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  1. #1

    Default Capillary Hoop Stove discussion

    First of all my apologies to "donthaveoneyet" for hijacking his "Cat Can Stove - Questions" thread here: http://whiteblaze.net/forum/showthre...tove-Questions

    This is new thread to continue the discussion of the CHS (Capillary Hoop Stove) and constructions techniques

    Quote Originally Posted by BirdBrain View Post
    It is all about balance. I explained the math elsewhere. If I can find it, I will copy and paste it here. The short of it is that as the cross-sectional area of the ribs increases, the pressure in the hoop increases. This causes what TetKoba calls "bubble bomb" (things get lost in the translation). Vaporized fuel is forced out the bottom. The hot vapors cause increased thermal feedback. If you are seeing any bubbles in the bowl of your stove, you have too great a cross-sectional area in your ribs. This is where things get ugly. One might think that if you increase the number of ribs, you will also increase the amount of cross-sectional area of the ribs. It is actually the opposite. I will not belabor this point. I will revisit it if I can find my diagrams that illustrate this.

    The reason it is important to "squash" the walls (as TetKoba would say) is to ensure uniform ribs and predictable cross-sectional areas of the ribs. If you do not detail the ribs, you will end up with a crap shoot of pressure in each rib.

    The secondary hoop is a JSB thing. It provides a secondary buffer against the "bubble bomb" and allows for faster initial priming.

    The slanted ribs provide for smoother wicking. Contrary to the name, this is not a capillary stove. It is a metal wick. Think about the effort it takes to lift a heavy object straight up. Then imagine having a slant to push it up instead.

    These things are very marginal. I do not expect builders to follow my obsessive nature. It is not reasonable. I have built many of these things and tested for every variable many times. I sought to build a balanced stove that would have as little thermal feedback as possible. I have only succeeded 3 times. I have used one of those successes hundreds of times. It shows no change in performance. That one is the one shown in the video of the stove in my hand. I figure between that one I am using now and the 2 spares, I have a life time of stoves on hand. I stopped building months ago.

    On a related note, I sent OMO a less than perfect stove. He did not get one of my 3. I have mailed so many of these things out, I have lost track. One that I was fairly happy with is now in Australia.

    Now I am off to see if I can find my cross-sectional area illustrations.
    BB, thanks for your description. I am definitely seeing "bubble bomb" and excess thermal feedback in my stove, hence it gets too hot to hold. I followed your reasoning about reducing the capillary area with more channels, and think it makes sense. I will try to clean up, tighten my channels and see if that helps.

    As for the JSB secondary hoop, I could see how such a feature would help ensure more uniform pressures in each of the channels/wicks/tubes, however it is not clear to me where and how to integrate this feature into the stove. Is it just formed in the space between the bent tabs (making an inner ring) from the collet and the bottom/outer bowl? Its some sort of azimuthal additional ring to balance channel pressures, but where is it and how is it made?

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    Quote Originally Posted by aero-hiker View Post
    First of all my apologies to "donthaveoneyet" for hijacking his "Cat Can Stove - Questions" thread here: http://whiteblaze.net/forum/showthre...tove-Questions

    This is new thread to continue the discussion of the CHS (Capillary Hoop Stove) and constructions techniques



    BB, thanks for your description. I am definitely seeing "bubble bomb" and excess thermal feedback in my stove, hence it gets too hot to hold. I followed your reasoning about reducing the capillary area with more channels, and think it makes sense. I will try to clean up, tighten my channels and see if that helps.

    As for the JSB secondary hoop, I could see how such a feature would help ensure more uniform pressures in each of the channels/wicks/tubes, however it is not clear to me where and how to integrate this feature into the stove. Is it just formed in the space between the bent tabs (making an inner ring) from the collet and the bottom/outer bowl? Its some sort of azimuthal additional ring to balance channel pressures, but where is it and how is it made?
    I do not have many of my construction pictures left (computer crash). Here are a few from an eFREVO (my design) that might be helpful. The eFREVO is about as powerful of an eCHS stove as can be built. It suffers in efficiency though. To achieve balance, you need to regulate pressure in the hoop. That can be achieved through variables in the construction of ribs, jets, aperture, and hoop. The hoop is the hardest to modify. Therefore, it becomes the limiting factor. Everything is done to make the hoop happy. The slant of the ribs and the secondary hoop at the bottom are the least important. I like them for esthetics. They do serve a purpose, but it is very marginal. Anyways... here are some pictures. I use cuticle cutters and needle nose pliers to make the tabs. Lose the 95% ethanol. It can be used if you create a stove with a huge hoop and tiny aperture. Other than that, stick to methanol.

    ~
    IMG_20140430_191556_120.jpg

    ~


    I am yet to fully figure out Windows 10. It is a crap shoot how pictures get attached. They are there. Oh well.
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    Last edited by BirdBrain; 11-04-2015 at 13:32.
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    Quote Originally Posted by BirdBrain View Post
    ... Lose the 95% ethanol. It can be used if you create a stove with a huge hoop and tiny aperture. Other than that, stick to methanol.

    ...

    I am yet to fully figure out Windows 10. It is a crap shoot how pictures get attached. They are there. Oh well.
    Why do you suggest methanol over 95% ethanol (is yellow Heet methanol?) From what I understand the energy density of the 95% ethanol is higher, so you should be able to boil more water with less fuel... assuming the stove is equally efficient with both fuels... maybe this is the issue?

    Ah windows.... how I don't miss thee. (I use Mac for work reasons but much prefer Linux...)

    Thanks for clarifying about the second hoop. I think I can see what's going on now.

    I just tried to make my ribs smaller and more uniform... Most of them are small, but there are two or three that seem really large and don't want to get much smaller without buckling the crease. I think I might have to buy some more red bull and give it another shot. Now that the top is crimped over, and there's JB weld in it, I don't think I can really save my ribs.

    I too have my jets in a counter clockwise arrangement... or at least I tried. My thinking was that since I plan to use it mostly in the Northern Hemisphere, I might get a low order vortex stabilization or enhancement from the Coriolis effect. However, I would guess that the Rossby number is pretty big for a stove (relative to a weather system in geostrophic balance, for example) so in practice inertial forces will dominate Coriolis forces.

    (Actually let's perform an order of magnitude analysis: Ro = U/(Lf). Here in DC, and around the middle of the AT, CDT, PCT, I'll assume a latitude of 39 deg N. This doesn't really matter, since f is bounded between 0 and 2 times the angular frequency of the earth's rotation. This is a very small number. Therefore f ~ 2*omega*sin(phi) = 2( 2*pi/(24*60*60) )sin(39) = 9.1e-5. I should really be using sidereal days, but this is a rough order of magnitude calculation and I can't be bothered to look up the length of a sidereal day in seconds. Now L is order cm, 1/100 and U is order 1... Ro ~ 100/9e-5 ~ 100/1e-4 ~ 1e6. Therefore, it should make no difference which way the jets go... yay I love my dimensionless parameters!)

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    As far as the purpose of the secondary hoop goes, it can be explained by a parallel to water hammer. Have you ever been in a house where you hear a bang when you shut a faucet off? That is cause by "water hammer". To combat water hammer, plumbers put spring devices or air gaps in the system (not going into detail here). The secondary hoop combats what I would call "vapor hammer". In the following video watch the flame pattern of the stove at the upper right at 0:50 and at 1:20.


    https://www.youtube.com/watch?v=BepwaJrYpcM

    TetKoba's preference of construction methods and dimensions has produced a situation where the middle stove is the most balanced.

    A CHS stove with a small aperture is more efficient, but can suffer from vapor hammer. Vapor hammer occurs when the hoop is starved for fuel. To combat this, I construct a secondary hoop at the bottom. This secondary hoop does not interfere with the wicking, but does provide a level of resistance that combats the vapor hammer. Stopping the vapor hammer also balances the delivery of the wicked fuel. This decreases boot time and balances the flame. One could just increase the size of the aperture. Doing so makes the stove more powerful and decreases efficiency. Each builder must decide for themselves what is important. It is not a matter of right or wrong. It is a matter of preference. I prefer a more efficient stove (within reason of power offset).
    In the end, it's not the years in your life that count. It's the life in your years. - Abraham Lincoln

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    Quote Originally Posted by aero-hiker View Post
    Why do you suggest methanol over 95% ethanol (is yellow Heet methanol?) From what I understand the energy density of the 95% ethanol is higher, so you should be able to boil more water with less fuel... assuming the stove is equally efficient with both fuels... maybe this is the issue?
    The CHS family of stoves are too powerful. They need to be calmed down a bit to achieve balance. Ethanol whips the wild stallion. It becomes hopelessly out of control unless you construct a huge hoop and tiny aperture to calm it back down.
    In the end, it's not the years in your life that count. It's the life in your years. - Abraham Lincoln

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    Quote Originally Posted by aero-hiker View Post
    I should really be using sidereal days, but this is a rough order of magnitude calculation and I can't be bothered to look up the length of a sidereal day in seconds.
    I had a physics teacher wanting to slap me one day when he assigned a question that dealt with the rotational motion of the earth. He said day. I interrupted and asked if he meant sidereal day or solar day. He said, "I can see that you are going to be a problem". I said, "I am just trying to know how to define the environment".

    Counter clockwise to follow the conventional direction of rotational motion. I, II, III, IV, repeat.
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    Quote Originally Posted by BirdBrain View Post
    I had a physics teacher wanting to slap me one day when he assigned a question that dealt with the rotational motion of the earth. He said day. I interrupted and asked if he meant sidereal day or solar day. He said, "I can see that you are going to be a problem". I said, "I am just trying to know how to define the environment".

    Counter clockwise to follow the conventional direction of rotational motion. I, II, III, IV, repeat.
    Unlike physicists, we electrical engineers like clocks to run clockwise. We wind up using j instead of i for sqrt(-1) (j = -i) and do our vector cross products with a left-hand rule. It's always interesting, translating stuff for the physicists.

    I used to have a counterclock in my office, but the movement failed years ago and I got rid of it. It was a nice conversation piece while it lasted.
    I always know where I am. I'm right here.

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    Quote Originally Posted by Another Kevin View Post
    Unlike physicists, we electrical engineers like clocks to run clockwise. We wind up using j instead of i for sqrt(-1) (j = -i) and do our vector cross products with a left-hand rule. It's always interesting, translating stuff for the physicists.

    I used to have a counterclock in my office, but the movement failed years ago and I got rid of it. It was a nice conversation piece while it lasted.
    You electrical engineers believe in negative forces. You guys iz backwards.

    I am glad you are reading this stuff. I firmly believe if you would jump into the construction fray, you would have things to offer that I would gladly steal.
    Last edited by BirdBrain; 11-04-2015 at 14:49.
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    I suspect that the CHS-u has a considerably larger hoop than the eCHS due to the mating of the bottom of two cans to form the hoop. Either way, I'll try to pick up some Heet, and test my stove again now that I have "squashed" the ribs--perhaps too literally...

    The next stove I build I'll also use the pilot hole technique and probably won't drill the angled holes until it's loosely assembled, so that I can get the angles the way I want them. I'll start with small jets, then increase the size if needed, since it's pretty hard to make a hole smaller!

    One other variable I was wondering about: for the CHS-u, as TETKoba constructs it, the aperture is convex (cut from the upside-down bottom of a red bull can...) but I was wondering about the merits of making it concave. My thinking is that the jets might be inclinded to attach (like a boundary layer) to the solid portion of the convex aperture. If the aperture were more concave, better mixing might be able to develop at the jet-air interface... have you ever given any thought to that?

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    Quote Originally Posted by BirdBrain View Post
    You electrical engineers believe in negative forces. You guys iz backwards.

    I am glad you are reading this stuff. I firmly believe if you would jump into the construction fray, you would have things to offer that I would gladly steal.
    FYI I am a PhD candidate in Mech/Aero Eng... I specialize in high-speed turbulent boundary layers, studied with numerical simulation (DNS). Also, I am an aspiring stove maker/UL enthusiast.

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    Quote Originally Posted by BirdBrain View Post
    You electrical engineers believe in negative forces. You guys iz backwards.

    I am glad you are reading this stuff. I firmly believe if you would jump into the construction fray, you would have things to offer that I would gladly steal.
    Maybe I'll play with stoves over the winter, since it looks as if I'm off hiking for an extended period. But I've other projects to work on as well, so no promises.
    I always know where I am. I'm right here.

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    Also, BB, I'm curious why you draw the distinction between wicking and capillary action. Even though I'm a fluid dynamicist/aerodynamicist, my understanding of creeping flows, surface tension effects, etc. is fairly limited, since I focus on the high speed side of things. If I understand correctly capillary action happens when surface tension forces draw a fluid up a small tube. Is that not what happens between the collet and the tub in the CHS? Or is alcohol a fluid with a convex meniscus? (I don't think this is the case as it is a super "wetting" fluid...) Both capillary action and wicking are just intermolecular forces drawing up a fluid, are they not?

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    Quote Originally Posted by aero-hiker View Post
    I suspect that the CHS-u has a considerably larger hoop than the eCHS due to the mating of the bottom of two cans to form the hoop. Either way, I'll try to pick up some Heet, and test my stove again now that I have "squashed" the ribs--perhaps too literally...

    The next stove I build I'll also use the pilot hole technique and probably won't drill the angled holes until it's loosely assembled, so that I can get the angles the way I want them. I'll start with small jets, then increase the size if needed, since it's pretty hard to make a hole smaller!

    One other variable I was wondering about: for the CHS-u, as TETKoba constructs it, the aperture is convex (cut from the upside-down bottom of a red bull can...) but I was wondering about the merits of making it concave. My thinking is that the jets might be inclinded to attach (like a boundary layer) to the solid portion of the convex aperture. If the aperture were more concave, better mixing might be able to develop at the jet-air interface... have you ever given any thought to that?
    TetKoba prefers the CHS-u. I prefer the eCHS. I suspect their cans are thicker than ours. I was never satisfied with the jets I could produce with the CHS-u method. The tops of the cans are a harder grade of aluminum and are folded over on themselves. This makes for superior jet construction. The material does not degrade as quick as well. Therefore, I have given little consideration to other aperture construction methods. I did make a radical version out of an aluminum Coke bottle. It was built in an effort to see how efficient a stove I could construct. It can boil 2 cups starting at 70 using 10 ml of methanol. It is not practical for the field because it takes too long to heat the water. There would be diminishing returns as the ambient temperature dropped. In the field, you need a powerful stove as well as an efficient stove. I will post those pictures when I get home... if I can find them.
    In the end, it's not the years in your life that count. It's the life in your years. - Abraham Lincoln

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    Quote Originally Posted by BirdBrain View Post
    TetKoba prefers the CHS-u. I prefer the eCHS. I suspect their cans are thicker than ours. I was never satisfied with the jets I could produce with the CHS-u method. The tops of the cans are a harder grade of aluminum and are folded over on themselves. This makes for superior jet construction. The material does not degrade as quick as well. Therefore, I have given little consideration to other aperture construction methods. I did make a radical version out of an aluminum Coke bottle. It was built in an effort to see how efficient a stove I could construct. It can boil 2 cups starting at 70 using 10 ml of methanol. It is not practical for the field because it takes too long to heat the water. There would be diminishing returns as the ambient temperature dropped. In the field, you need a powerful stove as well as an efficient stove. I will post those pictures when I get home... if I can find them.
    No need to post... I think I saw it on the ****** thread... it looks like a real PITA to build, so I think I'll pass on the coke bottle one (although I just saw some of those bottles at a CVS around the corner...)

    What cans that can be found in the mid-atlantic US would you recommend for an eCHS? All the ones I've seen seemed like the tops were too small... I've enjoyed messing around with my CHS-u so I'll probably try to make one or two more at some point, perhaps in addition to the eCHS once I source some decent cans.

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    The truth behind right and left handed helicies is that I am right handed so I hold the drill in my right hand and the stove in my left hand and at that angle, the jets always point counter clockwise (when viewed from above). However I would point out there is a significant hemisphere effect that has nothing to do with Coriolis. In the WESTERN hemisphere my flame rotates counterclockwise but in the EASTERN hemisphere my flame rotates anticlockwise ;-_

    My Stove definitely looks more like the 46 mm stove. I have a whole bag of cans to play with (pulled them out of the recycling dumpster at work) so I may have to experiment a bit more. I don't have an aperture cutting rig - I rip it by hand using the procedure I described on the other thread, which probably isn't optimal but I can try to make one that better approximates the medium sized aperature. I may also have to get myself a pin drill so I can try the 0.6 mm jets, which I can't do with my monster power drill. Also I've considered trying to fabricate some sort of crimping tool to create more reproducible ribs, since that also seems to be somewhat critical. Has anyone thought about or used a tool to crimp the ribs?

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    Quote Originally Posted by Odd Man Out View Post
    The truth behind right and left handed helicies is that I am right handed so I hold the drill in my right hand and the stove in my left hand and at that angle, the jets always point counter clockwise (when viewed from above). However I would point out there is a significant hemisphere effect that has nothing to do with Coriolis. In the WESTERN hemisphere my flame rotates counterclockwise but in the EASTERN hemisphere my flame rotates anticlockwise ;-_

    My Stove definitely looks more like the 46 mm stove. I have a whole bag of cans to play with (pulled them out of the recycling dumpster at work) so I may have to experiment a bit more. I don't have an aperture cutting rig - I rip it by hand using the procedure I described on the other thread, which probably isn't optimal but I can try to make one that better approximates the medium sized aperature. I may also have to get myself a pin drill so I can try the 0.6 mm jets, which I can't do with my monster power drill. Also I've considered trying to fabricate some sort of crimping tool to create more reproducible ribs, since that also seems to be somewhat critical. Has anyone thought about or used a tool to crimp the ribs?
    That Greek or Italian guy (forgot his name etc.) who has a video about making TETKoba's CHS-u (but with outward facing jets) uses some kind of home made die/jig setup. He puts the can over a wooden cylinder with a notch cut in it, and then uses some kind of hand held tool to press the crimp into the can/collet. Other than that, that's all I've seen. Not sure what other techniques one could try...

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    Quote Originally Posted by aero-hiker View Post
    That Greek or Italian guy (forgot his name etc.) who has a video about making TETKoba's CHS-u (but with outward facing jets) uses some kind of home made die/jig setup. He puts the can over a wooden cylinder with a notch cut in it, and then uses some kind of hand held tool to press the crimp into the can/collet. Other than that, that's all I've seen. Not sure what other techniques one could try...
    I was thinking something simple, like a needle nose pliers with some sort of bar on one side to make and indentation and some soft rubber on the other side (replicating my finger, which is what I currently press against). No attempts yet - just ideas.

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    Quote Originally Posted by aero-hiker View Post
    Also, BB, I'm curious why you draw the distinction between wicking and capillary action. Even though I'm a fluid dynamicist/aerodynamicist, my understanding of creeping flows, surface tension effects, etc. is fairly limited, since I focus on the high speed side of things. If I understand correctly capillary action happens when surface tension forces draw a fluid up a small tube. Is that not what happens between the collet and the tub in the CHS? Or is alcohol a fluid with a convex meniscus? (I don't think this is the case as it is a super "wetting" fluid...) Both Icapillary action and wicking are just intermolecular forces drawing up a fluid, are they not?
    I will answer when at home. Off to work.

    Edit: I have a moment and will say this much. I do not feel that capillary is a sufficient word. The fuel is not delivered purely because of dimension or material. Wick is not sufficient either. Cloth wicks absorb fluids via cappilary action. There are other forces in play. Anyone who has ever tried to weld, braze, or sweat pipe with water in it understand those forces. Water, not just pressurized steam, is drawn to the heated metal. The water is drawn because of the heat. The forces that deliver the fuel is not cappilary action in and of itself. If it were, fuel would cycle out the jets prior to heating. The vehicle that delivers fuel during priming and running differ. During the burn, it is what I would call a metal wick. I can see the reasoning of calling it cappilary action. I just feel that word is incomplete to describe what is happening.
    Last edited by BirdBrain; 11-04-2015 at 17:02.
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    Quote Originally Posted by Odd Man Out View Post
    I was thinking something simple, like a needle nose pliers with some sort of bar on one side to make and indentation and some soft rubber on the other side (replicating my finger, which is what I currently press against). No attempts yet - just ideas.
    If you happen to know a siding contractor, HVAC, or gutter guy- not sure if it would be too heavy for an aluminum can or be the right shape- but this is the tool they use to do the tapered crimps to fit ductwork or gutters together. The siding guys also carry an assortment of cutters for J channel and F Channel that may be modified.

    http://www.tools-plus.com/malco-c5r.html

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    Quote Originally Posted by BirdBrain View Post
    I will answer when at home. Off to work.

    Edit: I have a moment and will say this much. I do not feel that capillary is a sufficient word. The fuel is not delivered purely because of dimension or material. Wick is not sufficient either. Cloth wicks absorb fluids via cappilary action. There are other forces in play. Anyone who has ever tried to weld, braze, or sweat pipe with water in it understand those forces. Water, not just pressurized steam, is drawn to the heated metal. The water is drawn because of the heat. The forces that deliver the fuel is not cappilary action in and of itself. If it were, fuel would cycle out the jets prior to heating. The vehicle that delivers fuel during priming and running differ. During the burn, it is what I would call a metal wick. I can see the reasoning of calling it cappilary action. I just feel that word is incomplete to describe what is happening.
    I wonder if the cohesion-tension theory of water flow in plant xylem is similar to what you are describing?

    http://science.jrank.org/pages/6949/...on-theory.html

    https://en.wikipedia.org/wiki/Xylem#...tension_theory

    http://www.cropsreview.com/transpiration-pull.html

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