MadSci Network: Physics

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


    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 as 

    "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.",5716,80998+1+78866,00.h

    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.

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 
your biggest challenge.

Steven Miller
Undergrad - Mechanical Engineering
San Diego State University

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