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Post by coastalrocketeer on Jan 12, 2018 15:19:56 GMT -8
Thank you both @karl and peterberg for these interesting posts based on your individual expertise and experience! Whether or not they further my of my goal of fleshing out what parts of gas flow are easily understandable, and the relationships between, and dependence on, various static and dynamic variables, which might be used to intentionally and intuitively design structures to maximize desired events and effects in various parts of our stoves throughout the dynamic range of a burn cycle, (if that's even possible,) I am very much enjoying the discussion and what I am learning from everyone contributing, so once again, Thank you both!
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Post by coastalrocketeer on Jan 12, 2018 22:07:33 GMT -8
How did you cool the gas ? (...) The effect of condensation on buoyancy is very small. Remember condensation releases relatively high amounts of energy. Perhaps the condensation increases the transfer of thermal energy out of the gas, reducing volume and velocity simultaneously. (velocity loss from the shrinkage would occur, no?, and increased total mass in the stack to be "lifted" as the gas shrinks, if that mass change is more than the loss of water caused?) This could be increasing the mass of the gasses in the stack to be lifted, and this has some effect that causes a stall, against such minimal pushing/pulling forces in the system? The water condensing on the cooler surfaces of the chimney presumably is both carrying/losing embodied heat to the chimney, and "losing" some of it to the phase change requirements, and causing more loss of volume and increase in mass in the stack, the heat the water in the gas provided? This is all happening in what is still a gas rather than steam, at WELL below the boiling point presumably, unless we create condensing conditions, intentionally or otherwise? I presume the steam we see from the exhaust of a rocket is occurring due to all the embodied wood and intake air moisture of the stream, hitting the "dew point" as it mixes with cooler and drier outside air... combined with how "dirty" the outside air and gassss are with "condensation nucleating particles"? Much more of which, would/should be coming from the gas stream, than the outside air, unless you live in Mexico City or Beijing of 1990's? This is all very interesting to me, so please correct or confirm my wonderings above, or add what you are inspired to, to the conversation as you are able :-)
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Post by peterberg on Jan 13, 2018 2:08:35 GMT -8
How did you cool the gas ? BTW a temperature difference to the outside of 50C in a 5 meter chimney creates less than 1 Kg per square meter or less than 0.1 g per square cm. That is a very weak force and with the fire door inside a warm house the force is likely only a bit more than half a kilo per square meter, which is hardly enough to stabilize the system The mass of carbon dioxide in the flue gas is roughly 2.5 times the mass of water vapor and the density more than 4 times higher. Considering the fact that flue gas is denser than air reduces the very weak force even more. The effect of condensation on buoyancy is very small. Remember condensation releases relatively high amounts of energy. I tried two ways to cool the exhaust further. At first I made the existing bell higher with bricks and very thin seams. To rule out possible moist I took off the extra bricks and fired the heater to dry out again. After that coupled a small drum to the bell instead, acting as a second bell. Same effect. I might have the theory wrong but most of what I know is found out by experimenting. As opposed to ruling out possibilities on theoretical grounds without testing in real life. I know in what circumstances a chimney stall occurs, in fact it's very predictable. The reason for such a stall could well be the shrinking of the total gas volume by condensation of the water vapor so that CO² becomes suddenly a much larger (and dense with that) part of the gas volume.
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Post by coastalrocketeer on Jan 13, 2018 6:41:52 GMT -8
I tried two ways to cool the exhaust further. At first I made the existing bell higher with bricks and very thin seams. To rule out possible moist I took off the extra bricks and fired the heater to dry out again. I might have the theory wrong but most of what I know is found out by experimenting. As opposed to ruling out possibilities on theoretical grounds without testing in real life. I know in what circumstances a chimney stall occurs, in fact it's very predictable. The reason for such a stall could well be the shrinking of the total gas volume by condensation of the water vapor so that CO² becomes suddenly a much larger (and dense with that) part of the gas volume. Thank you Peter... I VERY much respect all the hands on research you have done to examine the parameters that effect the system stability to maintain proper gas flow and promote clean combustion, with novel designs of your creation. Can you point me to any posts here you can think of, where you describe the conditions and factors you observe that upset the balance and stall chimneys? Or provide the most important observations from your point of view (if you think there is anything relevant and informative, that does not require too much expenditure of your time to describe/explain?)
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Post by Deleted on Jan 13, 2018 7:18:02 GMT -8
Peter do you remember the amount of O2 at the moment of stalling ? The O2 content allows to estimate the amount of excess air. Theory of Wood Firing www.videncenter.dk/groenne%20trae%20haefte/groen_engelsk/kap_06.pdfJudging from the testo charts the excess air is mainly in the range of 2-3 and only seldom slightly below 2. From my raw calculation resulting per kilo 1584g carbon dioxide = 36 mol ~ 806 liter at 0°C . 4002 g nitrogen ~ 143 mol ~ 3203 liter at 0°C. Water = 624g ~ 35 mol ~ 784 liter at 0°C. With excess air of 2 additional 4002 g nitrogen ~ 143 mol ~ 3203 liter at 0°C. 1077 g oxigen ~ 33.6 mol ~ 754 liter at 0°C Total ~ 11.3 Kg which gives just about 5.5 % water content of which only a rather small part will condense. Very likely the excess air is significantly higher which reduces the relative water content even more. After all the working forces are already extremely weak so that condensation and stalling may be mere coincidence. A well-known statistical example is that married men live longer lives. But why ? Men in secure financial conditions are on average healthier. Healthy men in secured finances are more likely to be married and men who get sick while married are left more by their wives. Statistic also says that men with lots of friends live longer lives, which is mere coincidence as well.
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Post by peterberg on Jan 14, 2018 3:44:53 GMT -8
Or provide the most important observations from your point of view (if you think there is anything relevant and informative, that does not require too much expenditure of your time to describe/explain?) There's one aspect which is easy to describe and repeatable. In my case, the temperature of the exhaust gases inside the chimney close to the heater needed to be above 60º C within 20 minutes after lighting the fire. When it was not, I could expect a chimney stall within 2 minutes, like clockwork. Later, I've seen this effect at different locations and chimneys. The only thing that appeared to be different was the timing, between 15 and 25 minutes depending on the quality of the chimney stack. That is to say, every stack seemed to have its own time frame, better chimney meaning later stalling. All occasions were accompanied by dripping or very wet chimney bottom end. Just before stalling the temperature and the O² dropped sharply. O² levels at the moment of stall were generally low, around 4% in most cases I'd say.
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Post by Deleted on Jan 14, 2018 6:32:38 GMT -8
A stable fire with O² levels around 4% is only possible with very small pieces of wood, pellets or powder, fired in large plants optimized for this kind of fuel and usually operating with blowers. The seemingly obvious reason ( condensation or what ever ) may not be the real reason.
You have forced you system in an extremely unstable state in which a collapsed was unavoidable. If you had used a graphite blower as heatsource, which does not produce water, the result would have been the same.
Condensation releases heat and evaporation cannot occur in an amount required for a sharp temperature drop.
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Post by peterberg on Jan 14, 2018 8:21:38 GMT -8
As I said, just before stalling there happened to be a sharp temperature drop, whatever the reason. When I acted really quickly, heating the chimney through the inspection hatch with an electrical paint stripper caused the temperature to go up again and no stalling occured.
To sum up: there was a signal at several occasions that a chimney stall was immediate and I found a remedy to counteract it. This effect has been confirmed by Erica Wisner a couple of years ago. Whatever the reason, this is what happened irrespective of what you think of it.
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Post by coastalrocketeer on Jan 14, 2018 12:55:08 GMT -8
Just before stalling the temperature and the O² dropped sharply. O² levels at the moment of stall were generally low, around 4% in most cases I'd say. Thank you Peter! Just spitballing, but Perhaps the O2 goes up as the gasses into the system (and combustion rate) slow before the full stall?
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Post by coastalrocketeer on Jan 14, 2018 14:09:29 GMT -8
Condensation releases heat and evaporation cannot occur in an amount required for a sharp temperature drop. Could the heat the condensation is releasing(taking from the gas?) be transferring to the chimney pipe more than the gas and thus cooling the gas more than it would if it was just occurring within gas? Very interesting discussion and I appreciate both your and Peter's input very much!
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Post by Deleted on Jan 14, 2018 14:55:37 GMT -8
Water molecules do not change temperature in a phase change, but only the energy level. The energy freed by condensation will be released to everything that can serve as condensation core.
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