### Re: How can I use wind and the Venturi effect to cool a building?

Date: Thu Oct 19 15:05:47 2000
Posted By: Steven Miller, Undergraduate, Mechanical Engineering, San Diego State University
Area of science: Physics
ID: 967236301.Ph
Message:
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Jerry,

While I can't say that it is impossible to use the venturi effect to
cool an architectural structure I will suggest that it would be
impractical.  The venturi effect is described at Britannica.com as
follows:

"Bernoulli's theorem implies, therefore, that if the fluid flows
horizontally so that no change in gravitational potential energy occurs,
then a decrease in fluid pressure is associated with an increase in fluid
velocity. If the fluid is flowing through a horizontal pipe of varying
cross-sectional area, for example, the fluid speeds up in constricted
areas so that the pressure the fluid exerts is least where the cross
section is smallest. This phenomenon is sometimes called the Venturi
effect, after the Italian scientist G.B. Venturi (1746-1822), who first
noted the effects of constricted channels on fluid flow."
http://www.britannica.com/bcom/eb/article/8/0,5716,80998+1+78866,00.h
tml?query=venturi%20effect

Although I am not familiar with the method of making ice with the
wind, that process is viable under the laws of thermodynamics (with
respect to the Venturi Effect) for two reasons - the increased velocity of
the wind could improve heat transfer (forced convection) from the water
and the decreased pressure could lower the freezing temperature of the
water (much as the boiling temperature of water decreases with altitude).
Consulting a P-T diagram (general) for pure substances
(Cengal, "Thermodynamics - An Engineering Approach", 3rd ed. Page 61) we
see that for water (a substance that expands on freezing) the freezing
temperature decreases as pressure increases.   So while this would
demonstrate that the venturi effect could help one make ice by lowering
the freezing temperature - it is irrelevent from a cooling perspective
because although the water exhibits a phase change, the temperature
doesn't change from what it is - just because it's ice at subatmospheric
pressure it's not necessarily colder.  Mountaineers face this problem when
cooking at altitude - even though the water is boiling at high altitude
does not mean it is as hot as at sea level - therefore your food may never
cook even though the water is boiling when in the mountains.

As to the ice cave, I was unable to find that particular one on the
internet but I did find one with similiar characteristics in Iowa.
http://www.state.ia.us/par
ks/decorice.htm

They note on the website that the cave stays frozen well into the summer,
but do not elude to the venturi effect.  Instead, they describe how the
cave (below ground and insulated from the Sun's heating effects) freezes
the water from the spring thaws and that this water remains frozen well
into the summer months.  This may not be the same reason that the ice cave
in South Dakota remains frozen but it does show that the venturi effect is
not required to maintain sub terrainian ice into the summer months.

Because the wind exhibits primarily horizontal motion and the venturi
effect requires a horizontal pipe of varying cross-sectional area we have
two obvious alternatives - one is a hallway as you mentioned of varying
cross section and the other would be a duct of varying cross section,
either being open at both ends to the outside and running through the
building in the direction of wind flow.  The hallway would be out for the
obvious reasons - the temperature drop would only occur in the region of
increased wind velocity hence this would have to be the workspace - how
could you keep your papers on your desk?  The duct would be a more logical
solution.  However, without getting into the Bernoulli equation and the
compressible nature of air, we can determine that in order for this system
to work we would need to transfer the heat out of the building using the
cooled air in the duct.  How do we transfer the heat? (if you channeled
the cool air into the offices, it goes back to atmospheric pressure and it
warms up again.) Conventional heat exchangers in the duct would restrict
flow and would destroy the venturi effect by restricting the flow through
the constriction in the duct.  Minimizing the footprint of the heat
exchanger would minimize the restriction but would also dcrease the
exchanger's efficiency) If the wind changed direction the ducts would be
ineffective.  I'm not saying that this would be impossible but the
evidence points to the unlikely.

Sorry for chickening out on the Bernoulli analysis for a compressible
gas - but we can save that for after the transfer problem is worked out!
It's taken me too long to answer this question as it is!  You could
explore this idea further - the Bernoulli equation and conservation of
mass would help to determine the velocities and pressure changes necessary
as well as the duct dimensions (just make sure you account for the
compressability of air), but I believe the transfer of the heat will be

Sincerely,
Steven Miller
San Diego State University

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