I agree with Jacob - for a radiator, you want the fans pulling cool air into the case through the radiator, not exhausting warm air from the case out through the radiator.
for my setup i have 4x 4'' fans on the Rad (CPU cooler) pulling air in through the top of the case, 2 5'' fans on the front pulling air in (I put filter screens over both of these sections) and one 5'' exhaust fan out the back.
Like jacob said, this keeps a positive air pressure, combined with the filter screens it should keep anything nasty from getting in.
I would say mount your NHD15 as an intake fan at the front of the case, and move one of your Noctua 140mm to the back as an exhaust. Does your front panel have a filter? I would suggest putting a screen filter in front of your intake fans to keep you Rad and case clean.