Gallery, Projects and General > The Design Shop
Why does smoke go faster up a taller chimney?
Lew_Merrick_PE:
You are missing the simplest explanation. When you fly a kite, it gets more lift/drag the higher it goes because, generally, wind speeds are greater the higher you go in the atmosphere. The wind passing over the top of a smokestack creates a Bernoulli vacuum draw. The higher the wind speed, the greater the draw. The taller the smokestack, the faster the wind passing over its exit moves.
vtsteam:
Mine draws in a calm. And the length relationship works whether the wind is blowing or not.
awemawson:
--- Quote from: Lew_Merrick_PE on February 27, 2013, 01:36:34 PM ---You are missing the simplest explanation. When you fly a kite, it gets more lift/drag the higher it goes because, generally, wind speeds are greater the higher you go in the atmosphere. The wind passing over the top of a smokestack creates a Bernoulli vacuum draw. The higher the wind speed, the greater the draw. The taller the smokestack, the faster the wind passing over its exit moves.
--- End quote ---
But a flue draws very well even on a calm day so long as it has warmed up. The solution I reckon is mainly gravity driven. The lighter hot gases rise.
John Hill:
There is a column of air from the surface of the earth to the top of the atmosphere, the pressure at the bottom is about 15 psi. If you built a chimney right up to the top of the atmosphere and filled it with a gas which weighs half that of air the pressure at the bottom of the chimney would be half, about 7.5 psi. So the difference between inside and outside at ground level would be 7.5 psi.
Go to 12,000' ft or so and the air pressure will be half sea level, about 7.5 psi and the pressure inside our imaginary chimney will be 3.75 psi. We can see then that the difference between inside and outside the chimney is reducing with altitude.
Warm air as in a chimney is lighter than cold air and therefor rises by the effects of displacement. The air in the chimney would shoot out the top very fast if it were not for the pressure of air outside and as we know the pressure difference between inside and out decreases with altitude we can see that the chimney air will rush out faster from the top of a high chimney.
John Hill:
--- Quote from: vtsteam on February 27, 2013, 11:50:25 AM ---Does the pressure of a jet engine compressor have to exceed the pressure in the combustion chamber when the engine is not moving forward and gaining ram effect?
--- End quote ---
Yes, the compressor stage of the jet engine is the major part of the engine, it takes up about half the length of something like a RR Avon and starting the engine involves spinning the compressor stage to get pressure in the combustion chamber(s).
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