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Post by icarus on Mar 7, 2019 18:56:56 GMT -8
Peter, what is the "worst case scenario" at O2 levels below 8%?
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Post by peterberg on Mar 8, 2019 1:22:15 GMT -8
Chances are higher the CO would rise far above 5000 ppm which is in 90(ish)% of cases accompanied by a lot of dark grey smoke. General conclusion: below certain low O² percentages like 6% there's a greater possibility for the combustion process to become unstable.
There are some cases where the O² dived below 5% and still no disasters happened, but these are quite rare. So to declare a design as stable, it need to stay above 6% O² most of the instances. And in order to achieve that, I'm aiming for 8% as a safe margin.
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Post by DCish on Mar 8, 2019 10:34:14 GMT -8
Cool developments! This exchange about O2 levels as related to CO has me thinking it's time for me to get a little better acquainted with the 2022 targets. What I found were the following requirements for residential biomass burners (cordwood, not pellets): - CO below 1500, (below 300 for pellet stoves!) - NOx below 300, - PM below 40, and - "Organic Gaseous Compounds (OGCs)" below 120. Here is my best attempt to interpret your measurements -- can you correct any errors? - CO in all the graphs is well below 1500 in most of the burns for most of the time -- seems solid - PM is too expensive to test regularly, though you've mentioned at least one batch box being tested and doing well, - NOx - if I recall correctly, we're not too worried about NOx since we're burning wood at temperatures below where we would be producing lots of it. - OGCs -- I don't know anything about this requirement. Do you know if there is any reason we should be concerned about these emissions? Interestingly, since it hasn't come up in a while, the US EPA standard is "2 grams per hour of smoke" by 2020. I poked around more and found that this is measured by the "dilution tunnel" method -- capturing the smoke and mixing it with ambient air to cool it and condense out any evaporated hydrocarbons, then running it through a filter, drying the filter, then weighing it to see how many grams of matter it picked up along the way. I don't see how this is easily replicated for us. Here is an interesting article about how commercial European PM instruments were compared to the US dilution tunnel method in preparation using these portable instruments to test entrants in the 2013 Wood Stove Decathlon. ( www.forgreenheat.org/docs/WSDCFinal.pdf) Incidentally, for newer members of the forum, Matthew Walker participated in this event ( donkey32.proboards.com/thread/1908/woodstove-decathalon-washington).
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Post by Orange on Mar 10, 2019 2:01:55 GMT -8
if I calculated correctly, DSR is around 25cm/10'' shorter than the standard BB with riser 8*B.
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Post by peterberg on Mar 10, 2019 3:56:28 GMT -8
The last couple of days I have been fooling around with making the exit port in the top box wider and narrower again and see what effect this would have. And I tried frantically to contact one of the rare people "in the know" to find out what the expected results would be. Interestingly, this guy told me two venturies in the same gas trajectory tend to limit the gas velocity in the first. All depending on distance and shape between those two, size and volume of the space before, between and after. And last but not least: the size of the second or in other words the proportions between the venturies play a very important role. The larger the second, the less effect on the gas velocity in the first.
So I made it wider first, no discernable effect. Made it smaller, nearly the same size as the first and the thing became very sluggish, hard to get up to speed. Made it a bit wider again, still unlike a Roadrunner or Speedy Gonzales but perfectly doable. The first port staying at the same size, proportional to the square riser stub: 57%. All tested configurations were done at least twice.
Best performance was achieved with the second port at 78% of the square riser's csa. Due to coming up to speed slowly, the chance of overfuelling is deminished a lot, I'd say. In the coming days I'll push it a bit harder and see how that goes.
I don't know who at first mentioned making the exhaust port smaller than system csa, this might be Trevor or Brian. The argument was providing back pressure, and for all I know that would be the same effect as my knowledgeable guy said it would. Thanks guys, for providing an alternative route of thinking.
Another thought: what would happen when the top of a normal vertical riser is restricted slightly?
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Post by gadget on Mar 10, 2019 8:13:33 GMT -8
Peter, I know this is probably not possible but do you think this heater could scale to an 80mm chimney? I was just thinking its about the size of a water heater and standard water heater chimney is about 3" or 80mm. If so, how about using an old water heater for the barrel?
I'm kind of a prepper and was thinking this could be a possible emergency way of keeping a home from freezing in a grid down type scenario. Allot of homes have gas water heaters with a triple wall chimney. Its just very tiny.
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Post by Vortex on Mar 10, 2019 10:54:46 GMT -8
The last couple of days I have been fooling around with making the exit port in the top box wider and narrower again and see what effect this would have. And I tried frantically to contact one of the rare people "in the know" to find out what the expected results would be. Interestingly, this guy told me two venturies in the same gas trajectory tend to limit the gas velocity in the first. All depending on distance and shape between those two, size and volume of the space before, between and after. And last but not least: the size of the second or in other words the proportions between the venturies play a very important role. The larger the second, the less effect on the gas velocity in the first. So I made it wider first, no discernable effect. Made it smaller, nearly the same size as the first and the thing became very sluggish, hard to get up to speed. Made it a bit wider again, still unlike a Roadrunner or Speedy Gonzales but perfectly doable. The first port staying at the same size, proportional to the square riser stub: 57%. All tested configurations were done at least twice. Best performance was achieved with the second port at 78% of the square riser's csa. Due to coming up to speed slowly, the chance of overfuelling is deminished a lot, I'd say. In the coming days I'll push it a bit harder and see how that goes. I don't know who at first mentioned making the exhaust port smaller than system csa, this might be Trevor or Brian. The argument was providing back pressure, and for all I know that would be the same effect as my knowledgeable guy said it would. Thanks guys, for providing an alternative route of thinking. Another thought: what would happen when the top of a normal vertical riser is restricted slightly? I don't know if Brian came across this earlier, but when I was doing my summer experiments last year, I found that a secondary port/throat, in the form of a narrow 180 degree switchback at the exit of the afterburner, (around 87% CSA) gave me a nice steady smokeless burn without any over-fueling problems. It seems to depend a lot though on how much back pressure/resistance to gas flow there is in the mass it's heating, because later when I installed the afterburner on my inside stove, I found the best exit port size was 1 CSA. That stove has over 1 ton of mass though, with 18 feet of channels and 13 90 degree turns, so it has quite a large resistance already. That's what led me to the conclusion that the back pressure was an important factor. Having a defined insulated secondary combustion chamber between the two ports, where the gases can expand, mix and burn, seems to be as important as the gas speed limit that the back pressure creates.
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Post by DCish on Mar 10, 2019 19:43:11 GMT -8
The last couple of days I have been fooling around with making the exit port in the top box wider and narrower again and see what effect this would have. And I tried frantically to contact one of the rare people "in the know" to find out what the expected results would be. Interestingly, this guy told me two venturies in the same gas trajectory tend to limit the gas velocity in the first. All depending on distance and shape between those two, size and volume of the space before, between and after. And last but not least: the size of the second or in other words the proportions between the venturies play a very important role. The larger the second, the less effect on the gas velocity in the first. So I made it wider first, no discernable effect. Made it smaller, nearly the same size as the first and the thing became very sluggish, hard to get up to speed. Made it a bit wider again, still unlike a Roadrunner or Speedy Gonzales but perfectly doable. The first port staying at the same size, proportional to the square riser stub: 57%. All tested configurations were done at least twice. Best performance was achieved with the second port at 78% of the square riser's csa. Due to coming up to speed slowly, the chance of overfuelling is deminished a lot, I'd say. In the coming days I'll push it a bit harder and see how that goes. I don't know who at first mentioned making the exhaust port smaller than system csa, this might be Trevor or Brian. The argument was providing back pressure, and for all I know that would be the same effect as my knowledgeable guy said it would. Thanks guys, for providing an alternative route of thinking. Another thought: what would happen when the top of a normal vertical riser is restricted slightly? I don't know if Brian came across this earlier, but when I was doing my summer experiments last year, I found that a secondary port/throat, in the form of a narrow 180 degree switchback at the exit of the afterburner, (around 87% CSA) gave me a nice steady smokeless burn without any over-fueling problems. It seems to depend a lot though on how much back pressure/resistance to gas flow there is in the mass it's heating, because later when I installed the afterburner on my inside stove, I found the best exit port size was 1 CSA. That stove has over 1 ton of mass though, with 18 feet of channels and 13 90 degree turns, so it has quite a large resistance already. That's what led me to the conclusion that the back pressure was an important factor. Having a defined insulated secondary combustion chamber between the two ports, where the gases can expand, mix and burn, seems to be as important as the gas speed limit that the back pressure creates. It's definitely something I took note of in 2016 when I was exploring a riser stub followed by a baffle with a circular hole in it. Although I wasn't thinking of it as back-pressure, I was focused on shaping the flame path to create maximum turbulence and dwell, observing that without a second restriction, gasses tended to simply stream straight through the afterburner space. Looking at that thread now I see pictures, but none of my videos -- argh! Anyway, when Trev recently posted his double vortex afterburner and mentioned that sizing the post-afterburner restriction was crucial, that absolutely rang a bell with me based on my 2016 experiments. Sometimes a thing has to be discovered from a couple of different angles before it is well defined and its function is recognized! Thanks for putting your time into exploring this one, Peter.
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Post by ronyon on Mar 11, 2019 19:23:43 GMT -8
The last couple of days I have been fooling around with making the exit port in the top box wider and narrower again and see what effect this would have. And I tried frantically to contact one of the rare people "in the know" to find out what the expected results would be. Interestingly, this guy told me two venturies in the same gas trajectory tend to limit the gas velocity in the first. All depending on distance and shape between those two, size and volume of the space before, between and after. And last but not least: the size of the second or in other words the proportions between the venturies play a very important role. The larger the second, the less effect on the gas velocity in the first. So I made it wider first, no discernable effect. Made it smaller, nearly the same size as the first and the thing became very sluggish, hard to get up to speed. Made it a bit wider again, still unlike a Roadrunner or Speedy Gonzales but perfectly doable. The first port staying at the same size, proportional to the square riser stub: 57%. All tested configurations were done at least twice. Best performance was achieved with the second port at 78% of the square riser's csa. Due to coming up to speed slowly, the chance of overfuelling is deminished a lot, I'd say. In the coming days I'll push it a bit harder and see how that goes. I don't know who at first mentioned making the exhaust port smaller than system csa, this might be Trevor or Brian. The argument was providing back pressure, and for all I know that would be the same effect as my knowledgeable guy said it would. Thanks guys, for providing an alternative route of thinking. Another thought: what would happen when the top of a normal vertical riser is restricted slightly? Peter,this post is so clear, informative, and gracious.
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Post by peterberg on Mar 12, 2019 10:01:25 GMT -8
Monday morning another test run was completed succesfully. By that time, a question kept nagging in the back of my head: which feature is doing exactly what? Is the downhanging brick essential, the trip wires left and right under that brick or the restriction of the end port? It might also be a combination of all this, and in what proportion? That afternoon I decided to investigate this and to concentrate on the restricted end port to begin with. I ripped out the hanging brick, trip wires, even the chamfered rear corners of the riser. The end port size and the floor channel remained untouched, the ceiling of the top box just straight horizontal. The resulting diagram of the first testrun of this configuration couldn't be shabbier. It went in thermal overdrive almost immediately, O² got down to 4% and CO was out of sight for 10 minutes. No slow start at all like the previous one. I figured that the end port could be restricted a bit further in order to slow the process down sufficiently. But restricting to what size? I'd cut some pieces of insulating brick and decided there and then it would be best to mimic the csa of the first port. And try bigger or smaller port sizes according to the results. EDIT: In hindsight, the port happened to be slightly larger instead. This results are due to an end port that's about 5% wider as compared to the riser port that's 57.5% of the square riser's csa. First run this morning I used 5 pieces of oak and some pine on top and the results were remarkable, to say the least. No thermal overdrive, no overfuelling at all. The CO could be lower at places but the numbers were very good overall so I won't complain. When something seems too good to be true, it usually is. So this afternoon the thingy was cooled down enough to load again and off it went again. This one placed itself in the same league as the first one, coming up to speed nicely without overdoing it. Mark the O² comes down lower than in the morning diagram, maybe due to using 6 smaller pieces of oak and 6 pieces of not-so-dry willow. I'm aiming for a weight of 3.5 kg (7.72 lbs) of fuel for every run, by the way. This is looking very, very good, I expected it to produce at least one peak of CO because of the smaller fuel but it didn't. I need to fetch more of the oak, ash or beech fuel in order to continue this test series, hopefully it will be as succesful. I'd think there's some work ahead but we are definitely on the right track, not to mention using the simplest build I could imagine. No fuss, straight corners, rectangle boxes with some rectangle openings here and there, the floor channel being the most complicated part of the whole build. Apart from the door assembly and air inlet of course, but that's for later.
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Post by peterberg on Mar 13, 2019 3:51:29 GMT -8
This morning I tried to get an answer to another question: is in the floor channel the proportion 2 to 1 in horizontal to vertical part really necessary? The horizontal part is made by welding two smaller tubes side by side which are open to each other the last 4" just before where the vertical stub is. So I blocked off one of the tubes, creating a channel consisting of the same csa throughout.
This run failed miserably, 10 minutes into the burn the O² dropped sharply to 3.5%, sending the CO sky-high. At the moment the CO rose I pulled the plug of the blocked duct and it took another 10 minutes to reach a stable state again. From there on it was plain sailing.
Conclusion: the 2:1 proportion of the floor channel really do provide more air flow as opposed to the straight channel.
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Post by Dan (Upstate NY, USA) on Mar 13, 2019 13:06:52 GMT -8
I also think that metal down the middle of the floor channel would be more surface area to heat the secondary air. Common heat exchange pricipals.
I would like to know if there would be a difference between the same size channel made of one piece vs two.
Can you imagine the whole floor made with side by side channels? All feeding the same port?
Image the slow secondary air flow and the heat transfer!!!
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Post by peterberg on Mar 13, 2019 14:15:26 GMT -8
I also think that metal down the middle of the floor channel would be more surface area to heat the secondary air. Common heat exchange pricipals. I would like to know if there would be a difference between the same size channel made of one piece vs two. Can you imagine the whole floor made with side by side channels? All feeding the same port? Image the slow secondary air flow and the heat transfer!!! First: the part of horizontal metal exposed to the fire is as wide as the port, no more. Because of two triangular strips left and right that covers part of the metal. Second: on top of the exposed channel there's a layer of ash that's insulative so the direct heating of that horizontal part is subdued. And why should the secondary air flow would be slowed down while heated? Normally while coming up to temp the secondary air flow is accelerated instead, driven by the under pressure of the port. Problems commonly arise within the first third of the burn. Once that's past the tester is able to lean back and wait it out.
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Post by Dan (Upstate NY, USA) on Mar 13, 2019 16:42:31 GMT -8
If the whole floor was channels it would be slow due to that fact that each channel is feeding a small port.
I was not speaking of the heating itself slowing down the air. I was speaking of the air flow down each channel.
The speed at the exit would be the same except the dwell time of the secondary air in each channel would be greater hence the more heat exchange. Therefore hotter secondary air.
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Post by peterberg on Mar 14, 2019 4:02:37 GMT -8
If the whole floor was channels it would be slow due to that fact that each channel is feeding a small port. I was not speaking of the heating itself slowing down the air. I was speaking of the air flow down each channel. The speed at the exit would be the same except the dwell time of the secondary air in each channel would be greater hence the more heat exchange. Therefore hotter secondary air. It isn't that straightforward, I'm afraid. All this depends on the air velocity in the port, the floor channel's vertical stub is acting as a kind of venturi itself to name a factor, since it is a temporarily restriction.
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