I am by no means an expert in air circulation. To me, the main (APU) fan has the purpose to push air onto the radiator fins. The front fan pulls extra fresh air into the case, while the back fan pulls hot air out of the case. Of course the air flow is altered inside the case, but that does not seem to matter much from a practical POV: I get 7-8ºC lower temperatures than with just the original Noctua.
I think both the original Noctua and your aftermarket fan are “static pressure” fans, so it’s not a problem, I was replying the above comment by convexproblem.
“Airflow” fans typically have thin blades with high angle of attack, creating faster airflow at a given RPM, but the blades will stall at lower speeds, reducing efficiency. “Static pressure” fans have thick, “curvy” fan blades where the angle is shallow at the beginning and steep at the end, to force air from a very low velocity to areas with air resistance such as radiator fins. According to the photo you posted at OP, your fan is already “static pressure” so there’s no problem.
The shape of the heatsink is very mismatched to a 140mm fan… That’s what I meant.
How’s that? Can you explain? And would it be a perfect match for a 120?
I got similar delta temps when I was using the 140 mm fan as I get with the current 120 Phanteks. If you were right, that would not have happened.