Hello:
Interesting article Ron. From that article:
# Fan rotation speed is a major factor in sound propagation
dB 1 = dB 2 + 50log 10 (rpm 1/rpm 2)
Thus, a fan moving at a 20% slower speed should exhibit a 5 dB drop in sound level
# Other factors:
- Vortex shedding: turbulent eddies in the wake of the fan blade tips
- Turbulence due to the obstructions in the intake or exhaust wake
- Fan motor vibration
- Harmonic resonance associated with the number of blades
- Fan interaction with compressor noise
The first point is of particular interest, of course -- and it points in the direction of getting more air flow from lower RPM's.
And the point that you mentioned, as well as the "vortex shedding" both seem to indicate that the captured blade ends is a good thing.
As for why any sweep is needed -- from what I can glean from quiet fans fans that I've seen, is that the leading edge needs to have a gentle angle of attack (to minimize turbulence above the top surface of the blade), and then the surface needs to curve to accelerate the air. The angle of the trailing edge is what is going to determine the velocity of the air flow.
What the sweep does is it allows this acceleration to be less abrupt -- since the length of the curve (of the section, or airfoil) of the blade is longer that is possible with a straight blade (with no sweep), the acceleration can either take longer and/or get to a greater angle at the trailing edge. (I'll try and draw this in DataCAD later on, after I boot to Windows; I'm running Linspire right now.)
The number of blades is important -- I think there is a big benefit to having an
odd number of blades. This seems to be borne out by the fact that almost
all fans have 3, 5, 7, or 9 blades. The only exception I can thing of is the AeroCool with 16 blades.
Why does this matter? My best guess is that an even number would cause the harmonics of the
sound produced by
each individual blade to
sum up and
increase the sound pressure level -- because the even harmonics are going to reinforce each other. I.E. the peaks of the second harmonic will line up with of every other peak of the fourth harmonic. The odd harmonics are going to be out of phase with each other: the peaks of the first harmonic lines up with the trough of the third, and therefore it subtracts from the sound, rather than reinforcing it.
[Edit] I was thinking more on this: it may have more to do with the RPM and the harmonics produced by each blade. If there are an even number of blades, and the fan turns at 1000RPM, then the pitch of the fan (1kHz) will have strong pitches at the even multiples: 2kHz, 4kHz, 8kHz, etc., to be emphasized, and these are the same frequencies as the harmonics. If there are an odd number of blades, then the frequencies are distributed over a greater number of nodes. [/Edit]
Now I could be all wet on this! But I think, at least that there is
some good reason to have an odd number of blades. It also seems that the larger the fan, the more blades it can have and still be practical, and so it can provide a larger surface area actually doing the
work of pushing air.
Lower RPM fans can have more blades and more surface area. High RPM fans need to have fewer blades in order to avoid "cavitation" when air cannot move fast enough to move into the low pressure areas after each blade passes by...