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Post by Vortex on May 20, 2019 9:07:15 GMT -8
Hi Vortex (Trevor?), first of all I'd like to thank you and everyone else who shares their ideas and findings on proboards. Hi, Yes it's Trev. My present setup has no supplementary air supply into the afterburner. I tried many different versions but have never found any benefit from them. I have 3 air supplies into the firebox; Normal primary air supply. A two position door catch which leaves a 5/64" / 2mm gap around the edge of the door, and a 1.5" round hole in the ashbox door which allows air up through the ash trap. The size of the gap under the cooktop is 3" high by 11" wide. Narrower gives better heat exchange to the hotplate but creates more resistance to flow. Don't go less than 1 CSA. My shelf is made of a of 1" ceramic fiber board with 1" CF blanket on top. It sits on a ledge either side and rear, so it can be slid back and forth to tune it to the mass. Yes. A wide rectangle of at least 1 CSA. Mine is 3" x 9" The configuration of channels in the mass of my new stove was designed to try and maximize the heat exchange in a very small dense mass. Bells are great but don't seem to work well below a certain size as there isn't enough room for the gases to stratify. This sketch shows the gas flow in my new stove (ignore the right side that was the pre-Aryan setup). There are 2 layers of storage heater bricks, the gases flow across the top, down, back across between the 2 layers, down again and back across underneath the bottom layer, then into the bottom of the chimney. The new one works well but has quite high resistance to flow, so it needs a bypass and a proper insulated straight chimney.  I look forward to seeing your stove thread. |
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Post by fishalive12345 on May 26, 2019 3:06:35 GMT -8
Thanks for your prompt and detailed answer. I certainly have a much better understanding of your system now. I also realised that the answers to some of my questions had already appeared in your thread.
You mentioned the minimum size requirement for bells. I was in fact thinking about using a bell as the heat storage element. Is this issue discussed anywhere here?
I'm trying to get my ideas about the stove that I plan to build into some kind of communicable form before I start my thread or, then again, maybe I'm just procrastinating.
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Post by Vortex on May 27, 2019 0:30:34 GMT -8
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Post by marcios on Jun 20, 2019 10:08:21 GMT -8
Hi Trev,
Besides log position, you've thought other way to minimize the problem of port-fire distance variation,
as to lower the firebox roof or something else?
Do you think this factor doesn't affect efficiency so much?
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Post by Vortex on Jun 21, 2019 1:18:57 GMT -8
Hi Marcios, Lowering the firebox roof would make the firebox smaller, unless you widened or lengthened it, both of which would make that part farther from the port than they already are. The present size seems to be about the optimum. If you stand the logs verticaly leaning against the rear wall of the firebox it's actually quite easy to keep a farely constant distance between the top of the fuel and the port right up until the start of the coaling phase. Look at the last few seconds of this video and you'll see what I mean: www.vftshop.com/Video/32degrees.mp4 |
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Post by marcios on Jun 21, 2019 16:10:46 GMT -8
Very nice video.
This burn doesn't seem to have been fully charged, or the logs at the back always end up like this last?
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Post by Vortex on Jun 22, 2019 2:06:29 GMT -8
That was a full load. if you stand the logs vertically the back ones are always left like that at the end.
My loading technique is to stand 3 large logs vertically against the back wall, then put one split horizontally against their bottom (to stop them falling over as the firebox floor slopes toward the ash trap) then I lean medium sized pieces standing vertically against the back ones, gradually getting smaller in size towards kindling and paper at the front center.
I light the paper through the primary air inlet. The fire burns up the front quickly igniting the whole top surface, which gradually burns down through the fuel towards the bottom, but the largest ones against the back wall are always the last remaining because of their size and being against a wall.
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Post by marcios on Jun 23, 2019 8:17:08 GMT -8
My question regarding lower roof sugestion was about a way to maintain heatest zone by reducing air stream above fuel towards the port, if, as supposedly, downdraft gasifier efficiency come from (considering "DSR2 similar to wood gasifier?" discussion), as well as for lighthen the problem of primary and secondary air supply control throughout the cycle.
Nevertheless, your loading technique shows a very practical and balancing solution for that core model, so we can continuously move forward in refinement by air inlet configurations, dynamic controlers and lining materials!
What port size do you recommend for a 12 cm (internal diameter) 5m isolated chimney ?
Bypass to could start?
Any backsmoke (or it smell) occasionally?
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Post by patamos on Jun 23, 2019 9:58:13 GMT -8
Having querried more than a few old time heater builders... I get the sense that fuel quality (eg well seasoned and stored) and how we load it, are really the biggest factors in day to day efficiency. No matter how well designed a fire box is, if the operator doesn't have their act together, it will not perform well. For example, a common masonry fire box with under air supply is less efficient in testing than the newer highly touted eco-box. But it generates a stronger draft and initial burn rate than the eco-box. So, if the wood is damp the eco-box will have trouble ramping up and will not do as well. There are a lot of people out there burning green/wet wood... so... take your pick. The potential for changes in fuel load height location can be a factor in any heater with a top-exit throat. Trev solves this by careful loading as described above. Other designs solve it (somewhat) by having a smaller, longer lower firebox. Matt Walker's side exit with a split riser probably solves the problem better than anyone. But that is a small firebox that may require multiple reloads for larger heating applications. Then again, it offers a great cook top and oven... The way i see it, Trev is optimizing the middle ground between masonry heaters and rockets. Mid size fire box. 'Trick re-burn' features downstream that make it burn quite clean with good fuel and ideal loading. BUt also the kind of thing you could just throw a fairly large volume of variable quality fuel into and it will do well enough. my 2c  |
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Post by Vortex on Jun 23, 2019 10:21:09 GMT -8
My question regarding lower roof sugestion was about a way to maintain heatest zone by reducing air stream above fuel towards the port, if, as supposedly, downdraft gasifier efficiency come from (considering "DSR2 similar to wood gasifier?" discussion), as well as for lighthen the problem of primary and secondary air supply control throughout the cycle. Nevertheless, your loading technique shows a very practical and balancing solution for that core model, so we can continuously move forward in refinement by air inlet configurations, dynamic controlers and lining materials! What port size do you recommend for a 12 cm (internal diameter) 5m isolated chimney ? Bypass to could start? Any backsmoke (or it smell) occasionally? It has no secondary air supply to the afterburner, so the air has to come via the firebox. The hottest lightest gases should theoretically rise to the top and get mixed in the afterburner. Around a 50% CSA port seems to be the best size. Mine is a little under now I have the bow on the front inside edge of the port. I've yet to try scaling this core up or down, but a 120mm system is a scale factor of 0.8, so your port should be 140mm x 40mm. I highly recommend a bypass. I don't get any smoke-backs or smell, unless I try reloading mid burn which I almost never do. |
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Post by marcios on Jun 23, 2019 15:03:46 GMT -8
Thanks Trev, starting the odyssey as soon as possible!
What would you think about scaling 0.8 except 305x305mm firebox?
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Post by Vortex on Jun 24, 2019 2:41:06 GMT -8
I would think you'd have to scale the firebox down as well. Otherwise wouldn't the burn rate have to be slowed down, which would lose the efficiency?
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Post by marcios on Jun 26, 2019 4:38:11 GMT -8
Keeping the same flow (velocity through port), the lower the burning rate, the higher production of wood gases (question a) so the efficiency would increase (question b) (up to a certain point and with enough oxygen for the secondary combustion of course). Does that make sense?  |
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Post by Vortex on Jun 27, 2019 1:48:10 GMT -8
I don't know, I've never tried scaling down the stove but not the firebox. The CSA of a 120mm pipe is only 0.64 the CSA of a 150mm pipe. So if you have the same size firebox, for the fire to burn at a similar rate the gas velocity will have to increase proportionately, but the surface area of the inside of the pipe in contact with the gases is proportionately grater as well, so there is a lot more resistance to gas flow.
Rocket stoves don't work well below a certain gas velocity. I think what you're proposing to build would be like trying to light my stove with no bypass, when the mass is cold. The fire would be too slow and sluggish to ever heat the afterburner enough to establish the conditions for proper secondary combustion. Think of trying to light a kerosene pressure stove without much pressure and without lighting the pre-heater - will it burn efficiently?
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Post by marcios on Jun 28, 2019 11:57:05 GMT -8
A thought about how to see things more clearly:
As a divide to conquer strategy, we could consider the configuration from the exit port of the secondary combustion chamber to the end of chimney as the "second part problem", in which severals things can be considered to obtain the required back pressure with the maximum heat transfer/recovery, including a larger pipe if necessary.
That way we also can view a given port&secondary chamber as a "power system" with specific behavior in relation to gas velocity, composition and temperature from "fuel system" (from firebox and eventual secondary air).
So, for this functional decomposition, the "first part problem" would be how firebox could better feed the power system for overall performance, assuming that the gas velocity is kept within the best range (by air supply and backpressure controlers).
Then, yes, how to reach and maintain high temperature in secondary chamber with less overall burn rate, if the fire would be too slow and sluggish in the firebox?
Some ideas:
1- loading and firing :
-secondary burn as quickly as possible with little sizer pieces on top surface up to the port
- light fire at the front top and mantain top to down burn as possible, maybe with that 3 logs vertically against the back wall but others pieces horizontally and gradually larger to bottom (and maybe with upper top air inlet open after starting stage?); need try variations
2- high reflectivity ceiling material (some especial ceramic or coat?)
3- when secondary combustion is stable, maybe the gas temperature from firebox becomes a factor of lesser influence than the wood gas volume proper mixed with preheated air supply?
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