[CoolROD]

I am talking about axial fans in series, but counter-rotating.

One of the reasons that a fan stalls is that the angle of attack of the airfoil increases as the axial airflow slows (backpressure builds). As the fan approaches a "dead head" condition there is no laminar airflow over the top part of the airfoil.

1) An axial fan imparts a rotational velocity to the airstream (in essence the air corkscrews out of the fan). If fan1 imparts more axial velocity to the airstream than rotational velocity, fan2 will have a more favorable angle of attack. This will help shift the stall point of fan2.

2) The rotational velocity imparted on the airstream by a single fan is energy wasted. If you have a counter-rotating pair of fans then fan2 sees the rotating airstream at an apparently higher rotational velocity. This is a way of recovering some of the wasted energy from fan1 and increasing the efficiency of the system. There is a significant increases in the static pressure that a fan generates with increased rpm (DC voltage).

3) There may be another increase in fan2's efficiency if fan1 is oversped and pressurizes the inlet of fan2 and increases the air density. I just want to play with that, though.