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Post by Donkey on Apr 21, 2010 15:50:46 GMT -8
Thanks Donkey and Peter for your answers. So, if I am understanding this correctly, I could have a single bell with an oven on the top of the first bell? Would this oven actually be part of the first bell with nothing to separate the bell from the oven...(wouldn't this be considered a black oven), or would it work to have it a separate chamber....separated by a ceiling/floor of firebrick. Would there also have to be air circulating around the top and sides of the oven in order to heat it up sufficiently? A "black oven" is one in which the fire, or exhaust (in this case) circulates THROUGH the oven itself, in a "white oven" it does not. With a "black oven" you risk smoke in your home when you open the door. If you build a "white oven" (which I recommend) you will need to build a tight oven box, where the heat will circulate all around it. I'd put it inside the first bell as it's hotter there. Essentially, the second bell (or more) is for harvesting any heat that the first bell has let through.. The first bell will run hotter, the second (and subsequent bells.) cooler. Using more bells tends to harvest more heat over-all, as heat that leaves the first will move to the second, heating it and so on. It's a good idea to build at least one, if not more, outside first. Work it out outside where it's safer, then build 'em inside. |
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Post by zoidberg on Jan 22, 2011 1:01:33 GMT -8
... I'll have to add that I am using a feature which is called a "trip wire" at the sides leading to the bottom. The function is to disturb any laminar streaming. These factors do contribute greatly to the vigorous and much needed turbulence which is taking place in there. That is to say, that's what I do think. Peter, Would you post more detail about the "trip wire" feature ? It doesn't seem to be drawn in the Sketchup model, and I am curious about it. Superb work, by the way. Thanks very much for sharing your valuable findings. May I ask if there have been any improvements to the design since the last posts ? Thanks a lot. |
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Post by peterberg on Jan 23, 2011 13:36:27 GMT -8
Would you post more detail about the "trip wire" feature ? It doesn't seem to be drawn in the Sketchup model, and I am curious about it. The trip wire phenomenon was discovered by Ludwig Prandtl en.wikipedia.org/wiki/Ludwig_Prandtl in 1904. It's all about boundary layers and drag factors and why golf balls have dimples and tennis balls a stiff fluffy outer skin. It is digestible described for mere mortals like me in this article: ecomodder.com/forum/showthread.php/crash-course-aerodynamics-introduction-turbulence-1132.html Actually, I've used it as a mini turbulence generator. Inside the rocket syphon, I've tried to make the sharp corners smooth by introducing a 45 degree angle. In all three corners, which killed part of the good results I had until then. Not to mention the rocket sound, which disappeared for the best part. By making a trip wire in the refractory casting of the syphon, the excellent results were back again.  This picture do show one of the pieces of one experiment, with the 45 degree angle and the zig-zag trip wire. I couldn't hear nor see a difference using these parts, only by measuring the gas stream the effect could be proven. And here another photo:  The wire itself is about 1/8 of an inch high and wide. The function is to disturb any laminar flow running over that particular area. Thank you! Yesterday I was visiting a young man in Belgium, he did built a stove based on the drawings. There were some discrepancies with the drawings, and I've discovered an error. Not a major one, but important enough to edit the drawings. Not ready yet, it will be completed in the coming week I'd think. Maybe it'll be a complete new implementation, that will take some more time. |
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Post by zoidberg on Jan 25, 2011 9:43:25 GMT -8
Good explanation. Me being a mere hobbit rather than mortal, I think I get the idea  Inside the rocket syphon, I've tried to make the sharp corners smooth by introducing a 45 degree angle. In all three corners, which killed part of the good results I had until then. Not to mention the rocket sound, which disappeared for the best part. By making a trip wire in the refractory casting of the syphon, the excellent results were back again. You decided not to get the square corners back, I guess soot would stick in them, or something in that line ? Yesterday I was visiting a young man in Belgium, he did built a stove based on the drawings. There were some discrepancies with the drawings, and I've discovered an error. Not a major one, but important enough to edit the drawings. Not ready yet, it will be completed in the coming week I'd think. Maybe it'll be a complete new implementation, that will take some more time. The corners in the syphon maybe ? I noticed in the Sketchup model they are square. I am eager to see the new iteration, keep up the good work! |
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Post by peterberg on Jan 26, 2011 7:51:13 GMT -8
You decided not to get the square corners back, I guess soot would stick in them, or something in that line? The goal happened to be something else... The roaring sound of rocket stoves is a very pleasant extra in my opinion, but at a cost. It will inevitably take some energy from the gas stream. In other words: when there's a pulsating sound of some sort, there will be friction. And yes, soot will stick in the corners, unless the whole contraption is hot as hell... To reduce both effects, I've been smoothing the corners. The loss of the roar seemed to coincide with the loss of efficiency. One possible cause could be laminar flow i.e. an undisturbed boundary layer along the walls of the syphon. To combat that, I've utilized a trip wire with, that has to be said, good results. Much to my own surprise, by the way. A large percentage of my conclusions has been proven terribly wrong in the past. To extend the idea of laminar flow, in my opinion there shouldn't be a smooth surface inside the tubes of the rocket mass bench. In order to extract as much heat as possible from the hot gas stream. Just my two pennies worth. ;D See the explanation above. Too difficult to implement in firebrick, and the turbulence is much needed for mixing of the unburnt gases and fresh air. |
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Post by zoidberg on Jan 26, 2011 8:56:13 GMT -8
Ah ok. I thought the "rocket" noise was an inescapable feature of these stoves. See the explanation above. Too difficult to implement in firebrick, and the turbulence is much needed for mixing of the unburnt gases and fresh air. Not sure if *I* got my question right, I meant asking what the error in the SketchUp models was. Well, I am just too curious but I can wait for the new model to see myself |
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Post by peterberg on Jan 27, 2011 3:46:13 GMT -8
I'm sorry, understanding the finer nuances of the English language isn't easy sometimes.
The error was in the air ducts, they should start right behind the inlet valve. But the drawing showed them about 2.5 inch to the back. In that way, the intake volume of fresh air is a bit smaller than it should be.
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Post by zoidberg on Jan 29, 2011 2:25:09 GMT -8
I'm sorry, understanding the finer nuances of the English language isn't easy sometimes. You tell me! The error was in the air ducts, they should start right behind the inlet valve. But the drawing showed them about 2.5 inch to the back. In that way, the intake volume of fresh air is a bit smaller than it should be. Ah yes, I guess they should look more like they do in RB-PD5.skp, am I right ? How that change would have an effect on air intake ? While we are on it: I believe you stated somewhere in this thread that distance from top of the riser to top of the bell should be 1 metre or more. My guess is that the resultant volume would facilitate adecuate expansion and cooling of the gases coming out of the riser, then ensuring adecuate downdraft. Am I on the right track ? I am sorry if I am a bit of a bore, with so many questions. I am quite the obsesive type and when I bump into something really fascinating like your design (and "rocket" devices in general) I cannot rest until my curiosity is satisfied.  |
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Post by peterberg on Jan 29, 2011 7:40:23 GMT -8
Ah yes, I guess they should look more like they do in RB-PD5.skp, am I right ? How that change would have an effect on air intake? You are right about the drawing without the door frame. Also: as in RB-PD4.skp. With the ducts further back from the door, the front air will get too much emphasis. The one meter away from the heat riser end is to facilitate a top lid of non-refractory materials. Like concrete pavers without rebar in it. Closer to the riser is possible with refractory materials only, like firebrick or refractory castable. This is just the explanation: you are on the right track concerning the rest. |
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Post by peterberg on Jan 30, 2011 8:52:23 GMT -8
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hpmer
Full Member
Posts: 240
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Post by hpmer on Jan 30, 2011 10:00:36 GMT -8
Peterberg,
Any magic formulas or ratios as to burn tunnel vs bells? Meaning is there any ideal relationship in relative volumes that you've found?
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Post by peterberg on Jan 31, 2011 8:19:05 GMT -8
Very tricky to use magic formulas, since there isn't such a thing as magic or witchcraft...
But yes, there are some rules of thumb. It all boils down to open space inside the bells. More open space, more wall to transfer heat into.
I am not sure about whether up- and downscaling will be linear, but this are the results. With a chimney exit of 6 inches, the syphon can be as large as 4" x 8". A single bell where the syphon is dumping its heat is large enough with a volume of 625 liter. A double bell with a volume of 425 liter is large enough when served by the same firebox and syphon. A triple bell with a volume of 300 liter is large enough again.
Grosso modo, when moving from a single bell to a double variant you can subtract about 30% of the volume. When using a triple bell, subtract 30% again. All three variants had the same efficiency. The smaller ones where not as heavy so the capacity to store the heat was smaller also.
Very interesting stuff I would say.
Now the volumes in North American terms: single bell 22.1 cubic feet, double bell 15 cu ft, triple bell 10.6 cu ft.
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Post by zoidberg on Feb 3, 2011 3:56:01 GMT -8
... Grosso modo, when moving from a single bell to a double variant you can subtract about 30% of the volume. When using a triple bell, subtract 30% again. All three variants had the same efficiency. The smaller ones where not as heavy so the capacity to store the heat was smaller also. I feel like I am asking a silly question but, where does the extra heat (which is not stored) go ? Up the chimney ? Or is it radiated through the room (so room temperature is higher) ? Very interesting stuff I would say. Indeed! |
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Post by peterberg on Feb 4, 2011 2:16:35 GMT -8
I feel like I am asking a silly question but, where does the extra heat (which is not stored) go ? Up the chimney? Or is it radiated through the room (so room temperature is higher? The less material (in my case brick) is used in the bell system, the sooner the chimney stack temperature is rising. In other words: the system sooner will be "full". Not within one fuel charge starting from cold, by the way. The triple bell system is smaller with less mass compared to the double bell system. The ability to absorb heat is the same up to a certain point. After that, the stack temp of the triple will slowly rise. The double version will keep for considerable time the stack temp lower. I very much hope this is clear to the reader. |
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Post by zoidberg on Feb 4, 2011 2:36:23 GMT -8
Perfectly clear, many thanks.
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