square or round heat riser

I am starting a build of a rmh based on Ernie and Erica's 6" Annex. I have a supply of soft (kiln) firebrick. As well as old soft fired red brick. I could cut the soft kiln brick into small peaces that would make a 12 sided tube (6" diam) and stack those rings to make the heat riser. (it would be quite a bit of work like building a barrel out of wood) Or I could just go with a square brick formation with a 5x5.5 opening. They have very close to the same cross section. Is it worth the time to go for a round heat riser?

thanks
Alan

Alan, cross sectional area of a square, is theoreticaly higher, than it's inscribed circle.

But in real life, with gases flowing through theses, it doesn't work like that. Having more side surface per CSA the square has more drag. Even more in the corners. Because gases are sticking via friction to the materials, forming a boundary layer, which is slower than the rest of the stream of gas. So the corners of a square don't contribute much to the flow.

So if you are going to make a square riser, make it 6" square. If you want to make a roundish riser, people have found that an octagon is pretty close to a circle in effect, and much easier than a 12-sided figure, as it only takes 45 degree angle cuts.

Glenn Herbert wrote:So if you are going to make a square riser, make it 6" square. If you want to make a roundish riser, people have found that an octagon is pretty close to a circle in effect, and much easier than a 12-sided figure, as it only takes 45 degree angle cuts.

And if building a batch-style, vs. a J-style, PeterVDB has mentioned a number of times a preference for the octagon, instead of a square; and a circle CSA ideal. But again, if one is building out of brick, the octagon has been reported as good enough. People that go round seem to either cast the riser or use another round material (ceramic tube, or something - I forget what exactly).

So the preference would be (1) round riser, (2) octagon riser, and (3) square riser.

All of which begs the question... what is the difference between these shapes? How great is the loss in efficiency?

If a round riser is set to equal 100% efficient, is the octagon 80% efficient, and the square 50% 100:90:60%

I have no idea. Just wondering how great the difference really is.

I think the best approach would be to ask Peter van den Berg for his opinion... he could probably give a guess at least.

Glenn Herbert wrote:I think the best approach would be to ask Peter van den Berg for his opinion... he could probably give a guess at least.

Yes, I agree. I just thought I'd throw it out there and see if it catches anyone's eye. Peter's done the most careful analysis of which I'm aware. There aren't many of us experimental private builder's with a Testo meter!

I'm planning my re-build to move from J-style to batch, and I'm rolling around these kinds of questions. Undecided right now. Square is obviously easy. Round I think is very unlikely right now. I'm not going to buy the expensive refractory tubes, even if they were available locally. I'm not sure I want to mess around with casting refractory just yet either. So I'm looking at brick. Most likely. Unless I change my mind ;P

Well well well.

Peter's opinion about square risers on batch rockets, is, that they are slow into getting up to operating temps, when the ram horns can be seen, and are dying off more quickly, giving worse results at the begining and end of the burn. Prety sure you can find this explained at Donkey's.

Tho, they ram horn.

http://www.youtube.com/watch?feature=player_embedded&v=tjzQyNXBKAA
I think octagon and round are prety much the same.

Thanks for sharing that video. I've embedded it below.

Satamax Antone wrote:I think octagon and round are prety much the same.

Aerodynamically speaking, the square is about as good as its inscribed circle. Although the square is about 1.27 times as large as the circle but the fast streaming core in the heart of the vertical tube is more or less round, the corners are more like friction areas. The larger csa is thought to be compensating for the losses.

From another perspective, the double vortex is forming in an earlier stage when the riser is round and will last longer. The octagon is a close approximation which is very little larger than its inscribed circle. And an added bonus could be that the walls are not smooth so that the octagon presumably is mixing the gases better with air when the double vortex is spinning round.

One of the insights I learned from Larry Winiarski last week is that every change in direction and every path narrowing costs pressure loss in the combustion system.