Thread: Discussion The Truth about ducted fans
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Old Aug 03, 2006, 07:08 PM
winmodels is offline
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Wareham, Dorset, UK
Joined Dec 2003
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I'm still working on that article. The most work however are the drawings.
The funny thing is that I have a text in mind which needs some sketches to explain. Then I jot down a spec which gives me hints of what I actually want to show in the pictures. When those are done I have forgotten what I actually want to describe .
Anyway, here are two sketches for the saga. At least I can tell you what they show: the planes give an indication of the flying situation ( start or high speed straight and level). Above is a graph which shows the air velocity at any point in the plane's duct and EDF and beyond. The vertical scale of the green line is the velocity, starting in front of the intake until it leaves the nozzle. The interesting thing for the high speed case is that the air is stopped dead at the very tip of the plane's front end. This only applies to the very centre of the stream tube, not to the whole. From the tip onwards the air is displaced side wards, which causes an acceleration of the air past the front side where it can reach higher velocities than the free air speed.
In the take off case the air is sucked into the intakes from all sides. I have assumed that there is no flow separation inside the throat.
The lilac line shows the total pressure outside and inside the duct. Total pressure since it is the static and the dynamic pressure we are concerned with. This will then make the connection to the first sketches in this article which referred to the total pressure developed by the fan. Details of the pressure development in the fan stage does not interest here, but we can see that it is the only source where energy is added to the air flow.
Duct losses and other losses of pressure are shown by the fall of the pressure line on the way through the duct.
The two pictures shown here now deal with arrangements designed for minimum loss. There will be more similar pictures which show losses from other sources ( flow retardation in a diffuser) in a more explicit way.

Have fun with fans

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Last edited by winmodels; Dec 21, 2006 at 06:04 AM.
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