stack effect

[struct_eeyore]

Hi all,

Not a fluid engineer, so forgive my ignorance. I'm hoping someone can direct my to the proper way to calculate air flow rate in a open ended stack. Stack is 100' high, bottom is about 10' away from ground surface, 10' in diameter; a 6 ft/sec wind is blowing across the top. Temperature differential is negligible.

 

[gerhardl]

For a laminar air flow (not exceeding the speed of sound) you will get out of the pipe what you put in (or force through with a fan), less friction loss, and adjusted for pressure and temperature difference at input and output. For most normal purposes (your case?) you can neglect both friction and pressure/temperature differences.

 

[IRstuff]

I think you're asking about the Venturi effect; however, the bottom of the pipe is fairly close to the ground, so there's probably some ground effect as well. I didn't run across a clean example of what you want, but if you assume there is some level of constriction as the wind travels across the top of the pipe, that'll give you a pressure differential that you can then apply to the height of the pipe to determine how much flow you actually get. BUT, being that this pipe is rather large, it's unclear to me how well the math really fits the problem.

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[gerhardl]

Thought about 'natural draft', but could not imagine a real use for the construction. Could you give us more details?

 

[SlideRuleEra]

Temperature differential is negligible.

Flow will be negligible, too. No energy to drive the system, Second Law of Thermodynamics... there is no free lunch.

http://www.SlideRuleEra.net

 

[struct_eeyore]

Well, we utilize this principle in perfume bottles, so it certainly works.

 

[LittleInch]

Without some sort of eductor, aerofoil device or wind powered ventilation spinning hat your flow is negligible - but it might make a nice humming noise.

Like this

https://hvacsolutionsdirect.com/cat...f-Caps/FAMCO-WIND-DIRECTIONAL-Vent-Cap-SKU722

or this

https://www.askthebuilder.com/roof-turbine-vents/

stack effect is when you have a temperature and or density difference in the air in your stack compared to the outside air.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[btrueblood]

"we utilize this principle in perfume bottles"

Never seen a stack on a perfume bottle, but ... realize if you had perfume in a bottle, the vapor above the liquid is something like 90% alcohol, and may have a fairly low density relative to air, the driving force is the vapor pressure of the fluid at the ambient temperature, and the low concentration of alcohol vapors in the surrounding airstream (thus diffusion comes into play too).

 

[LittleInch]

Most perfume bottles I've ever seen incorporate some sort of pump action which forces liquid through a small nozzle vapourising the contents in the process into a spray.

So not the same thing at all.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[btrueblood]

Yeah, LI, but olde-thymey perfume bottles had a rubber bulb that blew air through a venturi, like what IRstuff was saying...more like a carburetor.

 

[gerhardl]

I am waiting for structeeto bring some structural order to the discussion.

By the way: perfume consists of oil and a solution of alcohol, which will evaporate and rise. I believe the contraption is not going to be used at a perfume factory.

 

[struct_eeyore]

I was always under the impression that a faster moving fluid will have a lower static pressure than a slower/stationary one - so a Venturi effect of sorts, except at the interaction between two streamlines in this case. Also, am thinking there might be some sort of "shear drag" that might pull air out (is that even a term/mechanism>).

 

[SlideRuleEra]

structee - The the air in the stack has to be "lifted" out the top; since air has weight (634 pounds of air in the 100' high x 10' diameter stack), energy is needed.
There is no temperature differential to provide energy.
There is no means to extract energy from the wind.
Therefore, no energy to do the "lifting".

Before the stack liner is installed on a large stack, such as this 600' high shown under construction at one of our generating stations, there is essentially no air flow up and out of the concrete shell early in the morning. Later in the day, there is a noticeable air flow - the shell has become an inefficient solar collector. Air inside the shell is heated a little bit, then rises out the top, drawing in cooler air at the bottom.

After the liner is installed, and the station is operating normally (at a high temperature differential), natural draft will replace provide air flow equivalent to a fan powered by a motor rated at a few hundred horsepower.

http://www.SlideRuleEra.net

 

[itsmoked]

So if there is 6ft/sec across the top and 0ft/sec at the bottom there is still no draft?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SlideRuleEra]

Keith - No draft... for the conditions stated for this thread. The change in horizontal wind velocity with elevation is horizontal wind shear. Since the problem for this thread specifies negligible temperature differential, any heat production by friction from wind shear has to be ignored.

http://www.SlideRuleEra.net

 

[3DDave]

If there was a motivation from this air would move up on its own.

Now, if there was a barrier, like an elbow aligned with the downstream airflow, then that might induce some flow, but cut off at right angles to the flow - that's the alignment for a static pressure measurement, which is lower at the top of the tower by the delta-P due to the mass of the air in the tower.

 

[Compositepro]

Gravitational effects are not relevant because air inside and outside the stack are both affected equally.
"Stack effect" refers only to pressure or flow caused by temperature differences.

 

[IRstuff]

I think the OP was assuming 6 ft/s velocity at the top and zero at the bottom. Additionally, the air flow must compress to get around the stack, so the velocity is even higher.

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[itsmoked]

"Stack effect" refers only to pressure or flow caused by temperature differences.

This is the real deal breaker. It is the definition.

Slide you're saying there could be air movement because of venturi effect at the top but NOT due to 'stack effect' because there's no heating going on due to the wind?

Thanks Slide and Comp!



Keith Cress
kcress -

http://www.flaminsystems.com

 

[SlideRuleEra]

Slide you're saying there could be air movement because of venturi effect at the top but NOT due to 'stack effect' because there's no heating going on due to the wind?

For the problem, as defined by the OP, there is no venturi effect. There is no venturi:

Any stack effect air flow would have to be caused by a temperature differential, but there is no temperature differential.
Therefore, no stack effect and no air flow... no matter how fast the horizontal wind velocity or how much horizontal wind shear.

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