A shroud increases performance by reducing or removing the dead spot that occurs directly underneath the center hub of a fan. If you mount a 120mm fan directly to a heatercore, even if that core were the exact dimensions of the fan, there would be a dead air spot with no flow under the hub. Most 120mm fans hubs are around 2″ across. Also, this particular heatercore is wider and taller than a 120mm fan. So, if the fan were mounted directly to the core, there would be waterchannels and fins that received none of the fans airflow. A shroud acts, in addition to solving the dead spot problem, acts as an adapter between the size of the heatercore’s face and the size of the 120mm fan – it brings all of the core’s surface area into play.
general rules for picking a good shroud –
at minimum, a shroud should have 1″ of standoff between the fins and the fan. Absolutely optimal performance is usually found when standoff is approximately the same as the diameter of the fan hub but that often makes the core + shroud + fan larger than can be fit inside a case. A good balance is around 1.5″ or standoff.
fan cutout – a nice 4.5″ diameter round hole is fine on a shroud if you intend to mount the fans so they pull air through the heatercore but you need a more optimized cutout to get the full benefit out of a fan in push configuration. A fan cutout for push configuration needs to closely mimic the hole that is in the fan’s housing which is usually a box shape with rounded corners that looks similar to the one in the attached image.