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Post by pinhead on Jan 2, 2014 13:26:25 GMT -8
Re: J-feed, L-airflow rationale (with a final question for the experts):
As it stands with the generic rocket system, turbulence (via trip-wire, etc) in the burn tunnel (as in a swirl-type automotive combustion chamber) certainly helps with the mixing, and the extreme heat of the riser (like catalytic convertors / Mazda's "thermal reactor" exhaust manifold) burns off pollutants; and maybe it could be argued that it doesn't matter so much where the burn actually occurs, so long as it happens somewhere along the way; but strictly speaking, aren't these (tunnel/riser) kind of the second/third stages of combustion by these points? That is to say, aren't significant improvements also likely to be found more towards the beginning of the combustion process (i.e., in the bottom of the feed, where oxygen is actually first meeting fuel - (analogous in cars to fine atomization of gasoline / strong spark and the velocity / direction of intake charge, maybe)?
I'm by no means an expert. One big advantage of burning liquids is in their ability to atomize and mix with oxygen combined with their relative uniformity of composition. In other words, liquid fuels burn at a relatively even rate and temperature. This makes it much easier to customize the required burn time. Wood, on the other hand, is a mixture of greatly varying components, each of them burning at different temperatures and most of them needing to go through a plethora of chemical reactions to reach the final "full burn" in which CO 2 and H 2O are the byproducts. Re: J-feed, L-airflow rationale (with a final question for the experts):
Conceivably, if combustion was more uniform / complete in the bottom of the feed, maybe such high temps might not be required further on in the system in order to attain full efficiency / low emissions. And maybe we wouldn't have to subject ourselves to as many problems presented by scaling metals, cracking refractories and the necessity of more expensive specialized materials, etc.
I believe it was (again) Chrisburge who suggested that everything that needed to burn in a woodfire for maximum efficiency could be burned within considerably lower temperatures than what rockets are often producing. Are we making things too unnecessarily difficult for ourselves from a materials perspective, by in a way relying excessively on secondary or tertiary stages of combustion in our rocket stoves?
Just brainstorming here...
-Eric
Your analogous comparison to the auto industry is fairly accurate, though due to different reasons than you suggest. The reason Lotus (and modern engine designs) run cooler isn't due to a cooler burn; contrary to popular myth, a lean fuel-air mixture inside a combustion engine does not burn hotter. The hottest burn temperatures are seen at the stoichiometric air-fuel ratio (~14.7:1 air:fuel by mass for petrol). Leaner air-fuel ratios dump more heat into the engine due to the slower rate of combustion, not due to elevated burn temperatures - this is why exhaust valves are the first to go in an engine that "ran lean" for too long. As you said, modern engines eliminate this problem by speeding the rate of combustion. This is accomplished through fine atomization (analogous to chopping your wood into thin sticks), homogenous mixtures (eliminating "lean spots" and "rich spots" in the combustion chamber - the Peter Channel introduces preheated air to relatively fuel-rich areas), turbulence (Peter's batch box induces turbulence in the "throat" of the stove and J-Tubes use tripwire, etc.), high compression ratio, and strong spark/ignition. Since we're dealing with solid fuel instead of liquid, further "processing" must take place in order to change that solid into something which can be mixed with oxygen - either "grind" the fuel into a powder and attempt to "atomize" it to the perfect stoichiometric mixture (think grain dust explosions) or heat it to the point of chemical decomposition. The internal combustion engine does both. Wood-fired stoves are only able to do the latter. This is why efficient stoves must do the processing further down in the combustion zone - that first step takes quite a bit of time and energy. Turbulence is why we need more time than an ICE. After all, without turbulence, the laminar burn rate of gasoline is only ~13M/s. I personally consider the feed tube, burn tunnel, and heat riser to be "combustion zone" analogous to the combustion chamber of an ICE - and the barrel/mass to be the "power extraction" analogous to the pistons/crank. As long as the burn is finished before it leaves the riser, we can have an efficient stove. When considering efficiency, you must take into consideration two types of efficiency in both machines - chemical (% of fuel consumed) and mechanical (amount of energy that does useful work - IMHO catalytic combustion in the exhaust isn't doing anything particularly useful when trying to move a vehicle down the road). |
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jrl
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Post by jrl on Jun 23, 2014 12:52:35 GMT -8
Re: J-feed, L-airflow rationale (with a final question for the experts):
No great understanding of the science after one single, crude build (nor without gas analyzer numbers), so just surmising here and piecing together bits of what I've read / thought through, in light of this experience. The experts may feel free to correct me wherever necessary:
Typical "J" Problems:
1. In a traditional "J" built by the book, small systems would be constrained by equally small feeds ("system size" rule). My system is 3-1/2", but a 3-1/2" feed would really be too small to be of much use, unless there's an awful lot of nice straight 1/2" diameter scraps available to burn (preferably long ones, since they'll burn fast, too). And unless you've got plenty of wood burning, the small riser doesn't really create enough draw to keep things lively, i.e., to create temps hot enough for the desired levels of efficiency. Member Chrisburge seems to have correctly identified this as one of the inherent problems / challenges of small systems.
2. "System size" is a sort of questionable thing when it comes to "J" feeds anyway, since depending on the shapes / sizes / load quantity of wood, widely varying amounts of your "system" CSA are going to be blocked with wood, (Actual CSA = system CSA minus total CSA of loaded wood) and airflow down through and around that wood is going to be inconsistent; Problematically, when you have more wood packed in there/burning is when you need correspondingly more oxygen to get your air/fuel ratios right - BUT unfortunately what you get is exactly the opposite - more restricted airflow / less oxygen since more wood is blocking more of your inlet. The P-channel as originally conceived seemed partly to prevent pieces in the feed from blocking the leading edge of the tunnel, thereby providing an (uniformly) unrestricted portion of the inlet for free airflow. But it doesn't seem to be getting oxygen any nearer to the fuel (wood) itself, and it's not adjustable to compensate for load quantities / wood types (fast-burning vs. slower burning), etc, so might not always be idealized at 6-7% or whatever the figure was (obviously 6-7% of a 1/2-full (as %CSA) feed is not going to represent the same actual percentage of airflow compared to a feed that's 3/4 full).
3. The downward draft airflow in a "J" is not necessarily going to be directed towards the tips of all the wood pieces equally; in fact, the "path of least resistance" is probably going to be towards the leading (drum-side) edge, since it's easier for air to travel through a few pieces of wood there than through ALL of the wood. Thus, pieces at the leading edge receive more airflow / oxygen, and pieces at the back (further from the drum) receive less. This might cause inconsistencies of temps in the lower feed area, and more coals to accumulate in these lower-airflow areas, and cause unequal burn rates of equal-length pieces, as well, complicating re-load timings.
4. A taller feed-tube helps wood auto-feed better (i.e., not get jammed up / stuck) since pieces won't be splayed out at different angles as they can be in a lower tube; It also prevents wood from being loaded on any angle that puts the burning tips end up in weird places - like further into the burn tunnel or too close to the sides, where temperatures would tend to be cooler and airflow less. BUT the taller the feed, the more draft of its own it creates, counteracting the draw created by the heat riser. I think this is why it's been recommended that only an equal-size feed tube of no more than 1/3rd the height of the riser be used. If the feed tube were of larger diameter than system size, it could create even more riser-counteracting draw at an even lower height.
How this design MIGHT be avoiding these problems:
1. My feed is a little taller than 1/3rd riser height; but as no airflow is required (or in fact allowed, since it's capped) to pass through it, it creates no reverse-draft effects AT ALL. And no corresponding tendency towards smokeback. I've observed that system runs much more "rockety" when pulling through the end of the burn tunnel than when pulling down through the feed, in support of this assertion.
2. My feed, as it flows no air, can drop the "system size" rule and be opened up to 5" diameter, allowing adequate amounts of wood to be loaded / burned with plenty of air gaps around them; combustion thus improves, keeping temps & efficiency up. Both the extended height and the taper to a smaller diameter at the top help keep pieces "centered" and parallel (not splayed), i.e., more uniformly accessible to oxygen inflow; the taper also keeps (downward) air velocities up (at the feed opening) during uncapped reloads, so that even if longer wood pieces are still present / burning a bit in the feed, the smoke won't escape.
3. Another benefit of the feed-tube cap would seem to be that since there's no cooler intake airflow passing by it, wood is very well pre-heated and closer to combustion temperature by the time tips break off and pieces auto-feed down. This would seem to contribute to an overall cleaner burn, since fresh wood ignites immediately and burns at closer to ideal temp from the very start. And yet, since there's no air/oxygen flow present in the capped feed, wood doesn't typically start burning upward into the feed, as could happen in some cases with well-insulated (hot) feed tubes and very dry / fissured / bark-clad wood (like I am using).
4. Since primary air enters through the end of my burn tunnel, it must pass through ALL of the burning wood tips quite equally - so the temperatures / combustion characteristics in the bottom of the feed area should be relatively uniform from front to back and side to side and moreover from top to bottom, since additional secondary air enters through the grate below the wood tips. So it seems everything burning is exposed to roughly similar amounts of direct airflow. Thus efficiency improves and coals buildup is probably also reduced on account (the reason I can burn 9hrs. continuously through a small tunnel without it getting blocked).
5. Peter had written somewhere that excess airflow / oxygen can be actually be counter-productive, and that when the system is really "roaring" it actually can be running less efficiently overall. With my burn tunnel being in the form of a sliding drawer, primary airflow is completely adjustable to compensate and idealize oxygen levels for changes in wood type / size / quantity; Changes in the quality / volume of the "roar" are readily apparent as the drawer is slid in either direction (an added benefit is that ash removal is extremely easy / mess-free - simply remove the drawer entirely and dump it in the dustbin - that's it!).
All this has got me wondering now about the technical function(s) of the P-channel (I have not yet utilized one) - so if someone can refer me to an exact explanation for its efficacy, I'd appreciate it (been through Peter's early thread at a couple points); I'm assuming that it's partly providing a guaranteed, unrestricted portion of fresh intake air (oxygen) in a system where varying wood loads can variously block the burn tunnel - thus it helps prevent over-rich burn conditions (I used to get telltale black smoke just after re-loading, when I ran primary air through the (3-1/2") feed only); It also provides that fresh air in a pre-heated condition; So is this simply promoting elevated temps in the tunnel, thereby burning off measurable pollutants caused by incomplete burning / inequal temps / inconsistent burn characteristics of individual pieces of wood further back in the feed area?
I realize I'm going out on a limb here and kind of comparing apples to oranges; but I'm thinking back, a little analogously, to the 70's/80's when pollution controls were being introduced on U.S.-market cars: Most of the American automakers struggled with cumbersome and performance-reducing add-on devices like EGR (hot exhaust gases reintroduced into the intake) and spark-retarding and lean-burn systems (promoting hotter combustion temps) and catalytic convertors (hotter temps in the exhaust stream): essentially systems designed to cope with the reality of rather inefficient combustion characteristics of the mostly antiquated engines themselves. A totally different approach was represented by Lotus (the Elite, around '82-4, maybe?), which managed to pass then-current EPA emissions standards without such systems, on account of having a much better-designed cylinder head that burned everything uniformly/completely enough in the first place (not to mention being a lot more efficient in terms of fuel-consumption).
Just thinking that in a rocket stove (like in maybe any other combustion system), if you can find ways to promote some consistency / uniformity in 1) the amount of fuel actually burning, and 2) the oxygen flow around / mixing with every part of that fuel; then temperatures will be more uniform in every portion of the combustion area, and thus efficiency (in terms of both heat output and the burning off of pollutants) will be enhanced.
As it stands with the generic rocket system, turbulence (via trip-wire, etc) in the burn tunnel (as in a swirl-type automotive combustion chamber) certainly helps with the mixing, and the extreme heat of the riser (like catalytic convertors / Mazda's "thermal reactor" exhaust manifold) burns off pollutants; and maybe it could be argued that it doesn't matter so much where the burn actually occurs, so long as it happens somewhere along the way; but strictly speaking, aren't these (tunnel/riser) kind of the second/third stages of combustion by these points? That is to say, aren't significant improvements also likely to be found more towards the beginning of the combustion process (i.e., in the bottom of the feed, where oxygen is actually first meeting fuel - (analogous in cars to fine atomization of gasoline / strong spark and the velocity / direction of intake charge, maybe)? Conceivably, if combustion was more uniform / complete in the bottom of the feed, maybe such high temps might not be required further on in the system in order to attain full efficiency / low emissions. And maybe we wouldn't have to subject ourselves to as many problems presented by scaling metals, cracking refractories and the necessity of more expensive specialized materials, etc.
I believe it was (again) Chrisburge who suggested that everything that needed to burn in a woodfire for maximum efficiency could be burned within considerably lower temperatures than what rockets are often producing. Are we making things too unnecessarily difficult for ourselves from a materials perspective, by in a way relying excessively on secondary or tertiary stages of combustion in our rocket stoves?
Just brainstorming here...
-Eric Yes..... Yes. This is precisely the constraints of the traditional rocket stove J Tube design that I have been trying to engineer around. I'm glad you've had success with the larger, sealed feed tube and the air from the end of the feed chamber. That's encouraging. |
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Post by Donkey on Jul 2, 2014 22:54:58 GMT -8
Just to say; most of the burn cycle (of wood) is dealing with gaseous fuels. The major work tends to be done, down-stream from the pyrolization area (wood tips).
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Post by ringoism on Mar 9, 2015 8:55:24 GMT -8
Just thought I should check in here, as my last post on the thread goes back to end of Dec, 2013...
Prototype unit as shown in photos earlier is still running, for lack of time to build the next version (but still fully intend to).
Winters / snow last through March here, so ran it till at least then (bit beyond maybe, in light of having a newborn in the house) in 2014;
This winter we were at the in-laws' in India's much more pleasant Northeastern hills (Mizoram to be exact, sandwiched between Burma and Bangladesh), where I was in short sleeves all through Jan/Feb... Got back to the Western Himalayas five days ago, and it's snowed at least three times since (and both kids have colds), so the prototype has been put to heavy use again.
Very happy with the performance still; though as expected the main challenge might be the longevity of materials. Found that mild steel is pretty decent so long as it's not in direct contact with flames; my steel burn tunnel - only 1.5mm material (18ga?) I think - looks like it would do at least a couple more seasons; had maybe 5mm of 1200C ceramic wool sandwiched between its walls and the simple clay tiles lining the chamber, so it doesn't seem to take much to protect it, at least in my case. Moreover, even kitchen-grade stainless steel (404, 304, etc: magazine floor grate, burn tunnel roof, riser lining) seems to handle direct flame a lot better. Not going to last forever, but a big improvement over MS, and I'm thinking that if my basic tray/tunnel is kept simple, it's cheap enough to just newly fabricate / replace that piece annually, in lieu of utilizing more exotic (expensive), or heavy, or messy materials, etc. Still haven't found thin-cross-section refractory tiles (ideally something dimensionally like asbestos tiles but higher heat-range), so replaceable stainless might have to do it. Other suggestions welcome here.
Have yet to need to clean out the riser, drum, or roof-stack, which is to say it's pretty clean-running (local people have to start each day by banging on their fluepipes to knock all the crud loose).
As it always has been, still easy to start (no tricks required and wife manages it at least as easily as I do), easy to load, virtually no smokeback whether starting, running or loading, and a somewhat tunable setup re: burn rates / heat output. Feel very fortunate to have got my sizing seemingly just about right the first time.
Future improvements will focus mainly on improving longevity of materials (stainless burn tunnel/tray, maybe special grade in most critical areas; alternatively picked up some 1200C castable (1800C also available) and some ceramic fiber board to play with) and on enlarging the drum in an effort to reduce stack temps a bit and provide more area on top for cooking (we mainly boil single pots of water on it now);
Also want to work this year towards something commercially viable in the context, which at minimum means (as probably stated earlier):
-Portable; i.e., unitized and relatively lightweight
-Easy to install/start/run
-Sufficient for getting mainly small rooms of 100-200sqft. nice and toasty
-Preferably some sort of cooking surface, suitable for two pots.
-Preferably not too tall
-Minimum five-year service life
-Marketable within a range of $150-$300
First three basically achieved or easily achievable; while final four should prove challenging - but as a motivator, I had multiple requests from neighbors / friends to build them similar stoves to ours (wood getting scarce / expensive these days), and I know of at least four other people in the area who are now working on building some form of rocket stoves, so am getting the idea that its time has come in this place.
Hope to get into the forum a bit more these next weeks / months and see what's been happening the past year or so I've been out of the loop.
Regards,
Eric
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Post by Robert on Mar 9, 2015 13:35:55 GMT -8
and has consumed perhaps 7kg of dry 1-1/2"dia. hardwood in a full run of 6hrs. Hey Eric. Can you please say something more about how much wood you use? Here in Poland i put 7kg wood at one time in my stove (made from bricks). i light it it burns for 1,5 hours. and i am ok for 24hours at zero C outside. When minus 6 i light it two times so 14KG of wood at hardcore temps. What is your use? |
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Post by ringoism on Jan 22, 2016 11:12:02 GMT -8
Future improvements will focus mainly on improving longevity of materials (stainless burn tunnel/tray, maybe special grade in most critical areas; alternatively picked up some 1200C castable (1800C also available) and some ceramic fiber board to play with) and on enlarging the drum in an effort to reduce stack temps a bit and provide more area on top for cooking (we mainly boil single pots of water on it now); Also want to work this year towards something commercially viable in the context, which at minimum means: -Portable; i.e., unitized and relatively lightweight -Easy to install/start/run -Sufficient for getting mainly small rooms of 100-200sqft. nice and toasty -Preferably some sort of cooking surface, suitable for two pots. -Preferably not too tall -Minimum five-year service life -Marketable within a range of $150-$300 Early prototype still running and keeping us warm. Tentative design of version II finally (after a few years) done; questions of drum size and conductivity/emissivity of its materials has come up, so need to research that a little more before building. Otherwise have most of my materials ready or readily available. Probably will build a simple sheet metal brake so I can do all the work myself (vs. last time struggling with local fabricators). More soon, I hope. -Eric |
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Post by ringoism on Mar 11, 2016 5:36:52 GMT -8
Was going to post photos, but am getting a message when I try to attach: error: "this forum has reached its attachment space limit". Can I now only include photos with an external link (i.e., upload them on a photo hosting site first)? Will get some up as soon as I find out how.
Anyway, my second prototype has been built and initially trial-run a couple weeks ago, though as yet much lower performance than my original (mainly on account of the much shorter riser, I suppose). I knew I was pushing/passing the limits in terms of physical dimensions (needed a lower-height, small-footprint design since as mentioned earlier locals here generally are seated on the floor whilst cooking, and in generally small rooms), but wanted to give it a try anyway... I made the feed very high also (same height as my original prototype, but in one piece vs. two, and much higher in proportion to the riser now), mainly so that I could use sticks as lengthy as I am now, and re-fuel about as often... However, the draft / velocity being less now, I had (predictable) smokeback/burnback issues. Think the polished stainless was also the wrong choice there, since it's not going to dissipate heat well, and temps in the feed maybe get too high.
Still, the fabricated sheet-metal design seems about right, and if I can get my dimensions in order (even re-thinking my riser CSA at this point) I think this is going to fulfill my original goals quite well.
Got thinking maybe the low-riser (around 28", vs. the 38" of the previous) just wasn't going to work for a rocket, and wondering if I should be looking into alternatives, remembered the Walker riserless core; What a surprise when I went to Matthew's website and saw the new, lightweight, portable, modular, firebrick-lined, sheet-metal sheathed "Aluminum Series" cores... not that they're low-riser, but so similar in basic concept to what I had had in mind from the beginning (lightweight, portable, fabricated metal with high-temp innards), and had finally just built... Now if I could get mine to burn as well as his (and figure out how to get these photos up)!
-Eric
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Post by satamax on Mar 12, 2016 6:34:12 GMT -8
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