your heat sink model is NH-U9B SE2.

http://www.noctua.at/main.php?show=product...ts_id=32&lng=en

It operates by push pull method. One fan push air into the heatsink, one pulls air out from heat sink.

now is all pushing air into the heat sink, causing turbulence inside the heatsink and trapped inside. hot air no where out.

flipping one of the fan around will drop the temp. depending where you wan the hot air goes,to the front or back.

hope this helps