How a fan moves air?

Question

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?

Answer 1

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").