Expert: Paul Soderman - 10/28/2008

Question
QUESTION: Hi Paul- I'm a tyro, so will thank you in advance for your
forbearance. I want to maximize wind air flow through the
blades of a squirrel cage fan using a housing and baffles
to create areas of high and low pressure like those on a
wing. The system I envision has a forward "scoop" which
tapers into a venturi outlet at the top of the fan wheel.
On the backside of the wheel there is a baffle which causes
the air flow coming past the fan blades to tumble, creating
a lower air pressure in the housing below and behind the
wheel. Can you give me your basic thoughts on such a design
for that purpose? And are you aware of any
sources/resources which have perhaps addressed such a
design? I have a very rough and basic concept drawing which
may describe it better, and I'll send it along. Thanks for
your help.

ANSWER: Hi Bob
Not sure what a tyro is.  Your squirrel cage fan is unusual.  Most squirrel cage or centrifugal fans draw air into the center and exhaust out the perimeter (google centrifugal fans and see Wikipedia).  The centrifugal effect creates a nice pressure source.  Yours is more like a water wheel, which is not very effective since it just scoops the air along. Only a small part of the fan circumferance is working for you. I don't think tumbling air will help.  Your inlet scoop is ok.  Maybe I misunderstand your design.
Paul

---------- FOLLOW-UP ----------

QUESTION: Hi Paul
Thanks for your answer,it helped identify the underlying
fatal basic assumption in the concept (sigh). It was more
like a water wheel. A tyro is a beginner, a greenhorn.  

I did Wiki centrifugal fan, and have a question. If the
outlet pressure were lower than the inlet pressure, is
there a gain in torque?

What I want to come up with is a practical basic concept
which efficiently uses air pressure coming in to spin the
wheel and create torque at the fan axle, rather than apply
torque to the fan to create air pressure. I'm thinking that
if lower air pressure at the outlet increases torque at the
fan shaft I could use a conical scoop with a wing shape on
the exterior and direct the lower pressure air stream
across the exhaust outlet.  What do you think? Would there
be a gain in torque under those conditions?  

Thanks, Paul.

Answer
Bob

If I understand correctly, you intend to take a normal centrifugal fan (not a turbofan) force air through the fan in the normal direction and allow the fan to do work.  This is done by inducing a pressure drop through the fan.  The question then is - what energy source is driving the air through the fan.  Or what is causing the low pressure at the exhaust.  Whatever that is will require energy.
Paul