Should be fun 
I'm thinking about using a
but I can't find the equivalent of a boxed fan here in germany, only round ones (I want a boxed one!)