Originally Posted by yoshirama
This is more of a technical question than asking for advice. I've know that pc fan are always odd numbered due to reasons of balance and/or reducing resonance. However aside from your standard and common 7 blade fans, I've also seen 9 blade, 5 blade, 11 blade, and 3 blade designs. I just curious as to how different blade counts and designs will affect a fan's performance, from dB, cfm, and static pressure. Maybe someone familiar in fluid dynamics can fill us in?
As for the fluid dynamics: Pitch and efficiency (shape mostly) of the blades are what is primarily going to affect performance. The number of blades can also be due to the RPM range or the pitch of the blades, the more air the fan moves, or the faster it moves may affect the ability of blades to effectively draft air, hence adding or subtracting blades.
Shape and clearance between the blades and housing account for noise profiles (assuming well lubricated bearings) as they are the cause of vortices forming around the edges of the blades. The more effective the shape of the blade the less it will shed vortices which are generally the cause of the noise. If you think of a fan as a screw, the slower it turns, the less likely it is to tear up the material around it, and the smoother the airflow and noise will be. Start turning it too fast and the fluid begins to separate and make noise and the fan begins to lose efficiency. Fast blades can be efficient, but they need to be shaped differently for the desired RPM range.