Fans are of such common use that a basic understanding of the aerodynamics is appropriate. This understanding begins with the recognition that the blades of a fan propeller resemble the wing of an airplane, and as such follow the same aerodynamic laws. For example, a fan blade produces lift when the chord is elevated from the direction of the relative wind as shown in Figure 2.
Figure 2: Nomenclature for an airfoil
The elevation angle is referred to as the angle of attack (AOA). The greatest airflow delivery from a fan occurs when the AOA is at a minimum, but the pressure differential across the fan is zero. As the AOA is increased, the airflow delivery decreases and the pressure differential increases. The airflow can decrease to nearly zero, but will also deliver the maximum pressure differential in this condition, which is called the shut-off point. When an AOA is reached where the air will no longer flow smoothly and begins to separate from the blades, an "aerodynamic stall" condition exists.
Since a fan is a constant volume machine, it will move the same volumetric flow rate of air irrespective of the air density. However, the mass flow rate does change as the density changes. This becomes important when equipment is expected to operate at altitudes significantly greater than sea level. Therefore the volumetric flow rate required at altitude (low density air) will be greater than that required to achieve the same cooling as at sea level.
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