Tapered drill bits - diameter
Well, its not necesarily that the air in the center of tha channel delivers more volume to the but. part of what happens is that the pressure changes as the channel narrows. This along with the increase in velocity means that roughly the same volume of smoke is there, given a constant flow rate. What does happen is that the flow around the edges, nearest to the walls become less laminare and more prone to turbulence, which in turn allows the moisture suspended in the smoke to form dropletts and gunk up your pipe.