Diminished Return for Stacked Fans

[NexTerren]

So this will be my first rig in nearly 10 years, and my very first serious gaming rig, and I have a question about CFM for fan configurations.

I'm trying to create a positive pressure in my case, and I was looking at the NZXT Phantom. I want to use the H100i for cooling (I'm not quite ready to get into non-closed water cooling), and the Phantom supports mounting the H100i's radiator directly under its 2 200mm top fans.

Because the H100i's fans will effectively be pushing the air through the same opening that the two 200mm top fans are pulling through, I'd assume I'd see diminished returns on the case's net CFM. If so, by how much?


tl;dr: Stacked fans I'd reason would offer diminished CFM returns. If so, by how much? Any formulas out there?

Edit: I realized that my question might not be clear, so I'm going to clarify: I'm not trying to fight the diminished return, or optimize airflow. I'm trying to figure out the CFM so I can create a positive pressure, but I'm not sure how this stacking configuration would interact mathematically.

 

[steddora]

Honestly, I would probably just pull those 200's off and get a push/pull setup on the H100 if I was going that route. If you're working for CFM/Positive pressure you could certainly close off the extra space there. Either way, pushing air into a fan does change it's CFM rating. That's the hard part. The physics behind the change are probably too much for even the most highly qualified physicist to take the time to figure out. Sometimes pushing air into a fan can decrease it's CFM as it disrupts the pull of air the fan blades are causing.

Also with high size fans, it's harder to keep the CFM logical. It's eaiser to push air through a slow 200mm fan than it is to push air through an 80mm high RPM fan. So to get your answer, it's probably not going to effect it all that much in reality. I'd just make up for it with a little more intake CFM to compensate for "more likely" scenario's.