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Post by pinhead on Jan 11, 2018 7:41:50 GMT -8
Bouyancy is inherently related to temperature and volume, which according to Charles's Law have a direct linear relationship where as temperature of a gas increases, volume increases in direct proportion. And Charles is in Charge. Sorry, I had to!
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Post by coastalrocketeer on Jan 11, 2018 7:58:40 GMT -8
I think he is in that department... This page lays out the temperature/pressure/volume inter-dependence of gasses...
The linearity between temperature and volume of a specific mass of gas does not change with pressure, just the total volume occupied.
It still remains a linear relationship between temperature and volume, even at slightly less than atmospheric pressures.
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Post by coastalrocketeer on Jan 11, 2018 8:00:58 GMT -8
The slightly less than atmospheric pressure is entirely dependent on the "stack effect" and rocket stoves may be unique in having two... The riser, before the heat extraction system, and the chimney after.
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Post by coastalrocketeer on Jan 11, 2018 8:13:30 GMT -8
Anything I state here that seems stupidly obvious to anyone, please do not take offense or feel like I am assuming you don't know something that you do... All of these are being placed here for my and others future reference, because if I don't write it down, I may not remember what I was thinking and where I was going with it in a month or two, and I want to have all info here, to "flesh out the relationships, possibilities, and limits, including the obvious and well understood stuff.
I am just spitballing ideas and notions about systems and gas flow that I intuit from the various research and observation I have done here and in the physical world. Many of my assumptions/intuitions may be wrong, even based on easily identified scientific laws and physical rules.
So if you enjoy reading and thinking about these subjects and ideas too, your inspired comments on related concepts and ideas, criticisms, conflicting evidence, and confirmation are equally welcome!
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Post by pinhead on Jan 11, 2018 9:22:39 GMT -8
I think he is in that department... This page lays out the temperature/pressure/volume inter-dependence of gasses... The linearity between temperature and volume of a specific mass of gas does not change with pressure, just the total volume occupied. It still remains a linear relationship between temperature and volume, even at slightly less than atmospheric pressures. Yes, "A modern statement of Charles's law is: When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related." My Charles in Charge reference was a joke, referencing the TV show Charles in Charge.
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Post by coastalrocketeer on Jan 11, 2018 13:25:03 GMT -8
Yes, "A modern statement of Charles's law is: When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related." My Charles in Charge reference was a joke, referencing the TV show Charles in Charge. Thank you for that modern re-wording ... It implies that humidity likely changes the relationship. I suspect they specify dry gas because higher humidity increases specific heat capacity of a given volume of gas, increases it's mass, and likely thermal coupling/heat transfer capability too. Can you think of a tithing else that it could would change based on those or other factors?
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Post by pinhead on Jan 11, 2018 13:52:58 GMT -8
Also likely due to the fact that steam takes up 1800x the volume of liquid water, and phase change requires much more energy (and releases more energy) non-linearly.
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Post by coastalrocketeer on Jan 11, 2018 23:55:43 GMT -8
Duh... I used to know that. Good point!
And the boiling point of water would make that a very NON linear relationship to temperature above 212F
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Post by peterberg on Jan 12, 2018 2:03:02 GMT -8
Also likely due to the fact that steam takes up 1800x the volume of liquid water, and phase change requires much more energy (and releases more energy) non-linearly. Expansion of air due to temperature rise expressed as Kelvin is linear, that's true. See for that the " Good info" thread by Erica Wisner. And the existence of water vapor makes expansion definitely non-linear, that's true as well. A person much more knowledgable than me calculated the following: The chemical process of combustion produces 0.36 to 0.44 liter (or kilogram) of water for every kilogram of entirely dry wood. Wood normally isn't this dry of course, in normal circumstances 15% moist is still in there when stored outside. Most of the time woody fuel contains more water than that, 25% is very common. Together, the water that is exhausted as vapor is something between 0.46 and 0.6 kg or liter for every kg of fuel. On average, say half of the weight of fuel is exhausted as water. Expansion of water to vapor is 1:1700 according to this site. So the chimney of a 6" batch box rocket loaded with 5 kg (11 lbs) of fuel is exhausting 4.25 m³ (150 cu ft) water vapor in 45 minutes. Which is nearly 100 liter (3.53 cu ft) per minute of the burn on average. At the height of the burn it's much more, of course. I'm inclined to think the whole of it is rather more complicated than it appears to be at first sight, the ideal gas law doesn't seem to be valid on its own in this situation. Also, the heat extraction in a bell is greatly influenced by that volume of moisture I would say.
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Post by coastalrocketeer on Jan 12, 2018 2:22:42 GMT -8
Also likely due to the fact that steam takes up 1800x the volume of liquid water, and phase change requires much more energy (and releases more energy) non-linearly. Expansion of air due to temperature rise expressed as Kelvin is linear, that's true. See for that the " Good info" thread by Erica Wisner. And the existence of water vapor makes expansion definitely non-linear, that's true as well. A person much more knowledgable than me calculated the following: The chemical process of combustion produces 0.36 to 0.44 liter (or kilogram) of water for every kilogram of entirely dry wood. Wood normally isn't this dry of course, in normal circumstances 15% moist is still in there when stored outside. Most of the time woody fuel contains more water than that, 25% is very common. Together, the water that is exhausted as vapor is something between 0.46 and 0.6 kg or liter for every kg of fuel. On average, say half of the weight of fuel is exhausted as water. Expansion of water to vapor is 1:1700 according to this site. So the chimney of a 6" batch box rocket loaded with 5 kg (11 lbs) of fuel is exhausting 4.25 m³ (150 cu ft) water vapor in 45 minutes. Which is nearly 100 liter (3.53 cu ft) per minute of the burn on average. At the height of the burn it's much more, of course. I'm inclined to think the whole of it is rather more complicated than it appears to be at first sight, the ideal gas law doesn't seem to be valid on its own in this situation. Also, the heat extraction in a bell is greatly influenced by that volume of moisture I would say. I agree with everything you say... I was not necessarily looking for, or expecting the whole thing to be simple and "linear ratios", but to flesh out the the "rules played by, and desires" of a stream of gas flow. Not expecting it to describe all situations inside our stoves, but understanding the ones that turn out to be correct and solid, I feel, may wind up allowing designing certain aspects of our systems, like heat extraction zones,more intuitively, through being able to properly envision what gasses will do when encountering a certain shaped set of obstacles, for instance. This is more about knowing "higher moisture content/humidity will cause higher heat transfer to and from surfaces contacted, and create higher total heat capacity in the gasses flowing" than knowing specific values or exact change in heat transfer capacity (assuming some part of that last statement is not incorrect, which it may well be.) I live on the Oregon coast, where there is that much moisture in the air BEFORE we burn the wood! ;-) Thank you for your input Peter! I had not considered the amount of moisture in the gas stream as a variable. Though I agree that understanding the specifics of some of it, is as highly complex as some of the interactions and variation of states is... I still hope/think that some useful sets of the relationships can still be mapped, as long as one is looking for general rules, rather than specific data and numbers. And thank you for sharing the link to Erica's thread, which if I read was so long ago that I didn't specifically remember it.
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Post by Deleted on Jan 12, 2018 5:44:59 GMT -8
And the existence of water vapor makes expansion definitely non-linear, that's true as well. That is surely wrong. Water vapor is a gas and as such expands and shrinks linear. Only foggy air is not a dry gas in the sense of Charles's law and may undergo a phase change if heated, respectively dry air ( if dirty enough ) may become foggy by condensation if cooled. Clean air has been cooled to negative 40°C without condensation. The molar volume of a gas, which at STP (273.15 K, 1 atm) is about 22.4 L. Thus for water: 22400cm 3 / 18cm 3(g/mol) ~ 1244,4 times expansion. At the boiling point. (1244,4/273.15K) * 373.15K ~ 1700 times expansion Gas laws en.wikipedia.org/wiki/Gas_lawsA few years ago I have made a raw calculation of the amount and volume of gas created by wood combustion. Resulting gas volume per kilo wood donkey32.proboards.com/thread/759/resulting-gas-volume-kilo-wood
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Post by peterberg on Jan 12, 2018 8:24:39 GMT -8
And the existence of water vapor makes expansion definitely non-linear, that's true as well. That is surely wrong. Oh, is it? And when the exhaust gases of a wood heater are cooled down from 250º C to 25º C, is its volume still linear to Kelvin temperature?
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Post by Deleted on Jan 12, 2018 8:49:02 GMT -8
It becomes only nonlinear if a phase change occurs. Flue gas is likely dirty enough for a phase change. The heat released or required for a phase change influences the volume of the other gases. In the case of condensation the released heat would increase the volume of the remaining gases. One would need a full fledged CFD program and a powerful computer.
Without any excess air the flue gas of one Kg of wood occupies about 5232 liter at 25°C. From a table one may look up how much vapour can be hold per cubic meter at a given temperature, to calculate the amount of condensation and the new volume.
With excess air the water vapour accounts only for roughly 10% of the volume.
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Post by peterberg on Jan 12, 2018 12:03:02 GMT -8
It becomes only nonlinear if a phase change occurs. Flue gas is likely dirty enough for a phase change. The heat released or required for a phase change influences the volume of the other gases. In the case of condensation the released heat would increase the volume of the remaining gases. One would need a full fledged CFD program and a powerful computer. Without any excess air the flue gas of one Kg of wood occupies about 5232 liter at 25°C. From a table one may look up how much vapour can be hold per cubic meter at a given temperature, to calculate the amount of condensation and the new volume. With excess air the water vapour accounts only for roughly 10% of the volume. My first gas analizer, a Testo 327, was able to print a ticket via a thermal printer. That ticket mentioned the dew point, among other things. At the time, back in 2009, I was busy finding out all sort of mechanisms so I tried to cool the exhaust gases down enough to condensate the water vapor. It worked, water was seeping out the stove pipe's elbow when the temperature of the exhaust gases got as low as 40º C. The lower the excess air, the higher the dew point. But shortly after that moment I got a chimney stall caused by massive condensation in the brick chimney. That's how I found out chimney stalls are coupled to low chimney temperatures and condensation. Repeatable every time again in different situations, very revealing.
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Post by Deleted on Jan 12, 2018 14:52:22 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.
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