been looking at your sketches again, and i suddenly realised what would probably be the coldest parts of the chamber... the walls
Can't cheat Newton & Bernoulli out of their rightfull place
Anless i'm wrong, at the phase change point, the Butane gas will not go "straight up", but will have the tendency to "cling" to the walls (ok, maybe just a little) , then again, i assume the "hottest" part will be more to the center, so the gas will have to move across the surface towards the walls.
Performance might be better/different as expected, as part of the liquid butane "drip off" from the evap will evap before it reaches the (boiling) pool at the bottom.
of course, all this changes if you put a fan in the chamber
Anyways, i might be wrong on the above.. just thinking out loud