I am looking to get some noctua 140mm fans for my Corsair 750D but I am not sure which one is optimized for airflow. I do not have any restrictions blocking airflow. Here are the fans.
NF-A14 FLX
NF-A14 PWM
NF-A14 ULN
NF-A15 PWM
NF-A14 FLX
NF-A14 PWM
NF-A14 ULN
NF-A15 PWM