to get back to that sweet idea, I build a small setup with 4 fans, sadly different in power, speed etc

here you can see the fans

http://i.imgur.com/Qji5V1p.jpg
the upper left is really powerful, alot more than the others

built this

http://i.imgur.com/jmr1ZcO.jpg

fired it up

http://i.imgur.com/jTWBUcy.jpg

what can I say: it works, but the setup is not good for comparison. The entry is 24x24 cm² and the exit is 8x8cm².
A1*c1 = A2*c2 -> c2 = A1/A2*c1 = 9*c1.
So one would expect the air to flow 9 times as fast as it does at the entry. It doesn't, its actually kinda lame. The powerful fan just pushed the air right out of the weakest of my fans (yeah, who would have thought that?   ). I think when letting the fans suck the air out I would not get that effect of air moving the wrong direction, or maybe not that bad. Maybe I should try this with just one fan, doing the same in a smaller scale or get 4 equal fans.

Any thoughts?