# Shouldnt the wind generated from the fan blow the dust away

#### Evil

i just woke up and i looked at my electric fan...and noticed a lot of dust had accumulated on the fan...anyone haf any idea y? shouldnt the wind generated from the fan blow the dust away?

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#### Mentat

Actually, the fan sucks the dust in (from the back), obviously, but then the dust gets caught on the front of it. While some of it is blown off (noticed any congestion lately? ) some of it is just forced down (on to the vent panels) by the air.

#### Evil

but wont those that got stuck get blown off? afterall it is very near the blades of the fan where the wind is the strongest.

#### Phobos

Staff Emeritus
Gold Member
Believe it or not, the air velocity at the surface of the fan blade is zero (that's fluid mechanics 101). The air velocity quickly rises as you move away from the blade surface until you reach the full stream flow velocity. But a grain of dust is probably bigger then that zero-velocity boundary layer. So, perhaps there is some mechanical sticking (building upon imperfections in the blade surface and then dust sticking to dust like velcro) in addition to static-electricity sticking.

Let's assume that your common sense is right and 99.9% of dust is blown through the fan (no sticking). Running the fan for hours and hours moves a lot of air/dust volume through and that 0.1% of dust that sticks eventually adds up to what seems like a lot.

#### Evil

you mean the air at the blade is still? wow... can u elaborate more on the fluid mechanics 101 and the zero-velocity boundary layer part?
thanks

#### Loren Booda

Accumulation of dust can actually be a boon to those wishing to improve the aerodynamics of airfoils. First, by reforming the blade to the shape of the dust added, a more effective airfoil may be created. Secondly, weighing the dust between the initial and successive trials in a controlled environment gives a measure of how effective such reshaping might be. Thirdly, a highly charged shape may be introduced into a wind tunnel with fine particles meant to stick to, and gradually modify the shape of, its accumulating airfoil.

#### Evil

wat about charging the blades so that the air around it is ionised? that will kinda repel the dust particles and preciptate it out. the only flaw is that anyone one touching the charged blade will get a shock!

#### STAii

Actually i think that charging the dust particles will make them get more and more accumulated.
That is because each dust particle will not get a single charge, it will not be charged by induction (since it is not an inductor), therefore will have one side with a positive charge, and the other with a negative charge.
The thing that will make the opposite charged sides of the dust attract each other, accumulating more and more dust.

Another thing, the fan blows wind by making a change in pressure between the back side of the blades and the fore sides of the blades, now this happens because the blades are 'twisted' (not in a single plane).
Nearer to the middle of the blade (and very near to its surface), the speed of the wind will almost be zero, because the blade itself is blocking the wind, and the wind cannot have another path to this point (it will be a very hard turn for the wind).
This is only my explanation, not a proffesional one .

#### Evil

i was just wundering..are dust particles electrically charged?

#### STAii

By default, they are not charged.
But they can get charged very easily, by (i forgot what it was called, but it is a way of charging without making the two charges into contact ..)

(I know that my english sucks, but i am working on it ...)

#### Phobos

Staff Emeritus
Gold Member
Originally posted by Evil
you mean the air at the blade is still? wow... can u elaborate more on the fluid mechanics 101 and the zero-velocity boundary layer part?
Yep, on a microscopic level. You wouldn't notice unless you're particle-sized. Consider flow through a pipe. The flow is fastest through the middle and slower toward the edges where there is more friction.

Maybe I can elaborate when I get some spare time (and dust off my college textbook). For now, I hope this helps...
http://cvu.strath.ac.uk/courseware/calf/CALF/index/nindx.html [Broken]
http://web.cvut.cz/cp1250/fme/k212/personnel/tesar/skripta/i02%5Ea.htm [Broken]

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