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A Ridiculous Thought...

A Ridiculous Thought...

(OP)
Please forgive me for bothering people outside my supposed speciality, but the following thought occurred (and yes, before you ask; it was painful) that I would like to present as a question for discussion.

Is it possible, rather than controlling vortex shedding from the tips of ducted fan blades via close tolerances and consequently heavy construction, to trap those vortices and use that energy to amplify incoming flow?

As shown on the attached sketch, if the blade tip vortices were trapped in the shape of rotating torus around the periphery of the duct, could that rotating mass of air be used as an airfoil to accelerate and pack air into the duct center?

Intuitively, it would seem that the splitter distance from the blade disc would be on the order of the blade chord at that point, which would suggest that these large clearances could permit use of a fairly lightweight and flexible duct structure without huge leakage losses.

As drawn, the vortex ring is probably much too large in cross-section.  If this concept works at all, it would of course require substantial optimization.  Again, on an intuitive basis, it would seem that this might work better with low-order blade counts, making it more usable for larger, and less heavily loaded discs, such as recip engine powered ducted fans than for turboshaft applications.

For recip-powered ducted fans however, an insubstantial duct with non-critical clearances might provide some interesting options...

RE: A Ridiculous Thought...

Without going into the question that you'll need a variable shroud geometry for proper off-design, I don't think this will work (I'm not sure).

If the blade tip is close to the wall, the wall serves to prevent the formation of the tip vortex.

If there is no wall, the tip vortex will be there and will be the biggest of a set of concentric vortex rings that came into being due to your rotating blade. Exactly these vortex rings cause the flow to be pulled inside the compressor.

If you make an open ring around the blade tips as you do, you will probably only increase the rotational friction, i.e. the energy that your vortex uses to (eventually) acelerate the flow, comes from the axis of your rotor. This could be done more efficiently without so much vorticity, bringing the wall closer to the blade tip (I mean as a conical shroud without doughnut shape)

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