This is a Fan control plate design for creating a more laminar/focused flow on a fans exhaust. (This isn't a fan cover :-))
The turbulence is reduced, providing for more controlled flow and pressure.
Curved blades generated less noise but at the expense of loss in performance caused by the fact that curved blades stall (turbulent air flow) at a lower static pressure than straight blades.
Similar design can be seen in Noctua NF-F12 fans and SilverStone Air Penetrator fans.
Check this video out from SilverStone to see the concept: http://youtu.be/8m8fC809TK0
I've printed the 92mm and 120mm versions, and they work.
This design works on curved edge fans that rotate in counter clockwise direction (about 99.9% of fans).
I haven’t measured the change in the P-Q curve, but would imagine it creates and entire new curve.
20-August: Added 40mm V4 plate. This is a little tricky to print, because of thinner blades. But the performance is more in-line with the others. I tested on Fan FD244010HB and only had a loss of flow of 2-4% with a gain in pressure from 15-20%.
I sliced with Cura in Repetier Host V1.0.2 without problems.
Plate should be mounted on the exhaust, with the letter Q facing out.