Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 29/04/2012 11:25:01
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Darrell McCune asked the Naked Scientists:
Hello Chris,
Re: the Dyson Air Multiplier bladeless fan. Can the process be reversed and is there a theoretical limit to the size of such fans ?
In other words if a football field sized fan was constructed and a steady wind passed through the bladeless ring would a powerful air jet turn the impeller which could then be modified to generate electric power?
Best wishes ,
Mac.
What do you think?
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No.
How the Dyson Bladeless Fan Works. (http://electronics.howstuffworks.com/gadgets/home/dyson-bladeless-fan.htm) But it's way clever :)
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Or, maybe?
But not through that design, as I think.
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It could be said the Dyson design is kind of the opposite to the venturi effect...
..a high pressure low-flow pump inducing a lower pressure high-flow airflow.
On that basis the obvious equivalent in reverse is an enormous long tapered venturi with a relatively small turbine blade at the end.
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The Dyson bladeless fan actually does have blades in its base.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.dyson.ie%2Fimages%2Ftechnology%2Fddm%2FddmWhite.jpg&hash=ac14ff0510be777feea2c456ddd710b0)
It uses the so-called digital motor to spin its blades extremely fast, thus presurizing the space between its inner and outer rings. The pressurized air passes at very high speed thru a Venturi between the two rings at the back of the large opening. This generates low pressure around the opening, sucking in air from the back and blowing it out the front.
Because it depends on the very high velocity of the spinning blades in the base, its size is limited by the strength of materials in the motor. The size could be increased by using many motors, instead of just one.
I am skeptical of the claim that it is more efficient than an ordinary fan. I think large, slow blades move air more efficiently than small fast blades.
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The one big advantage of the bladeless fan is safety. Put your fingers in one and they come to no harm. Put your fingers in a bladed fan and you might find it hurts somewhat. You might even find yourself minus a few digits.
Apart from the aesthetic point of view, one of the major problems associated with wind turbines is bird strike. Now if you could have a bladeless turbine, that would solve that problem.
As far as aesthetics go, I find these turbines quite majestic, though I have to admit, I would not be keen to see the country side littered with them in every direction.
As for bird fatality, it is estimated that between 10 and 40 thousand unnatural bird deaths per year in the USA can be attributed to wind turbines. Not a very nice situation and hardly surprising that it is a situation cited against the installation of wind turbines. But I don't hear any such objections to communication towers, which are responsible for 40 - 50 millions bird deaths per year, or cars, responsible for 60 - 80 million deaths per year, pesticides 70 + million, windows & power lines, well over 100 million each and, of course, the domestic cat and the feral cats resulting from poor pet keeping, which are responsible for many 100's of millions of bird deaths per year.
Perhaps people need to get a little perspective into their lives.
Anyway, I don't think bladeless turbines are going to pop up at any time in the near future.
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No.
How the Dyson Bladeless Fan Works. (http://electronics.howstuffworks.com/gadgets/home/dyson-bladeless-fan.htm) But it's way clever :)
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Or, maybe?
But not through that design, as I think.
Based on that, no. You couldn't run it in reverse. The air is emitted from a small vent in the "ring" of the fan. To run it in reverse, you'd somehow have to channel the passing wind into that vent. If you did build a funnel to channel the wind into the right place, you'd be losing efficiency because the air would resist being compressed.
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I am skeptical of the claim that it is more efficient than an ordinary fan. I think large, slow blades move air more efficiently than small fast blades.
I think you are correct. That's why the fan jets of aircraft keep getting bigger and bigger. High speed blades produce a lot of wasted energy in the form of friction.