|MadSci Network: Physics|
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 your biggest challenge. Sincerely, Steven Miller Undergrad - Mechanical Engineering San Diego State University
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