I know this question doesn't quite fit the Permies view of simplicity and working with materials on hand, but if you could 3d print a rocket mass heater riser, feed tube, etc. using ceramics what shape would it be? Would there be a spiral twist in the riser? I guess for that matter you could just make one by had out of clay and fire it in a big kiln.
Alan, spirals have kind of proved to be counter productive because theses induce a drag. Impeding the draft. In a J tube, you can't do much more to improve mixing than a P channel, a trip wire and a kicktail.
