According to Paul :
Leo Lichtman wrote:
Fans are designed with curved blades (pitch changes from leading to trailing
edge) so that air flow across the blade is tangential. Putting a second fan
directly behind the first will put faster moving air in contact with the
leading edge, which will not be pitched correctly. It would be like trying
to push a car that is moving as fast as you can walk.

[ ... ]

I like your analogy, and that's sorta the way I though it might work!

Aside from that -- since I gather that you were considering them
to be both in series electrically and in the airflow sense, I see a
possible problem -- even assuming that they are identical fans. The
bearings tend to fail in these over time, and once that happens, one fan
stalls, putting more voltage across the other, and frying its windings.
(And that bit above about the blade curvature might mean that two close
coupled in airflow series would be unbalanced anyway.

Enjoy,
DoN.

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