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Re: [PSUBS-MAILIST] Dyson's Bladeless Fan

Actually the inefficiency of an ejector is caused by the need to transfer momentum from one fluid stream to another, the two initially having very different momenta. In the ideal case assumed in textbooks, the transfer is by interlaminar fluid shear, and already very lossy. In the real world things are much worse because there's turbulence. The ejector shroud and internal vanes (if any) can be designed to mitigate some losses, but not to eliminate those that are inherent in this type of momentum transfer. If you have to use an ejector, you're better off pulsing the primary stream.
If your concern is for the fish, I would just make a note in my head not to apply thrust willy-nilly, but instead to ramp up power gradually, giving the finny passers-by a chance to sense the disturbance and leave. Also might save cavitation damage to the propeller. 

Best,
Marc

On 4/5/2012 12:37 PM, Alan James wrote:

OK Mar,
I'll place a fish net behind the fan & have a burley dispenser.
If I remember rightly you've written a book on ducted fans.
Is the claimed efficiently purely due to the ducting as in a kort nozzle?
Alan


----- Original Message ----- From: "Marc de Piolenc"
<piolenc@archivale.com>
To: <personal_submersibles@psubs.org>
Sent: Thursday, April 05, 2012 4:04 PM
Subject: Re: [PSUBS-MAILIST] Dyson's Bladeless Fan



I'm not sure that eliminating exposed blades makes much difference to
sea life. In hydroelectric power, much study has been given to the
effects of turbines upon fish, and it was learned that fish kills by
turbines are the result, not of blade impact, but of pressure
gradients. An ejector like the Dyson "fan" has wicked gradients, so a
fish aspirated by it might well emerge apparently intact, but dead
just the same.

Marc

On 4/5/2012 11:48 AM, irox wrote:


Note, the "bladeless" fan does in fact have blades, but they
are tucked away in the base. It is effectively sucking air
into the base and pushing it along channels/tubs to be output
around the trim of the ring.

Maybe it has some use for underwater vehicles, such as an
environmentally sensitive area (no exposed blades to hit
sea creatures). But as Marc points out, it would come with
an efficient hit.

Cheers,
Ian.

--
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Ducted fans: http://massflow.archivale.com/



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--
Archivale catalog: http://www.archivale.com/catalog
Polymath weblog: http://www.archivale.com/weblog
Translations (ProZ profile): http://www.proz.com/profile/639380
Translations (BeWords profile): http://www.bewords.com/Marc-dePiolenc
Ducted fans: http://massflow.archivale.com/



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