[Stoves] Thoughts and questions about Buoyancy

[alex english]

Kirk,
You might want to refer to this chart.
http://stoves.bioenergylists.org/stovesdoc/English/Chimdr.jpg

Alex


On Mon, Apr 14, 2014 at 2:56 PM, kgharris <kgharris at sonic.net> wrote:

>   *Thoughts and questions about Buoyancy *
>
> I would like to pose some thoughts and questions to the group about
> buoyancy.  I have been trying to think this through for some time and
> would like to get some input.
>
>                I used a manometer with attached probe to test my TLUD for
> gas pressure variations, measuring to .01 inch of water column, hoping to
> learn more about what goes on inside the stove.  I found no variations in
> pressure.  The stove was at atmospheric pressure throughout.  I could not
> confirm any pressure variations within the functioning stove.  Can anyone
> confirm or disprove the results of this test?  Because I found no pressure
> variations, I had to find a mechanism which could move the gasses without
> pressure difference.  I am looking at buoyancy.  Heavy gasses fall and
> displace light gasses which rise.  This does not mean that light gasses
> have “lift” as in anti-gravity.  They are simply pulled down with less
> force by gravity than heavier gasses.  A light weight gas, like helium or
> fire gasses, placed in a vacuum chamber will fall, not rise.  Do heavy
> gasses push the light weight gasses up?  I am thinking of it as a field
> of gravity where heavier gasses are pulled more by gravity than light
> gasses, and so heavy gasses are pulled closer to the center of gravity than
> light gasses (ie down).  So buoyancy can only occur in a gravitational
> field and is an effect of gravity.  But then in a state of equilibrium
> the light gasses are sitting on top of the heavy gasses and thus must be
> exerting a force on the heavy gasses, and vise versa the heavy gasses must
> be pushing up on the light gasses.  This means pressure, and it would
> show up in the overall atmospheric pressure and so would not be seen in my
> manometer tests, since both ends of the manometer are subject to it.  The
> question I raise is “Does the air push up on the fire gases in the
> non-equilibrium situation inside the stove, or is it a displacement where
> the air flows in and the fire flows out all due to gravity?”  Either way
> it is clear that a small bubble of light weight gas in a sea of heavier
> gas, such as the situation with our stoves, cannot maintain its position
> and must rise.  If the heavy gas is pushing up on the light gas, would
> not a pressure area have been detected by the manometer test?
>
>                I have read that the raising fire is caused by the
> difference in altitude, that the atmospheric pressure on the bottom of our
> stoves is greater than the atmospheric pressure on the top and so the fire
> rises.  I find it difficult to believe that the pressure difference in
> two feet of stove height is significant enough to drive a high power fire,
> when the atmospheric pressure gradually changes over ten miles of height.
>
>                Exhaust gasses rising in a chimney do NOT create a vacuum
> below them and thus pull air in to the stove, as the word draft implies.  The
> atmospheric air falls into the stove and displaces the hotter and thus
> lighter weight fire gasses.  Another way to think of this is to think of
> a tube held in a U shape.  Both ends of the tube are at the top and both
> are open to the atmosphere.  If one leg of the U, say the right side, is
> filled with heavy gas, and the left is filled with light weight gas, the
> heavy gas will fall around the bend and then rise up the left leg to a
> point of equilibrium.  This is the same as what is happening in our
> natural draft stoves, the heavier atmosphere rushes into the stove and
> displaces the lighter gasses of the fire.  The primary air is able to
> rise through the fuel in the stove like the heavy gas in the U tube is able
> to rise in the left leg.  But since the incoming air is heated by the
> fire and becomes buoyant, it is itself displaced by more air from the
> outside.  A point of equilibrium cannot be reached until the fire is
> extinguished and the stove is cooled.
>
>                A taller chimney creates more draft because it contains a
> taller column of light weight gasses, which means a taller column of
> heavier atmosphere outside the chimney.  This gives more height
> difference for gravity to act on.  Buoyancy strength in this
> situation depends on the weight (density * gravity) difference between the
> gasses, and the column height.  A chimney two feet tall will have twice
> the weight difference of gas to act on as a chimney one foot tall.  Thinking
> in terms of electricity, stacking the volumes of gas on top of each other
> is like batteries in series and stacking them side by side is like
> batteries in parallel.  The batteries in series will double the voltage
> like the air volumes stacked above each other in a chimney will double the
> buoyancy head.
>
>                A fire temperature of 1600 F with air temperature at 50 F
> to 100 F creates considerable difference in density and weight between the
> fire and the air.  A hot air balloon rises at only 250 F, or a
> temperature difference of 200 F with the air, about 1/8th of the
> difference the fire in our stoves creates.  The density of the gasses in
> the fire is about 1/4th that of the atmosphere.  One could expect to see
> a very lively fire based on buoyancy as the driving force.  This is my
> conclusion, buoyancy is the driving force in our natural draft stoves, and
> most likely in an open fire as well.  I do not know how this will effect
> stove design, but it does help me to understand how a natural draft stove
> operates.
>
>
>
> Kirk
>
> Santa Rosa, CA. USA
>
>
>
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