Page 31 - Fluid Mechanics and Thermodynamics of Turbomachinery

12 Fluid Mechanics, Thermodynamics of Turbomachinery

FIG. 1.7. Range of pump impellers of equal inlet area.

speed implies that the machine design changes. Broadly speaking, each different
class of machine has its optimum efficiency within its own fairly narrow range of
specific speed.
For a pump, eqn. (1.8) indicates, for constant speed N, that N s is increased by an
increase in Q and decreased by an increase in H. From eqn. (1.7b) it is observed
that H, at a constant speed N, increased with impeller diameter D. Consequently,
to increase N s the entry area must be made large and/or the maximum impeller
diameter small. Figure 1.7 shows a range of pump impellers varying from the axial-
flow type, through mixed flow to a centrifugal- or radial-flow type. The size of
each inlet is such that they all handle the same volume flow Q. Likewise, the head
developed by each impeller (of different diameter D) is made equal by adjusting
the speed of rotation N. Since Q and H are constant, then N s varies with N alone.
The most noticeable feature of this comparison is the large change in size with
specific speed. Since a higher specific speed implies a smaller machine, for reasons
of economy, it is desirable to select the highest possible specific speed consistent
with good efficiency.

Cavitation
In selecting a hydraulic turbomachine for a given head H and capacity Q,itis
clear from the definition of specific speed, eqn. (1.8), that the highest possible value
of N s should be chosen because of the resulting reduction in size, weight and cost.
On this basis a turbomachine could be made extremely small were it not for the
corresponding increase in the fluid velocities. For machines handling liquids the
lower limit of size is dictated by the phenomenon of cavitation.
Cavitation is the boiling of a liquid at normal temperature when the static pres-
sure is made sufficiently low. It may occur at the entry to pumps or at the exit
from hydraulic turbines in the vicinity of the moving blades. The dynamic action
of the blades causes the static pressure to reduce locally in a region which is
already normally below atmospheric pressure and cavitation can commence. The
phenomenon is accentuated by the presence of dissolved gases which are released
with a reduction in pressure.
For the purpose of illustration consider a centrifugal pump operating at constant
speed and capacity. By steadily reducing the inlet pressure head a point is reached