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Why does smoke go faster up a taller chimney?

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vtsteam:
Thank you all. I feel I'm getting closer, but not quite there yet. Awemason is giving me a close to feasible mental picture. But still not quite there.

Archimedes decides to test this by sinking a pipe 200 mm in diameter and 3 meters long a meter deep under water. Ends capped. Horizontal orientation.

He opens both end caps and the less dense fluid flows out and water flows in. Now he does the same with a pipe twice as long.

Will the water flow out at a much higher rate just because it is longer? I have a feeling not. But maybe I'm wrong. I own a fish tank and when moving tubes around, length doesn't seem to be a factor.

Now you might say a chimney is vertical not horizontal. But I chose horizontal to negate the effects of the rapidly increased pressure in water as you work in the vertical orientation.

In air, pressure increase over 3 vertical meters is negligible by comparison. Unless you believe that such a small pressure differential is the cause of a very rapid increase in draft in air by raising a chimney 3 meters.

awemawson:
I think you have the wrong concept. The lighter smoke is floating in the heavier air as the log floats in the water. Air is a fluid don't forget. The upthrust on the smoke is due to it's floating as it is less dense. If you have twice as much smoke you have twice as much upthrust all other parameters being the same.

vtsteam:
I can easily visualize that a lighter gas rises within the medium of a denser one, until they mix.

What I don't understand is why when you isolate the two from each other in a pipe that they affect each other. And in proportion to the length of the pipe.

If you say it is at the top where the draw occurs, then why would it be stronger if the pipe is lengthened?

awemawson:
Isolating the flue gas from the air ensures that the density difference is maximised. The upthrust acts on each and every molecule of the flue gas, not just at the top.

vtsteam:
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.

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