J tube dimensions and the complete burn

Hello all.

For the last year I have been studying the book by Mr Evans, and just about every thread and you tube video available. The proportions of the feed, burn tunnel, and riser are not a problem - I have got my head around those.

However, at risk of appearing incredibly stupid and despite the research, I am unable to find, or have missed, an answer to this:

What ratio is the riser height (and therefore all other dimensions) to the cross sectional area of the burner tunnel?

Or:

Does the burning flame/gas/smoke length remain the same whatever volume (CSA of burn tunnel) of wood is burned?

For example:
Ianto says the riser should be twice the height of the burn tunnel length, but anything between 25 and 50 inches will work. But, a 4 inch burner tunnel will have a cross section of 16 sq in, a 6 inch = 36 sq in, an 8 inch = 64 sq in, a 10 inch =100 sq in. This would suggest to me that for complete combustion a 10 inch J tube would need to be considerably bigger in all dimensions than a 6 inch. Approximately 3 times as big? Probably not, but surely bigger.

If you take the Erica and Ernie plan for a 6 inch as being tried and tested dimension, the burn tunnel is 21 inch the riser 47 inch. Would it follow that a 4 inch (roughly half the CSA) work best with a burn tunnel of 10 inch and a riser of 25 inch, an 8 inch at say 36 inch and 84 inch?

Clearly this cannot be the case because I cannot find any RMH with those extremes of dimension.

Would it be safe to assume a state of complete combustion has not occurred if flame, rather than just hot exhaust products, are coming out of the top of the riser? Therefore the optimal dimension of the riser would be when the flame tip is level with the exit from the riser? This, to my simple mind, would mean the air to fuel ratio, and burn tunnel/riser dimensions, are correct because no secondary oxygen is being supplied (without the barrel fitted) to the flame exiting the riser and a state of complete combustion has occurred within the J tube. Am I right in believing this is the aim?
If so, this poses more questions. Does the resistance in the exhaust and barrel of the completed RMH hold back the burning gas and smoke inside the riser long enough to achieve a complete burn? Does this resistance increase when an incomplete burn (if it is) tries to draw oxygen back through the exhaust system to achieve a complete burn? Is this why some RMH systems fail with smoke back?

Or is none of the above relevant?

Thanks, in advance, for any comments. Please be kind I'm in Wales, UK, and this is my first post!

OK, my first post, I hope I'm doing this correctly. I have also, been searching for an answer to this question for a long time. I don't think anyone really knows. They all just use the size/dimensions that someone else posted. I too am looking for the CSA/Burn Tube/Feed Tube/Riser ratios. I assume it must be volume related, not length. I'm thinking the same as you. I want combustion that roars like a "rocket" and flame that just reaches the top, then I can cover it and the gases are now done burning at the top and starting to cool going down. I can't seem to get the ratio right to achieve the "rocket" sound. Hopefully, someone with experience in the ratios will see this post. There has to be a "perfect" ratio to achieve complete combustion and the "rocket" effect. Have you had any more luck with yours? I have started playing with a bunch of varied lengths of pipe to try to find a ratio that works. I'll let you know.

Thanks,
Bob

Bob Jones : Your definitive answer is just a couple of clicks away, Please go to rocketstoves.com to Download a PDF Copy $18.oo For the 3rd edition of Ianto
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This is a free endorsement ,I get nothing out of this except the time, and effort, and monies NOT spent in creating great 1st Builds for Permies members !

Think like fire, Flow like Gas, Don't be a Marshmallow! as always, your questions, and comments are solicited and Welcome ! PYRO - Logically BIG AL

Late Note : I do not know what happened to Paul Jones , and why he did not get help last Aug. While I was not available, this should have got an answer, perhaps it
was the scatter-gun approach that made dealing with a mountain of material too Daunting for his fellow members ! The Answers are there if you look for them ! A.L.

Bob Jones wrote:OK, my first post, I hope I'm doing this correctly. I have also, been searching for an answer to this question for a long time. I don't think anyone really knows. They all just use the size/dimensions that someone else posted. I too am looking for the CSA/Burn Tube/Feed Tube/Riser ratios. I assume it must be volume related, not length. I'm thinking the same as you. I want combustion that roars like a "rocket" and flame that just reaches the top, then I can cover it and the gases are now done burning at the top and starting to cool going down. I can't seem to get the ratio right to achieve the "rocket" sound. Hopefully, someone with experience in the ratios will see this post. There has to be a "perfect" ratio to achieve complete combustion and the "rocket" effect. Have you had any more luck with yours? I have started playing with a bunch of varied lengths of pipe to try to find a ratio that works. I'll let you know.

Thanks,
Bob

Personally, I doubt there is a "perfect" ratio. Under laboratory conditions, yes. In practical use, no. There are too many variable: wind, moisture content of the wood, building-draft (as opposed to chimney draft, which is also a variable), and on and on.

Therefore, the system that runs well I would expect to be the one that is built more robustly than ideal or perfect ratios. The real world is sloppy, and one must be able to overcome unexpected conditions, and to also build for the 20-year or 100-year storm kind of thinking.

Although, I would agree that knowing what the ideal laboratory ratios are would be extremely useful information. But they'd have to be beefed up. An example is load bearing in structures. An architect or engineer will calculate the load and identify the proper beam to take that load. But that is not the one that is specified in the real-world build: for example, a fudge factor of 2 may be introduced, so that the real-world build can theoretically take twice the planned load before failing.

So too with our RMHs. Of course, that's just my opinion.