How does a fan moves air towards you (I mean in 1 direction). Also propeller and fan have different shapes, does it mean they work different?
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1possible duplicate of What is going on in front of and behind a fan? – Kyle Kanos Sep 12 '14 at 00:52
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1While not an exact duplicate, the linked Q&A provide the answer to your query. – Kyle Kanos Sep 12 '14 at 00:52
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There's an interesting discussion to be had about Also propeller and fan have different shapes, does it mean they work different? and this isn't covered by the proposed duplicate. – John Rennie Sep 12 '14 at 04:57
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A fan changes the average velocity of air molecules. This can be seen as a molecular scattering process for very thin gases, whereby every single molecule hits the surface of the angled rotating fan blade in such a way, that an axial velocity component is imparted on the molecule. Since a single rotating fan blade can not do this without also imparting a radial component, such a fan is not very effective. Thankfully much of that radial velocity component can be transformed into an axial one by using a second angled blade that either stands stands still or does a counterrotation. This is the reason why technical fans are usually combinations of rotors and stators (the stators also serve as very important stabilizing elements of the fan housing).
For gases of sufficiently low density, this picture holds reasonably well if we replace the molecules with small volume elements of gas.
For dense gases propellers act a lot more like lifting surfaces, i.e. we have to calculate the interaction the same way we would for wings. That, unfortunately, is a significant fluid dynamics problem that, strictly speaking, also needs to take turbulence at the wing tip (i.e. the fan perimeter) into account. I am not going to get into that (for one thing I don't understand it well enough, for another, I don't want to kick off a fight about "how planes fly").
CuriousOne
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I think you should also mention the bernoulli principle-at least i think plays a part in moving the molecules in one particular direction. – soumyadeep Sep 12 '14 at 04:52
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If I mention the Bernoulli principle, we will have the "How does a plane fly?" fight... and I don't want to go there, because I am not in the school that buys into the hundred year old textbook explanation, but I don't have the authority to explain the actual physics of wings. – CuriousOne Sep 12 '14 at 05:11
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Bernoulli is right. Conservation of mass, conservation of the number of particles. He elegantly expressed that the velocity of the particles, combined with their density result in pressure. Therefore, particles traveling faster create low pressure. The pressure differential between one side and another causes the object to move. What part doesn't make sense? – user3533030 Sep 12 '14 at 05:21
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1One of the problems with Bernoulli is that you can make a very good plane out of a barn door (there should be plenty of videos of flat flying surfaces on the internet) by choosing a correct angle of attack. Planes can fly upside down just fine, too. And I have seen plenty of physicists who couldn't explain the function of a weather wane with it... in short, it oversimplifies a complicated dimension dependent problem. And that is all I am going to say about it. – CuriousOne Sep 12 '14 at 05:39