How does a fan produce air?

The very first thing that is to be known is that a fan does not actually produce any air on its own but it just pushes the air behind it towards you.

Ok, suppose you were to DESIGN a fan. You need to devise a mechanism to push air towards you. Everyone would have seen hand-fans at work (especially during IRRITATING power cuts). In hand fans, we just keep flapping it or waving it .

So, can we make this motion automatic by using a motor , using few connecting wires and supplying electric power. But practically, such a mechanism is very difficult to SUSTAIN. A more ingenious design was developed by SCHUYLER SKAATS WHEELER in 1882 which can be developed easily and is affordable. Let us try and understand this design.

Basically we have a motor which rotates the blades, and the motor is electrically powered. OK, SO WHAT? How is air actually pushed out? The answer lies in the DESIGN of the blades of the fan.

Ever wondered why the fan blades are bent at an angle? Why are they not flat?

 The very reason why it is bent is for pushing air! Eh? How does it do that?

Let us try and understand this process.  Consider the following diagram:

Here we consider the motion of one of the fan blades, say fan blade 1. When the blade cuts through air in the direction shown, it pushes the air molecules that it hits. For example, the air molecules A, B and C are hit by the blade and they are pushed in the direction shown. They are pushed partially in the downward direction and forward direction. In this way millions of molecules are pushed downward and we can sense this stream of air, standing below.

So what happens when the blade goes through this region completely?

If all the molecules are pushed down, is the region left to be empty? No. What happens is as such an “empty space” or “VOID SPACE is created, the air molecules behind/above the fan blade fill this space up immediately. We have a situation which is something like the schematic shown.

Then these molecules occupy the same region. In other words, instead of A, B, C molecules there will be some other molecules in the same place. Now these are pushed down by the next blade following blade 1. In this way, air is pushed downward with the rotation of the fan.

Now there is still another question. If the air molecules from the top of the fan keep filling this “VOID SPACE” constantly and if they are also subsequently pushed downward, then will there come a time when we have no air above the fan?? Well the answer is “NO” again.

This is better explained by the following schematic:

Hence, the air around you is just “re-circulated” but you feel the air from the fan because the air stream reaches you with high speed. The next part of re-circulation is not observed/felt because it occurs slowly, smoothly and automatically.

Now that you’ve got a glimpse (I hope you have!) of fan-working, try to see if you can reason out the following:

1. If I increase the number of blades, will the fan produce more air? Then why not 4, 5 or 6 blades instead of just 3?
2. What will be the problem with 2 blades? (Well, this has a little common sense to be used in it…nothing much to do with all the previous theory)

-         Article done by Ahmed Sammer Khan