How Fans Move Air: Propeller vs. Blade Shape

[bnvbnv]

If fan blades move in a circular way how is the air moving towards you (in front of fan) ?

Also propeller and fan have different shapes, does it mean they work different?

 

[ModusPwnd]

They create pressure on one side and reduce pressure on the other. A difference is pressure is what causes air to move. The shape of the blade pushes air on one side, creating higher pressure. On the other side a void is left, creating low pressure.

 

[bnvbnv]

Let's say u sit in front of a fan.
In a fan like that if I am not wrong it is moving in the right direction (clockwise). By the shape of blades I assume they create high pressure on a side closer to you but it would mean air would travel backward to a side further away from you (from high pressure to low pressure). I don't get it (or my reasoning is wrong).
http://www.mh-china.com/UploadFile/2006112014312518080.jpg

 

[rcgldr]

There is some outwards flow, but most of the flow will be forwards. There is an increase in pressure from behind the fan to in front of the fan, but the blades move fast enough to capture the air and force it to flow forwards instead of backwards despite the pressure jump. Behind the fan, the air accelerates towards the fan as it's pressure decreases below ambient. As the flow passes through the fan blades, the pressure increases above ambient, but the speed of the air right at the fan blades remains about the same. Then the air continues to accelerate as it's pressure decreases from above ambient back to ambient.

As to why the air flows forwards instead of outwards, it's because of the angle of the blades. If the angle of the blades was perpendicular to the direction of rotation, the flow would be outwards and not forwards, but the angle is much less than this, so the direction of the "pull" and the "push" from the blades is mostly forwards and somewhat circular and/or outwards.

 

[davenn]

Hi bnvbnv
welcome to the forums

By the shape of blades I assume they create high pressure on a side closer to you but it would mean air would travel backward to a side further away from you (from high pressure to low pressure). I don't get it (or my reasoning is wrong).

it can't trave back because of the air that is still getting pushed forward from behind it

instead it moves out into the lower pressure area where you are sitting

Dave

 

[A.T.]

it would mean air would travel backward to a side further away from you (from high pressure to low pressure).

Air has inertia so it cannot immediately turn around, when it passes the fan. In an enclosed room it does travel back, just not through the fan but around it.