<VV>Mo Mo Fan

[AeroNed at aol.com]

In a message dated 7/14/2005 4:49:24 PM Central Standard Time, pp2 at 6007.us  
writes:

Yes air  has weight but it is also compressible. This means that a given 
volume  will weigh more at a higher pressure than a lower. If you block the  
outlet, the internal pressure is going to rise which means the fan is  
moving in a denser medium.



I'm going to chime in on this one. If the outlet of the fan is blocked the  
air has no where to go. Sure the pressure builds up in the plenum but only as  
much as the fan can provide. After that the fan just churns the air. You can  
think of it like a cavitating prop in water (similar but not exactly the 
same).  The power required to turn the fan decreases. The amount the power decrease 
 depends on how well the outlet is sealed and a relationship between the fan 
and  its ability to churn the air versus compress it (basically fan  
efficiency).
 
Understanding how air flows is "easy" when the air is attached to the  object 
it's flowing over. Air can surprise you with what it's  capable of doing when 
it separates from the surface. 
 
Ned