Gallery, Projects and General > The Design Shop
Why does smoke go faster up a taller chimney?
Pete.:
--- Quote from: vtsteam on February 27, 2013, 09:54:41 AM ---You haven't explained why it acts and how it acts. Is it like a magnetic field acting through the pipe walls?
But I think I understand why, now.
The slug of smoke in a chimney is an object with boundaries. It is related along its length, not a disconnected length of random relationship. It is an object..
And the pipe, being fixed in position, is a representative of the external density to the object of hot gas.
And likewise the pipe is a representative of the internal mass to the surrounding air, minus its actual mass. Thus the pipe itself would have some upthrust, even though the ends are open. Not enough to lift it. But a small amount. If you sealed it and made it light, it would be a hot air balloon, and could rise.
The interesting concept to me is that the interface acts in a representative way to both pressures. And the contained smoke is an object.
Now I understand it.
--- End quote ---
The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.
andyf:
Thee's a sort of explanation here < http://en.wikipedia.org/wiki/Stack_effect >, though I don't pretend to understand it :scratch:
I suppose the second equation is only approximate for chimneys and flues (as opposed to simple air convection up a building from an opening at the bottom to one at the top) because the composition of flue gases differs from that of fresh air.
Andy
vtsteam:
--- Quote from: Pete. on February 27, 2013, 03:13:57 PM ---
The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.
--- End quote ---
Nope at the bottom of my chimney it's about 1000F and at the top about 300F.
The bottom ends in a roaring fire in my particular setup.
vtsteam:
I think what Pete means is that the air outside of the stove is colder than that in the flue.
To all the commenters, I'm pretty satisfied with my theory that the stove and stovepipe are pressurized by high pressure in the combustion chamber. And that low pressure at the top of the chimney caused by rapid cooling makes it directional, and therefore a flow.
And that a longer stovepipe represents a longer distance over which the force of pressure continually created in the stove can act on the mass of gasses within.
I do think that the difference in densities of warm and cold air also contributes -- particularly in starting. But I think the pressure generated in the combustion chamber is the primary force once the fire is going well.
If we take our stove and stovepipe and turn them on their side, and put a small blower in the grate we have a classical turbojet engine. I don't think turning it upright diminishes the contribution of thrust up the chimney of expanding gas products in the combustion chamber. I do think the difference in density takes the place of the fan, but like a compressor in a jet engine the directional force provided by the difference in density produces less energy than the combustion chamber provides -- a compressor's main function is enforcing a direction.
Like all theories it can be proven or disproven by an experiment designed to isolate those two factors.
Pete.:
--- Quote from: vtsteam on February 27, 2013, 04:50:38 PM ---
--- Quote from: Pete. on February 27, 2013, 03:13:57 PM ---
The less dense warm air in the flue is pushed up by the pressure of cold air at the bottom of the flue. If the flue is 10 feet long you have a 10 foot column of warm air thus a 10 foot head of cold air pushing warm air up the flue. If the flue is 20 feet long you have a 20 foot head of cold air pushing the warm air up from the bottom.
--- End quote ---
Nope at the bottom of my chimney it's about 1000F and at the top about 300F.
The bottom ends in a roaring fire in my particular setup.
--- End quote ---
Doesn't matter, it takes a fancy calculation to work out the rates but to all intents, the only force driving the smoke UP the chimney is the weight of cold air outside it entering the bottom. The fire just provides heat energy to lower the density of the air in the chimney. There's no 'pressure' produced by the fire or the hot air would spill out the air intake at the base. It's all about the density of the air when heated.
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