fluid_mechanics

672 Chapter 12 ■ Turbomachines
Head, ft
60
50
40
30
20
10
Brake Horsepower
Percent Efficiency
500
400
300
200
Head
Brake horsepower
Centrifugal pump
Axial-flow pump
100
80
60
Efficiency
40
20
0
0 10 20 30 40 50 60
Capacity, 1000 gal/min
F I G U R E 12.20 Comparison of
performance characteristics for a centrifugal pump
and an axial-flow pump, each rated 42,000 galmin
at a 17-ft head. (Data from Ref. 12, used with permission.)
head decreases, the speed increases, the impeller diameter decreases, and the eye diameter increases.
These general trends are commonly found when these three types of pumps are compared.
The dimensionless parameters and scaling relationships developed in the previous sections
apply to all three types of pumps—centrifugal, mixed-flow, and axial-flow—since the dimensional
analysis used is not restricted to a particular type of pump. Additional information about pumps
can be found in Refs. 4, 7, 9, 12, and 13.
F l u i d s i n t h e N e w s
Mechanical heart assist devices As with any pump, the human
heart can suffer various malfunctions and problems during its
useful life. Recent developments in artificial heart technology
may be able to provide help to those whose pumps have broken
down beyond repair. One of the more promising techniques is
use of a left-ventricular assist device (LVAD), which supplements
a diseased heart. Rather than replacing a diseased heart,
an LVAD pump is implanted alongside the heart and works in
parallel with the cardiovascular system to assist the pumping
function of the heart’s left ventricle. (The left ventricle supplies
oxygenated blood to the entire body and performs about 80% of
the heart’s work.) Some LVADs are centrifugal or axial flow
pumps that provide a continuous flow of blood. The continuous
flow may take some adjustment on the part of patients, who do
not hear a pulse or a heartbeat. Despite advances in artificial
heart technology, it is probably still several years before fully
implantable, quiet, and reliable devices will be considered for
widespread use.
Runner
vanes
Runner
vanes
Diameter D
Eye
A B C D
Type Centrifugal Centrifugal
Mixed flow Axial flow
N sd
Gal/min
Head, ft
Rpm
D, in.
D eye /D
1,250
2,400
70
870
19
0.5
2,200
2,400
48
1,160
12
0.7
6,200
2,400
33
1,750
10
0.9
13,500
2,400
20
2,600
7
1.0
F I G U R E 12.21 Comparison of different types of impellers. Specific speed for
centrifugal pumps based on single suction and identical flowrate. (Adapted from Ref. 12, used with
permission.)