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Why sqaure burn chamber over round?

 
Posts: 7
Location: Colorado
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I've been looking at round vs square burn tunnels and I'd like some feedback from the experts, or some advice on where this might have already been discussed. I understand that square is easier to build with bricks but I plan to cast my core so I can make it any shape I want. So here is my thinking:

If we say that we make a round chamber and a square one, both with the exact same cross-sectional area, then the round one ends up having about 12% less surface area than the square one. Wouldn't this decrease friction along the walls thus decreasing laminar flow? And if so, would that not improve the burn overall?

I'm sure I'm over-thinking it but I'm a nerd and enjoy that part of the process.

Thanks all,
Tobias
 
pollinator
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You sound like an amateur nerd.

A professional nerd would have already calculated the Reynolds numbers for the two cross-sections.
 
Tobias Brace
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Location: Colorado
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True I am very much a beginner nerd with regards to fluid dynamics. My professional nerdiness has more to do with the human body so the learning curve is not as easy as I expected.

So Reynolds numbers are not making complete sense to me when I looked it up because I cannot wrap my head around thinking about a gas or flame as a viscous substance. Care to throw me a bone? Is that what makes a square better than a round?
 
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Hi Tobias,

This is complete speculation since I have only recently been learning about RMH as well. I remember watching Ernie and Erica in a video or maybe Paul said this but if the wood gets into the burn chamber itself it can affect the airflow and it may not burn as efficiently. That being the case I would think that if you molded the burn chamber round you would end up with a very sloping bottom that may cause the wood to enter the burn chamber.

In Ernie and Erica's video they talk about the original RMH were metal and did not work as well and it was an accidental discovery that the firebricks worked so much better. The pores of the firebrick apparently help mix the air into the fire and thus make the conditions for a complete burn easier to attain. Given this fact I would think that it may not be a good plan to try and minimize friction as it would appear that this actually aids in a more complete combustion.

Anyway hope I was somewhat helpful, this is all info I have gleaned from permies in the last couple months. Hopefully someone with more complete knowledge will clarify if I am incorrect in my understanding or further elaborate on this concept.

Daniel
 
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Wouldn't this decrease friction along the walls



Therein is why square is a bit better - from my understanding, turbulence in the burn tunnel is a *good* thing, so increased friction (to a point) and even fins, "turbulators" and other obstacles can result in a higher efficiency. What we're after in the burn tunnel is a thorough mixing of hot gasses with oxygen rich air - when these reach the super-insulated heat riser ...well, rocketty reburn

Once we get to the heat riser, however, round and smooth wins
 
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I think that for most installations by DIY folks, it makes sense to go with whichever shape is easiest to construct. There are fine points in terms of optimizing, but some of those are probably finer than our skills can readily address as we are experimenting and learning. Keep to the rules about consistent size, make sure the manifold is large enough, and so on, keep the heat riser the right distance from the barrel top, don't make your flue too long and have a good height on your chimney.

Get those dimensions right and you should have a good stove. Then you can start looking at whether a cylindrical burn tube works better or worse, than a rectangular one, whether a slick surface or a rough one produces better results and on and on.
 
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