Coming in late here, and I'm not an engineer.. just an ol' HVAC guy.
Thinking about the first mock up, wouldn't it be better to actually have A choked down to the fire boxe opening? My rationale here is pressure differentials, and we know that air will flow through the A opening, provided the rest of the openings possess a lower static pressure drop. In this sense, the flow, to me, seems more elegant when flowing toward a lower pressure, as opposed to having a 'choke point' midstream within the cook chamber, that "chimney effects" upon exit. I don't own a RF, so grains of salt.. That said, I do like the circular reverse bend, as air flows better (less static pressure loss) across a round opening vs a square one. Theoretical ideals would have the cook chamber cosisting of two flat ovals, joined at the other end by a flat oval of equal area. Impossible to build in practical terms. I love your experimentation and theory push. stay busy!
edit to above: I suppose my thinking for makign the most ideal RF design involves the combo of air flow through lowest pressure loss, while also having the ideal air change rate (to maximize the heated air). In plain english, air flows in the chamber, stays as long as ideal for heat gain, then exits at a rate that just enough to fuel a fire. I can't conceive of a way to even test this though. So many variables, that, to the point of elimimating them would create a one-trick-pony smoker that was designed to burn at only 225 with the smallest amount of fuel. Which is so variable in its own right. ugh! nevermind! my head hurts.
Edited by GoinForBroke - 7/13/13 at 12:21pm