Physics of Air Fans: Number of Blades & Flow Rate

[C_S]

hey guys,

I'm new here but i was a bit stuck on thinking about how air flow is effected by a fan. it's probably a really simple question, but it's got my cobwebbed brain a bit stumped!

my question is in two parts:

1. how does air flow relate to the design of a fan? ie, an answer with respect to the number of blades and blade surface area is what i am after. i have a feeling that if you increase the fan blades' surface area in any way, you therefore increase flow?

2. following on, if you have two separate axial flow fans whose dimensions are completely different, but turning at the same speed and have the same overall blade surface area, will you see both fans flow the same amount of air?

thanks in advance for any help

cheers

 

[Danger]

Regarding #2, I think that one with 'steeper' pitch to the blades should have a greater flow rate at the same rpm. It bites off a bigger chunk of air during each revolution.

 

[Mk]

Those higher-tech new GE jet engines have really steep-pitched blades and very many. Jet engines all have very many blades close together. Why is that?

 

[AlephZero]

Those higher-tech new GE jet engines have really steep-pitched blades and very many. Jet engines all have very many blades close together. Why is that?

The reason for the steep pitch is because the engine has a velocity forwards, which is equivalent to the air having a velocity backwards relative to the fan. If the pitch was shallow, the blades would effectively block off the front of the engine. A typical "domestic" fan (e.g. a room fan, or the cooling fan on a car radiator) doesn't have air already flowing towards it at hundreds of meters per second, so the blade pitch is more shallow.

The number of blades is a compromise. When you design a fan like that you want to do work on ALL the air that goes through it. If there are more blades, each blade has to do less work, therefore the forces and stresses in the blade are smaller, so the blades can be made lighter. On the other hand if there are too many blades, they block up too much space and that restricts the amount of air that can get through the gaps between them.

You don't say which particilar GE engine you are talking about but most large jet engines have a similar number of fan blades - usually between about 22 and 26. It's not surprising the numbers are similar, all the fans are working in the same air at similar RPM and at similar aircraft forward speeds.