In the axial flow ventilator the air must blow in line with the axis of rotation and there must be blades with space between them.  With few exceptions, the axial blower or fan must not have a lot of back pressure and will work a lot better if the blade is shrouded.  One obvious exception is the turbine compressor used in jet engines, but this requires very tight fittings and very high speed so the air is being rammed in against the outgoing air much faster than it can move - it uses a lot of power and makes a lot of noise.  Shrouding works better because the greatest loss in an unshrouded fan is the air coming off near the tips and immediately turning out and back circulating only through the fan and not through the room. The most common shrouded fan people see is muffin style fan in computers.  An axial blower can have multiple sets of blades and the blades can even over lap so light is not visible through the fan.  Fans are used most often for moving large quantities of air between spaces at atmospheric pressure.