|
|
Post by mintcake on Nov 14, 2011 13:31:57 GMT -8
Dear Peter and all, I'd really appreciate comments on this tweak to Peter's design 6. I've done 3 changes: 1. It struck me that the hottest part of bell 1 was separated from the coldest part of bell2, not to mention the bottom of bell 2's chimney outlet by a single slab of concrete, and I thought that making that 2 slabs with some insulation would stop bell 1 loosing heat straight to the chimney. (Or was that deliberate, Peter?) 2. I then had a thought that if the space between bell 1 and 2 were open, then there'd be more heat transfer to the room, for warming the room quicker. The open "shelf" could be arranged with (insulated?) doors/side panels so that you could decide if heat storage or quick warm-up is what you want. I think I've seen that somewhere, but can't remember where. 3. I also moved the riser from closer to the front and turned it round. This means that the side clean-out should be able to access the whole floor of the first bell, I hope. Any thoughts? David Attachments:
|
|
|
|
Post by peterberg on Nov 15, 2011 4:51:47 GMT -8
Dear Peter and all, I'd really appreciate comments on this tweak to Peter's design 6. I've done 3 changes: 1. Yes, the stove is losing some heat straight to the chimney. Not deliberately done this way, just to keep the construction simple. The whole bell principle isn't losing much heat, even with the inlet and outlet open. 2. The open space between bell 1 and 2 could be a white oven as well. The top of the first bell will get awfully hot. 3. I'd think you mean the 1st upstream channel. Could be done this way, but you'll loose some heat to the second bell. That is because the syphon is ending in the top of the bell instead of at the bottom, which will kill the stratifying effect of a bell. My goal was to place the upstream channel in such a way that the end of the syphon couldn't "see" the exit opening. In effect, as far away from each other as possible. |
|
|
|
Post by jgreen on Nov 30, 2011 11:42:37 GMT -8
Hi. I have a few questions about something I read in the middle of this thread. I apologize if this has previously been addressed somewhere -- I did recently read all pages of both threads regarding the rocket bell project but may have overlooked a post that related to this. The bell concept is entirely new to me and very intriguing. My questions pertain to the sizing of bells, particularly in a stove with multiple bells, as you articulated here: 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. 1. For the sake of my own clarity -- are the quoted volumes referring to the cumulative volume of all the bells involved, or are they referring to only one of the bells? I was assuming that it referred to the total volume of bells in the system. 2. Follow up to the previous question -- If my assumption is correct, then are there any guidelines as to how the cumulative volume should be distributed amongst the bells? I'm asking for some kind of generalization, as I could just look at your sketch-up model to pickup the ratio of volumes for that particular stove. Or do you consider the distribution of volumes in your sketch-up model to be a good rule-of-thumb? (In that case, I can go do the calculations myself) 3. I started typing a third question, but then realized that it was too long and probably beyond the intended scope of this thread (though very related). So, I plan to start a different thread with that question as soon as I can finish writing it up. Thanks, -Jay |
|
|
|
Post by peterberg on Nov 30, 2011 13:00:47 GMT -8
jay, 1. The quoted volumes do refer to the total of the bells. I know, it do sound like a sort of magic. But this is what all the accumulated data suggest, I've been hesitating to write it down because of its complexity. It has something to do with the total surface area of all the internal walls, floor and ceiling of the bells. One large bell do hold a certain volume versus surface area of the walls etc. Two bells which are representing the same wall area as the single can be smaller by combined volume. A triple bell system with the same wall area as the single will be automatically smaller again. I have to add, this is what I've found with the data of the smallest system. That one could only held about 4 kg (8.8 pounds) of soft wood in a single batch. The up-scaled version could contain more or less double of that. 2. Is a good question. The majority of the experiments were done with a gradually stepping down of the bells further downstream. But, lately a man in the Netherlands has been building a double bell system with a comparatively small first bell and a very large second bell. The end temp of that construction happened to be so low he had to make a small permanent shortcut in order to have the end temp risen above 136 F (58 C). By the way, it was me who made the drawing and I think I've made a miscalculation. ;D
|
|
|
|
Post by jgreen on Nov 30, 2011 13:20:53 GMT -8
Thanks for the prompt response peterberg. Two thoughts in response:
1. Interesting that you bring up the relationship between total interior surface area and interior volume. I have been contemplating this from a theoretical, "pen and paper" perspective and it is a big part of my third question (to be a separate post when I finish clarifying my thoughts). I'm still struggling with comprehending all the factors involved (and their relative impact on the thermal behavior of the stove).
For my clarity again: It seems to me that you're saying the total volume figures were a function of the combined (interior) surface area of the bells. Meaning, you found a surface area which works for the system and the total volume of the bells is the result of sizing each bell such that the surface area requirement are met. If I'm understanding correctly: is this to say that the cumulative interior surface area of the system is the more critical factor?
2. What is meant by "one could only held about 4 kg (8.8 pounds) of soft wood in a single batch." -- I'm interpreting this to mean that after 4 kg of wood fuel, the thermal mass of the stove is at "maximum" heat storage capacity and will not store any more thermal energy. If I'm understanding correctly, would this be more related to the total mass of the stove material than the surface area/volume of the bells? [i'm thinking that if two stoves had the same thermal mass but different sized bells, they would merely take a different amount of time to reach their maximum heat storage capacity]
I'm finding the conceptual aspects of this part of the stove to be very interesting! Thanks again.
|
|
|
|
Post by peterberg on Dec 1, 2011 10:02:43 GMT -8
If I'm understanding correctly: is this to say that the cumulative interior surface area of the system is the more critical factor? Yes, it is. 2. What is meant by "one could only held about 4 kg (8.8 pounds) of soft wood in a single batch." -- I'm interpreting this to mean that after 4 kg of wood fuel, the thermal mass of the stove is at "maximum" heat storage capacity and will not store any more thermal energy. No, you are wrong this time. The firebox was very small, couldn't contain more. So, with a larger firebox the bells need to be larger as well, in order to absorb the greater amount of heat. Heat retention is a different story, this can be prolonged by adding more weight, i.e. more mass in the walls of the stove. |
|
|
|
Post by Donkey on Dec 1, 2011 17:29:47 GMT -8
Interesting, huh??
When you look up the physics (let's see if I understand this myself) you find the description is all about the surface area.. The rule is basically this: when a fluid flows from a smaller, confined pipe or space into a larger volume, (when the volume change is big enough) it sets up a turbulent condition where the particles will crawl all 'round the container. Each particle of stuff will (theoretically, MUST) touch EVERY part of the larger volume's surface area BEFORE going out (the given exit) again.
Remember, surface area is ABSOLUTELY key to heat transfer. Heat moves best when it moves through direct conduction, molecules bumping into molecules.. With laminar flow conditions (where all the molecules are going smoothly in the same direction) inside a pipe, you can have a drastic temperature difference between the center and the edges of the flow. Laminar flow conditions inside a smooth pipe are great for moving stuff through, it keeps friction low but it's terrible for transferring heat. Turbulent conditions, on the other hand, transfer heat very well. Hot molecules are constantly being mixed out to the edges where they can transfer their heat to the walls. 'Course, turbulence inside a pipe tends to creates friction which can slow or even stop up the works completely.
Bells create a highly turbulent condition without (most of) the attending friction problem. They provide ample surface area for heat transfer, etc, etc, etc.
What's beyond me though, is why placing a smaller bell first in the lineup would transfer heat better than the reverse..
Peter, do you know of anyone who has checked that? Have any experiments been done to compare what would happen if a larger chamber was used first or a smaller one (all else being equal)?
|
|
|
|
Post by jgreen on Dec 1, 2011 20:15:23 GMT -8
Re: Donkey- Completely fascinating is more like it -- the more I think about the physics (and math) that are at play in a stove the more I want to learn. Questions: Regarding: The rule is basically this: when a fluid flows from a smaller, confined pipe or space into a larger volume, (when the volume change is big enough) it sets up a turbulent condition where the particles will crawl all 'round the container. Each particle of stuff will (theoretically, MUST) touch EVERY part of the larger volume's surface area BEFORE going out (the given exit) again. 1. Do you know the term for this phenomenon (ie. is there a physical property or law that describes this somewhere?) 2. Interestingly enough, after reading your description I would think that this phenomenon seems to have more to do with change in volume than change in surface area (though I completely understand how surface area is the more critical factor in actual heat transfer). If you think about it, its the change of volume (and I'm assuming its actually the accompanying change in pressure) that sets up the turbulence. Though to be honest, this is the opposite of how I would intuitively think it would work. I would expect a corresponding increase in volume (decrease in pressure) to result in less contact with the sides. The contrary would be, if the volume were decreased, the pressure would increase and that seems like it would increase contact with the walls of the bell. Though I do understand that these things work in strange and unexpected ways  As a side note: I realized today that an assumption I had always made about the volume and surface area relationship of a rectangular solid was incorrect: I always thought that for a box with a given surface area, there was only one possible volume. It turns out that you can have at least two boxes with the same surface area and different volumes (perhaps this is obvious to other people but it was news to me). Anyhow -- it made me rethink a few ideas I had about the relationship of a bell's dimension to its surface area and volume. What's beyond me though, is why placing a smaller bell first in the lineup would transfer heat better than the reverse.. 3. If the phenomenon you mentioned is correct--that a fluid flowing into a larger volume results in a high degree of contact (and therefore heat transfer) with the greater volume's surface area--then the above quote makes sense to me with this logic: placing a smaller bell first which opens into a larger bell, sets up the turbulence that allows for a great degree of contact with the larger surface area of the second bell. If you were to put the larger bell first, then the turbulence effect is not going to happen during the transition between the two bells (unless the phenomenon occurs when the volume is reduced as well, which I said before is more intuitive to my brain than how it was stated). Anyhow, thanks for the food for thought. I would love to do some experiments with tracking heat distribution (and absorption) in bells with different combinations of surface area, volume and "shape" (proportions of the dimensions) with the intention of developing some "rules of thumb" about these relationships. |
|
|
|
Post by Donkey on Dec 2, 2011 11:19:07 GMT -8
1. Do you know the term for this phenomenon (ie. is there a physical property or law that describes this somewhere?) Oy.. For me, that one falls under the heading of "random bits of crap rattling around in the junk drawer in the back of the head", until someone like Peter comes around and says something clever, suddenly that random bit fits (somewhere) and can be dusted off and applied.. I went looking last night (when posting above) for the technical info, probably used an improper search term and ended up in a fluid dynamics surfing session where I acquired more bits of random junk but didn't find what I was looking for..  Sorry, can't remember. It has something to do with turbulence and chaos theory (I think). Yeah, I thought of that.. Except, the exhaust typically flows from the first bell into a pipe or channel to the second bell and so on. Each bell needs to be fed from the bottom and exhaust to the bottom and the flow will always pass through some connecting passageway. It's not like we have the one bell opening directly into the other. I do believe that the pipe or channel between the two bells tends to be what I've been calling (for lack of a better term) "system size". That would be totally FAR OUT.. Do it. But first, it makes sense to try to grok what Kuznetsov has to say about it.. |
|
|
|
Post by peterberg on Dec 3, 2011 3:52:00 GMT -8
What's beyond me though, is why placing a smaller bell first in the lineup would transfer heat better than the reverse.. Peter, do you know of anyone who has checked that? Have any experiments been done to compare what would happen if a larger chamber was used first or a smaller one (all else being equal)? No, I don't know of anyone who's sorted out the differences, if there are any. By the way, there's an important difference between any first bell and a second in the way we are working with the phenomenon. Inside the first bell there's the combustion unit, exhausting into the bell close to the ceiling of it. Each following bell is fed and exhausted more or less at floor level, that's the main definition of a bell, isn't it? So, the first bell doesn't qualify as a real bell at all, only a contra-flow compartment. Which is serving as a way to capture the high heat, and to lead the gas stream into a real bell. In short, a bell allows stratification, and the first one doesn't because of the ongoing arrival of hot gases in the top of it during the burn. |
|
|
|
Post by Donkey on Dec 3, 2011 18:50:15 GMT -8
I suppose you could flow through a down pipe, into the bottom of the first bell below..
Or make the first bell above the top of the heat riser (flow enters from below), then place the second bell below the first..
What would be the practical considerations for these approaches?
|
|
|
|
Post by peterberg on Dec 4, 2011 2:09:15 GMT -8
I suppose you could flow through a down pipe, into the bottom of the first bell below.. You'll run the risk of maintaining not enough temperature difference between the heat riser and the downdraft channel. Which would stall the stove, as described in " the drive behind the drive" and as rectifier found out. Or make the first bell above the top of the heat riser (flow enters from below), then place the second bell below the first.. Personally, I would think this could be done... But it's difficult to have it done the proper way. Critical, I would say. The first bell will be very hot, but the drive of the heat riser have to be strong enough to push the gas stream all the way to the floor again. On the other hand, the riser could simply be tall enough in order to end straight into the first bell. The height of the bell wouldn't matter so much then, and the down-stream channel from the first to the second bell would be plus-minus as long as the riser. The riser would be very much hotter than the down channel so this construction do stand a better chance of successful operation. In effect, to translate it back into RMH terms, the barrel would be very tall and made of brick. The bench would be another compact cupboard-like brick construction. My first experiments proved the rocket itself could be ran in a tall single bell, the space above the riser being enormous. |
|
|
|
Post by jseasky on Jan 17, 2012 11:56:59 GMT -8
To peterberg.
I have learned so much from your comment and want to make your design.
You indicate the syphon openning 100mmx260mm in sketchUp.
But in South Korea, the brick size is only 115x230x65.
So, it's expected so many cutting to make stove.
How about the syphon openning 115x230 instead of 100x260?
It seems critical. so I hesitate to make stove immidiately.
And if I want to make 7 inch system, what the figure of syphon openning and combustion chamber.
Thanks your good teaching.
|
|
|
|
Post by kwillets on Jan 17, 2012 23:18:04 GMT -8
jseasky, are you familiar with Ondol construction? I have some questions about traditional firebox design compared to peterberg's or the rocket tube.
(If you are, I will post them on a different thread. Also, I should mention that I've seen some of the designs on this thread being discussed on Korean blogs, but my vocabulary isn't good enough to get all the information.)
|
|
|
|
Post by peterberg on Jan 18, 2012 5:00:35 GMT -8
Hello jseasky, welcome to the boards.
Length and width of the opening is not critical. The only proviso is that the height has to be above a minimum, which is 90 mm. Your dimensions boils down to about the same cross sectional area and isn't below 90 mm so you'll be fine.
I do understand you want to make a slightly larger version. It's best to make the firebox a bit higher, 1/6 part of the original drawing. In that way, the other dimensions won't be altered.
My larger prototype has been running fine with a 100x200 mm syphon, as well as 100x300 mm. The smaller one did show a 3% better performance compared to the larger one. All in all, I'd think the larger firebox would be a match with the same 115x230 syphon.
The air-ducts in the drawing are 40x60x2 mm, you could opt for a larger one. Remember, the slit in the combustion chamber is half of the CSA of the duct, intake is half of the slit again. The first ratio is important, the other can be varied.
|
|
