This question is related to the fans and compressors in jet engines.

(Please Note: I know how a propeller driven airplane works. The propeller has the shape of the airfoil and when this propeller moves through air, a pressure difference is created between the front and the back of the propeller. This causes the airplane to move forward)

Most of the fans that I have encountered till now merely recirculate the air in an enclosed area (for example: the ceiling fan recirculates the air already present in the room)

I am comfortable with the idea that fans recirculate air when they rotate. But my intuition is failing me when I think of fans (exhaust fans) that suck air out of an enclosed area.

The exhaust fan sucks out air from an enclosed area (ex: room). The fan in a jet engine sucks in air from outside into a enclosed area.

I would appreciate it if someone could explain how these fans manage to suck-in/suck-out air. Does it have anything to do with the shape/orientation of the blades of the fan?

  • 2
    $\begingroup$ neither the room nor (and especially!) the jet engine is an enclosed airtight area. $\endgroup$
    – Federico
    Feb 2 '17 at 12:04
  • 1
    $\begingroup$ The air being pushed out the front of the fan has to come from somewhere. Put your hand behind your desk fan, you will feel a slight breeze there as air rushes in to fill the low-pressure void created by the spinning blades. The air flow won't feel as strong as it does in front of the fan because it's pulling from a large area, while the column of air in front of the blades is much more concentrated (for a while). As you move your hand away from the front of the fan you'll find an area where the air pressure feels about the same as it does right behind the blades. $\endgroup$
    – FreeMan
    Feb 2 '17 at 15:36
  • $\begingroup$ @FreeMan thank you for your answer. The desk fan explanation was quite good. If I find myself near a desk fan, I will check if there is a breeze behind the fan. $\endgroup$
    – vrn
    Feb 2 '17 at 16:00
  • 2
    $\begingroup$ @vrn That would certainly be easier than trying to feel a breeze above a ceiling fan! :-D $\endgroup$ Feb 2 '17 at 16:10

The answer in all cases is the fan accelerates air, creating a low-pressure area, and more air moves in to fill the low pressure.

Consider a ceiling fan. As it runs, the air in the disk of the fan is accelerated downwards. This means that the space where that air used to be is now at lower pressure, since some of the air has been removed from it. Air below the fan is moving downwards, away from the fan, so it's difficult for it to fill the low-pressure zone. So the air that moves in will come from the top and the sides.

If you put the fan inside a tube (a duct), the air can't come in from the sides, so it must come from above. You now have something conceptually similar to a vertically mounted jet engine, except that it's much simpler (actual jet engines have multiple fans) and it's powered by a little electric motor, rather than by injecting and burning fuel.

If you want a pump, the fan doesn't care if you put in a false ceiling that divides the space above the fan from the space below it, with the duct and the fan as the only connection between the two. Since no air can get into the area above the fan, the fan will pump the air out of that zone. Eventually, the pressure in above the fan will get so low that air the fan is accelerating downwards will start to flow back up into the evacuated zone. This is why any particular fan will only lower the pressure by a certain amount.

  • $\begingroup$ thanks for being the first person to answer this question. I have marked this as the accepted answer. The concept is very clear now. Thanks again $\endgroup$
    – vrn
    Feb 2 '17 at 16:04

It's very simple. It's the orientation of the blades and the direction in which they're turning. You notice that the blades are placed in a way that they push the air also causing a vacuum from the back obviously. Take ur fan for example and observe the position of the blades and the direction they rotate in. Use ur visualizing skills and see that as the blades rotate in that direction they push that invisible air causing a suction from behind.


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