CHAPTER 6
VELOCIS SOLUTIONS @2018
15
Axial pump
Axial pumps do not utilize centrifugal forces. Instead, the impeller blades behave more like the
wing of an airplane, producing lift by changing the momentum of the fluid as they rotate.
Figure 7.12: The blades of an axial-flow pump behave like the wing of an airplane. The air is
turned downward by the wing as it generates lift force FL.
The rotor of a helicopter, for example, is a type of axial-flow pump. The lift force on the blade is
caused by pressure differences between the top and bottom surfaces of the blade, and the change
in flow direction leads to downwash (a column of descending air) through the rotor plane. From a
time-averaged perspective, there is a pressure jump across the rotor plane that induces a
downward airflow. Imagine turning the rotor plane vertically; we now have a propeller. Both the
helicopter rotor and the airplane propeller are examples of open axial-flow fans, since there is no
duct or casing around the tips of the blades. The common window fan you install in your bedroom
window in the summer operates under the same principles, but the goal is to blow air rather than
to provide a force. Be assured, however, that there is a net force acting on the fan housing. If air is
blown from left to right, the force on the fan acts to the left, and the fan is held down by the
window sash. The casing around the house fan also acts as a short duct, which helps to direct the
flow and eliminate some losses at the blade tips. The small cooling fan inside your computer is
typically an axial-flow fan; it looks like a miniature window fan and is an example of a ducted axial-
flow fan.
Performance curves for a typical propeller fan is plotted in Figure 7.13. Unlike centrifugal fans,
brake horsepower tends to decrease with flow rate. In addition, the efficiency curve leans more to
the right compared to that of centrifugal fans. The result is that efficiency drops off rapidly for
volume flow rates higher than that at the best efficiency point. The net head curve also decreases
continuously with flow rate (although there are some wiggles), and its shape is much different than
that of a centrifugal flow fan. If the head requirements are not severe, propeller fans can be
operated beyond the point of maximum efficiency to achieve higher volume flow rates. Since bhp
decreases at high values of flow rate, there is not a power penalty when the fan is run at high flow
rates. For this reason, it is tempting to install a slightly undersized fan and push it beyond its best
efficiency point. At the other extreme, if operated below its maximum efficiency point, the flow