Known impellers or fans can include an arrangement of airfoils. By airfoils is meant a foil or blade which is substantially a version of a wing. A typical wing or foil has a shape which creates a greater distance over one side, which is usually the topside, than the opposite side.
This configuration of a typical foil or wing when driven forward with its thickest end foremost splits the ambient fluid, be it gases or liquids to cause a portion to pass over the top and a portion to pass underneath. The greater distance the fluid travels over the side with the greatest curve, which is usually the top, forces that fluid to a tendency toward being attenuated.
This substantial attenuation causes a lowering of pressure. The lowered pressure attracts adjacent fluid and the effect is to create an upward suction. If the wing or foil cannot rise, the fluid travels down to meet it and usually passes mostly behind the trailing edge.
In this type of foil or wing it can be seen that there is a direct relationship between each side of the wing or foil.
If, because of a too coarse pitch (nose up) the pressure underneath becomes too high and the pressure above becomes too low, the foil or wing will stall. In this case the high pressure fluid from the underside creeps around the Trailing edge and forward along the topside and causes detachment of the topside fluid flow. Upwards suction is lost or greatly diminished and therefore loss of lift occurs.
High pressure air also travels around the foil or wing tips and creates vortices, which detracts from lift and creates a drag on the foil near its tips.
A typical conventional fan is almost always a circular arrangement of these foils or small wings and is subject to the same factors which cause a loss of efficiency.
In a typical conventional radial flow fan, the foils or miniature wings diverge from each other from a medial to a lateral area. In this situation, each foil or wing relies on the lower pressure air travelling over the low pressure side of the foil or wing to substantially reach the trailing edged to rejoin the higher pressure air being flung radially by the high pressure side of the foil. So in this type of fan is subject to having its blades or foils stall if a back pressure or head pressure is generated. If this type of fan is driven to too high tip speed each foil stalls and in certain circumstances fluid can actually travel back between each set of foils along the low or suction side of the foils. In effect there is created a counter current of fluid between any two foils.