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Post by peterberg on Dec 8, 2012 13:04:36 GMT -8
Max,
No, this wouldn't work, for the channel to work properly you have to have the overhang. By this way, the air channel is always open to the port, even when you place accidentally a piece of fuel underneath it.
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Post by satamax on Dec 8, 2012 22:34:18 GMT -8
Ok, thanks Peter. I had a chance to "make it right", but nope, not possible. You confirmed what i thought.
Another question. Since i have access to thoses 30x30cm flue elements which seem all right for the burn tunel/batch box. But overpower the 16cm diameter heat riser. Do you think going to 20cm would make the things right? Not on this one obviously. But it's an idea which has been turning in my head for a while.
Thanks again.
Max.
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Post by peterberg on Dec 9, 2012 6:35:43 GMT -8
Yes Max,
Narrowing the width of the firebox to 20 cm would solve the back-puffing problems. On the other hand, mounting a simple door or attaching the thing to an adequate chimney would solve it as well.
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Post by satamax on Dec 9, 2012 6:57:45 GMT -8
Peter, i've done the door. It stoped the smokeback. I've made the grid part of the door the same as the CSA of the heat riser. Ataching to a proper chimney will come soon. But i was thinking about future models ;D
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Post by pinhead on Dec 10, 2012 10:52:01 GMT -8
I've also got a question about the P-channel, specifically with regards to the design in your "ugly" prototype.
How long do you suppose the P-channel would last if it were made of, say, 1/8th inch mild steel?
But would it perform better if it were made out of some type of insulative material?
If it were integrally cast into the burn box it would have to be pretty thick... If it were, say, 2" thick -- in other words, a two-inch-thick overhang -- do you suppose it would perform the same function? I suppose this would have the same effect as moving the P-channel further into the burn tunnel.
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Post by pinhead on Dec 10, 2012 11:25:50 GMT -8
I modified your Sketchup file to show what I mean... Attachments:
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Post by peterberg on Dec 10, 2012 12:09:09 GMT -8
How long do you suppose the P-channel would last if it were made of, say, 1/8th inch mild steel? The p-channel as drawn, is air cooled. My prototype is ran about 80 times on the top of its nerves up till now. I've checked the channel last week, doesn't show a sign of burning out, no destruction, warping or spalling. Mine is made of mild steel, a little bit more than 1/16" thick. It could take any number of years to destroy the channel this way, maybe by rust in the summer? But would it perform better if it were made out of some type of insulative material? No, it won't. The vertical piece inside will get hot, warming the passing air this way. The horizontal part is getting warm as well later in the burn when the refractory is heating up. The channel won't have the same effect when further down in the tunnel, that's for sure. |
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Post by pinhead on Dec 10, 2012 12:59:12 GMT -8
In other words,
For the P-channel to work optimally, the air entering the throat of the burn tunnel through the P-channel needs to be preheated.
Correct?
BTW, is your skp model drawn to scale?
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Post by peterberg on Dec 10, 2012 13:29:27 GMT -8
Yes, that's correct.
And yes, my .skp model is to scale, in millimeters. The two barrels on top of each other do seem huge, but coupled to an adequate chimney this isn't enough. The thing could drive a tower of three of those drums, easily.
You are able to convert it to inches by selecting the whole model, press Ctrl-C, open a new drawing in inches and import it by Ctrl-V.
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Post by tumppiw on Dec 15, 2012 13:28:24 GMT -8
Thanks for an extremely interesting development thread on rocket heaters! I've been fiddling a bit with RMHs and stoves here in Finland and gotten quite strong comments from people developing Finnish masonry stoves. They disliked especially the downdraft without secondary air control present in the Ianto J-tube rockets. P-channel and the narrowing of the throat before the riser bring rockets towards the most modern masonry stoves in regard to air injection. In regard to that, I put a few photos about how the secondary air is currently introduced in masonry heaters. They are taken from this report So preheated air is injected from opposite sides of the firebox, with c.a. 6 steel pipes embedded in the walls. Twelve 16 mm diam. holes are used to direct the air about 10-20 cm above the wood layer. (There is some discussion between manufacturers on how high to put the holes. )The idea is to generate as much turbulence as possible and keep the gasses off the walls. If it would be applied to rockets, compared to the P-channel, this would move the secondary air feed a bit later in the downstream and introduce it in smaller jets. Or maybe the ramhorns do enough of mixing already? Peter, what do you think? (I could test it with our RMH, but I don't have anything to measure it with.) Attachments:
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Post by peterberg on Dec 16, 2012 1:06:51 GMT -8
So preheated air is injected from opposite sides of the firebox, with c.a. 6 steel pipes embedded in the walls. Twelve 16 mm diam. holes are used to direct the air about 10-20 cm above the wood layer. This do sound familiar to me, a prototype of Heikki Hyyttiainen's stove exist in the Netherlands, utilizing this system. Some time in the future I could be in the position to test it but don't hold your breath, everybody is guarding this as a valuable secret. I don't feel the same way, throwing the facts open to the public will set us free. If it would be applied to rockets, compared to the P-channel, this would move the secondary air feed a bit later in the downstream and introduce it in smaller jets. Or maybe the ramhorns do enough of mixing already? Peter, what do you think? It's a very technical solution to the same old problem. A rocket stove is depending on other features to do exactly that, especially when using a P-channel and trip wire. In the J-tube the combustion zone is confined to a small space, very close to the primary combustion zone. The volatiles, rising from the fuel, will be immediately burned. Farther away from the fuel the temperature is lower, so this will pose a problem in the first part of the burn at least. On top of that, we do want the injection of the secondary air just before the afterburner flame, to keep things close together. That's why the P-channel is so successful in the J-tube. Injecting this air later in the process won't do the same thing and won't bring any improvement, rather the contrary. Believe me, I've done it and been able to do the measurements. More or less the same effect do exist in a batch box rocket, the confined space, high temperatures and a lot of turbulence. The ram's horn pattern is the visible part of the violent mixing of the gases. |
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Post by tumppiw on Dec 19, 2012 12:21:36 GMT -8
So in the box version, the secondary air is injected quite close to the heat riser (before the narrow part). And in the J-type rocket the p-channel is in the beginning of the burn tunnel. What if you have a burn tunnel of say 50 cm? Is the optimum position still nearer to the fuel tube or the heat riser? What is surprising in the p-channel compared to Hyytiäisen (Hyytiäinen's) version, is that it seems to inject the air in the same direction as the flames go. Based on a presentation I saw by Hyytiäinen, he had measured particle pathways and mixing, and observed that even with a narrow throat, in a traditional fireplace the flow is pretty much laminar and it's difficult to get turbulent mixing between combustion gas and secondary air. That was the logic for experimenting an injection with small streams crosswise to the stream. It might be that the insulated rocket elbow is the key to turbulent mixing. Perhaps the secondary air from the P-channel stays separate on top of the stream until at the elbow it meets a hot combustion gas flow which has to go change directions and go through the (now preheated) fresh secondary air. It would be really interesting to do a clear plastic mock up of a rocket, set up a fan to give draft and then play around with smoke as the secondary air. (I stole another picture from the report for some visuals as an attachment.) If the secondary air stays as a separate layer before the rocket elbow, it is effectively pre-heated to very high temperatures. By the way, how much wood are you burning in a batch? If they are about 5 kg batches going in 30 minutes, your stove output is around 50 kW. That's over double compared to an 8 inch J-tube! And still you have excess air and the air intakes at pretty low setting. With fully open air intakes and a good gasifier, the system could probably heat a small village! Attachments:
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Post by peterberg on Dec 19, 2012 14:00:14 GMT -8
So in the box version, the secondary air is injected quite close to the heat riser (before the narrow part). And in the J-type rocket the p-channel is in the beginning of the burn tunnel. What if you have a burn tunnel of say 50 cm? Is the optimum position still nearer to the fuel tube or the heat riser? There's a short answer: directly after the feed tube. During working on this J-tube and gradually optimizing, it became apparent more and more of the burn happened closer to the feed tube. The p-channel on its own cleaned up the exhaust gases but there was still room for improvement. After adding the trip wire about 2" from the feed virtual all combustion were confined to the tunnel. The only exception being the stove into pulse mode, in that case the flames were shooting out the riser like a flame thrower. The fuel consumption rose by a factor of two, by the way. And yes, the air is injected in the same direction as the flames go. Difference as compared to Heikki's stoves is the flames are running horizontally which isn't a normal configuration for a wood stove. Because the flames tend to go strongly upwards, restraining that will create lots of turbulence. The only part of the tunnel where seem to be laminar flow was the ceiling, effectively countered by the trip wire. By the way, how much wood are you burning in a batch? If they are about 5 kg batches going in 30 minutes, your stove output is around 50 kW. That's over double compared to an 8 inch J-tube! And still you have excess air and the air intakes at pretty low setting. The batches in the batch box rocket were about 4 kg of soft wood at a time, practically gone in 40 minutes. Including the charcoal, about 15 minutes more. That's when starting a cold stove. Running a very hot system is another story, the fuel consumption can be as high as 4 kg in 35 minutes, charcoal and all. And yes, the air intakes are quite small as compared to the fuel consumption. |
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Post by satamax on Dec 19, 2012 22:22:58 GMT -8
Peter, Daft questions:
How many BTU's is 4kg of wood?
And have you burned a P chanel Yet? I wonder how long one would last in a horizontal rocket.
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Post by peterberg on Dec 20, 2012 1:36:32 GMT -8
How many BTU's is 4kg of wood? To stay at the conventional side, 4 kg/h would generate 20 kW/h at 100% efficiency. Of course we won't have such a high yield, so at 75% that would be 15 kW/h. The start situation is 4 kg in 55 minutes so that would be 16.36 kW/h. Converted to Btu/h that would be 51200. When awfully hot, usually after one complete run, this particular stove will consume 4 kg in 35 minutes. That would be 25.7 kW/h, converted to Btu/h it will come down to 87700. Of course, when overall efficiency is higher than 75% it will be more again. And have you burned a P chanel Yet? I wonder how long one would last in a horizontal rocket. Being curious, I've just checked. No sign of burning out, no flakes, no spalling. It do feel a bit rough inside the overhanging lip though, like a slightly corroded surface. As long as I don't plug up the entrance opening it will probably last forever. Corrosion outside heating season will do more harm I would think. |
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