riser shape

Some risers are round, some are square, is one superior to the other?

Wyatt; If they are insulated ,I think that they are all close enough to be called equal. Myself I prefer round but we build our J tube's with square corners so... some think the riser should be the same. I have seen pictures of octogonal riser's made with brick that could be better yet. Most people who have easy/cheap access to firebrick use them ,those who can get fireclay & perlite easily generally cast a round one. Back to my opening statement if it's well insulated any will do the job !

Wayatt B. : Speaking generally it is hard to share the amount of conviction with which we hold to the ''Know your Cross-sectional Area- and keep it Wholly'' !

This is truly the most important reason behind what would be over-built Icons to Pyromania otherwise !

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built to date, this is ''The Book'', the one most followed, - ''The Book'' Was used in over 95% of all 1st builds (that worked). it is a Handy Guide that will save you
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For the Good of the Craft ! Big AL

Wyatt Barnes wrote:Some risers are round, some are square, is one superior to the other?

I am going to say Yes, but it is not a performance based response. Which one is easier and/or more cost effective for you to build with your available materials and skill set? That one is superior

I totally agree with you Peter, especially in times of haste, need or both but I expect I will be able to build the shape I want with equal difficulty. I don't suppose anyone has tried triangular? So hard to judge which is superior with out a scientific approach which is too time intensive for most people, myself included. Might be the thing for a workshop though, one j tube, barrel, thermal mass and stack and three or four competing risers interchangeable to the j tube top.

I would say triangular would not be a good idea.
The competing theories of round vs. square boil down to a round shape giving more efficient airflow vs. square creating beneficial turbulent mixing due to the corners. My feeling is that by the time the gases get to the riser, they should already be mixed well and there is little or no benefit to further turbulence from the riser shape. The burn will be partly to mostly done by the time the gases are halfway up the riser. It is undoubtedly important to have the riser as insulated as possible to keep it hot, and the smaller surface area of a cylinder will aid in that. Along those lines, a triangular riser would have much more surface to absorb heat from the gases, and its shape would create a lot of drag due to the corners trapping a part of the flow.

I do like the idea of a riser experiment with a few risers of the same material and different shapes swapped out in one system. It would need a Testo to record exact results to be definitive.

One could probably find some studies showing information about fluid flows through various shapes. Such information probably will not reveal the best performers for an rmh, but would probably eliminate a bunch of non-starters.

IIRC, a circle has the smallest perimeter compared to it's cross sectional area, so ia circular heat riser is bound to loose less heat via contact with the walls, have less friction too etc!

Very true so a circular riser would definitely be the choice except we might need turbulence. A circular with protrusions might fit the bill, or a square......hence my question. Maybe I asked the wrong question. I should have asked " What are the most important properties needed in a riser" and then chosen a shape that most fulfilled the properties. I am probably overthinking this and should just pick an easy build to start with that is modifiable.

Wyatt; Your last statement is probably the best answer. A core/riser is relatively easy to modify once it is properly connected to a DRY mass. Too much experimentation with a wet first build could be frustrating. I cant stress enough how a truly dry & warm mass with a vertical chimney will create its own pump. A good size mass takes weeks of daily burning before you get a dry/warm mass that will hold your heat. Once you have a warmed up dragon, relighting in the morning becomes a problem with your lighter blowing out from the draw! Experimentation with a dry build is FUN !

If we assume that the burn tunnel shape and its transition to the riser take care of fully mixing the gases (no more air will be introduced after the burn tunnel), the riser must allow the burning gases to rise strongly and stay hot. A highly insulated, low-mass cylinder sounds like the best way to do that, though it may be that a square does it well enough to be as good in real life.

This question fascinates me. Present build will feature a riser that starts out square, morphs to a wider circular shape, then narrows to a circle that has the same cross sectional area as the initial square. Also putting together an equal height riser that is just circular all the way, for a comparison.  What I've read about pulse jets and two stroke expansion chambers makes me think that the more complicated shape might really offer advantages as far as when the burn will reach high temperature and complete combustion. Wish me luck, this is going to involve some complicated cardboard forms and fancy ceramic/vermiculite work...🙏