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Circular shrouded blades? Why Not?
Posted: Thu Aug 23, 2007 8:15 am
I know that fans generate noise at the blade tips, that pressure is reduced by leakage between the blade tips and surround. It occurred to me - why not attach blade ends to a circular shroud and rotate the whole thing? This would remove tip noise altogether (because there aren't any), reduce leakage (because there are no gaps, or at least they are smaller and moderately labyrinthine).
The surround itself would have essentially no windage so should not introduce additional drag.
This shape could also work quite nicely with rim/tip driven rotation.
What have I missed?
Posted: Thu Aug 23, 2007 8:47 am
What have I missed?
While I was playing with the TMD fan, it struck me that for really optimal performance, a computer fan's blades need to not only be driven from the rim, but also supported there. The TMD fan gets rid of the bulky motor assembly in the middle, but the fan's spindle and bearings are still central. And so there still has to be a little boss in the middle of the fan and, more importantly, supporting arms coming in from the frame. The arms and the spindle base need to be reasonably thick, to give them some strength in case something whacks the impeller. The TMD fan's three spiral arms are thinner than usual - partly because none of them need to provide a wire-route channel - but they're still right next to the blades, and must therefore still cause considerable flow disturbances.
If the impeller were supported by a bearing around its edge - if the whole impeller were built as an extension of the inner ring of some kind of ball or roller bearing - then there'd be no need for the spider of support arms and the central bearing assembly. Thought I.
It then struck me that, unfortunately, this ain't likely to happen. Not at a reasonable price, anyway. Because stuff would have to move too fast.
The centre support bearing of the 5800RPM TMD fan has to spin at 5800RPM. Duh.
The fan's impeller ring, however, has a diameter of about 70mm (the whole fan's a bit big for a "70mm" unit). So its circumference is about 220mm, and it spins 5800-odd times a minute, or about 97 times per second. So the perimeter's moving at about 21.3 metres per second - around 77 kilometres per hour.
The inner ring of the two centre ball bearings is less than 5mm in diameter; it's unlikely to be moving faster than about 1.5 metres per second. Maybe 5.4 kilometres per hour, tops.
Now, the inner bearings are teeny, but unless the balls in them are really amazingly small, they won't be taking a great deal of punishment.
Replace the little bearings with a big outer ring and even if it used big beefy balls, they'd still be ripping around the bearing at a hilarious rate of knots. If you tried to avoid making a large precision bearing by putting a rail arrangement around the edge of the fan and supporting it with three or four smaller bearings (which would leave gaps between the supports for the drive coils, which would otherwise have some trouble getting at the rim magnets), then the little edge bearings would be spinning even faster. A 10mm bearing in contact with the edge would have to spin at more than 40,000RPM.
Centre bearings are, therefore, here to stay. Well, unless and until someone comes up with an air-pressure or magnetic or similar clever-clogs non-contact bearing design that'll work in a moulded plastic fan that sells for less than the price of a Rolex.
Posted: Thu Aug 23, 2007 9:26 am
Ah, I'd seen them a while back but not noticed that they had the shrouded blades as well as tip driving. Seems dumb that their sizes are off - I guess they need to make their corner drivers smaller. Might be interesting to see a centre-driven fan using it though.
As for edge bearings, it would seem an idea place to use magnetic bearings to suspend the fan. As that site says, using mechanical bearings at the edge is a non-starter.
Posted: Thu Aug 23, 2007 9:54 am
wouldn't stabilising the rotor in 3 dimensions be rather difficult with magnetic bearings alone?
Posted: Thu Aug 23, 2007 10:19 am
Probably. My main thing was really to see fans like the impeller the TMD ones use. It would seem an obvious way to improve throughput, pressure and noise, I just wonder why it hasn't been done.
Posted: Thu Aug 23, 2007 1:08 pm
As I wrote somewhere else here a lot of fan noise comes not so much
from the tips (perhaps at very high speed) but from the four ribs attaching
the motor to the frame.
The problem is that the ribs are too close to the trailing edge of each
blade which has an eddy swiping over the ribs, each blade creating
a hum with 4x the frequency of rotation (but phase shifted).
I have just bought a Silenx 120x120x38 mm fan with 60 cfm at only 14dB
and like most things from Silenx the 14dB is a lie and the actual hum
a very real and loud disappointment.
When I cut off the ribs and run it (just motor and fan) it is then 14 dB or even
less, mostly air woosh (and any motor rattle or whistle if present).
If you then bring up a bar under the fan the hum begins slightly at 10 mm
distance from the trailing edge and louder as you get closer.
Now you know why the fan is so loud.
The solution is to find a way to remount the motor with ribs (thin steel wire would
be better) 10 mm from the trailing blade edges.
A wooden disc glued on the hub or some simple clasp system (if you want to remove it)
and you have the quietest fan you ever can expect to have.
For most PC applications the frame is not necessary - just a circular wooden
or plastic disc to attach the motor hub, and a cross with attachment holes at
the ends for ribs. It would be perfect. On some quieter CPU coolers they have
adopted this approach to minimize noise.
A circular shroud might make a difference but not a lot. There is an energy
loss in air moving over the increased area as a penalty.
I am thinking of devising a simple remounting kit so that anyone can take
his typical noisy fan, cut it off the ribs and attach it to a new mount and
all the fuss is over.
The problem is with the standardized 25 or 38 mm fan depth which forces
any manufacturer to juggle fan blade depth (and volume flow) against the
inevitable eddy interaction with the ribs. If they would just think about it
there would be no forum like this but they are either fossils or afraid the
increased cost, or deviation from a standard, would make it a lemon.