Tuesday, 13 October 2009

The Bladeless Fan

I don't know if you've seen any articles about the new 'bladeless fan' (http://www.ft.com/cms/s/0/5ba62a00-b755-11de-9812-00144feab49a.html?nclick_check=1 for example), but I was challenged earlier today by a good friend to to explain how it works.
There are two key passages in the article:
First: "As a result, Dyson claims the bladeless fan, which works by forcing a jet of air out of a narrow circular slit and then over an aerofoil-shaped blade, is at least as efficient as its bladed counterpart, more comfortable and much safer"
Forcing air out of a narrow slit is going to speed the air up considerably, and then this faster air is going to be passed over an aerofoil shape. This is classic stuff for Bernoulli's principle which is what makes aeroplanes fly. Basically if you have a current of moving air flowing over an aerofoil (wing) shape, the air going over the curved top has further to go than the air going under the flat bottom so it has to speed up to make it to the far side in the same time.
When any moving fluid (air in this case) speeds up, Bernoulli's principle states that it's pressure goes down and vice versa. So above the wing the air is moving faster than it is below the wing, which means the pressure above the wing is lower than the pressure below it. Thus the wing experiences an upward force and voila - make the air go over the wing fast enough by whatever means and we have lift off.
Okay, so that's the background. How does this apply to the bladeless fan?
Consider the second passage: "The new fan works by drawing air into the base of the machine. The air is forced up into the loop amplifier and accelerated through the 1.3mm annular aperture, creating a jet of air that hugs the airfoil-shaped ramp. While exiting the loop amplifier, the jet pulls air from behind the fan into the airflow (inducement). At the same time, the surrounding air from the front and sides of the machine are forced into the air stream (entrainment), amplifying it 15 times. The result is a constant uninterrupted flow of cooling air."
Okay, so they got an "annular apeture". Annular means ring and for apeture read slit, so there's a slit all the way round the ring that's blowing a ring of high-speed air across the surface of the shaped ring and out the other side. We're told the air "hugs the airfoil-shaped ramp" so we know we're on the right track.
This moving air crossing the aerofoil is going to cause low pressure at that point, so there's going to be a ring of low pressure created on the rear side of the ring. This will suck air behind the fan forward and cause it to go through the ring and into the airstream - a process they call 'inducement'.
They've also got some 'entrainment' going on, and basically what they're saying here is that once this thing gets going there is a current of moving air pouring quickly out the front. Moving air equals higher speed equals lower pressure, so the quiet air surrounding this low-pressure airstream is going to be drawn into it as well, amplifying the effect.
They quote an amplification factor of fifteen times, meaning that the little blower in the handle can be fifteen times smaller than would be required to generate that amount of air movement by conventional means.
What an amazing little device!

No comments:

Post a Comment