In 2009, British tech manufacturer Dyson released a product called the Air Multiplier - a fan that was quieter, more power-efficient and safer than others, and to top it all off, it didn't have any blades. It sounds like technological witchcraft.
So how do bladeless fans work?
When you think of a fan you probably think of two or more blades attached to a central spinning hub, producing a torrent of air. These blades can slice off a wayward finger, though, and fitting a protective cage to the fan blocks some of the airflow.
The Air Multiplier works differently. Using a combination of clever physics and aerodynamics it "multiplies" the air it sucks in, so uses less energy and generates less noise in the process.
It all starts with air entering through slits at the fan's base. A small brushless electric motor runs a tiny fan with asymmetrically aligned blades which pushes air through a set of stationary blades that smooth the airflow.
The air is directed up into the hoop-like tube at the top of the device where it's forced out of a circular narrow slit running around the hoop.
At the base of the hoop, the passage is wide. But it narrows near the top of the hoop, squeezing the air and accelerating it, just as water sprays faster out of a hose if you place your thumb over the end.
This is the first point at which the air is multiplied.
The inside of the hoop-like tube is curved like the top of an aircraft's wing too, so as air is forced out through the slit it clings to the curved surface.
This means surrounding air is sucked through the hoop too, thanks to a principle of fluid dynamics known as inducement. Put simply, the airflow induces the air behind the tube to be pulled along too.
At this point, the fan has still not finished multiplying its initial air intake. By taking advantage of another principle of fluid dynamics known as entrainment, air surrounding the edge of the tube is drawn through in the same direction as the electrically propelled air.
Altogether, this allows the Air Multiplier to multiply its initial air intake by about 15 times. Dyson says the fan can draw in about 20 litres of air, about three times the total volume of our lungs, every second. This means it can pump out about 300 litres by accelerating just 20 litres - a massively efficient conversion.
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