R. G. CUNNINGHAM
Professor and Head ,
Department of Mechanical Engineering,
The Pennsylvania State University,
University Park, Pa.
A. G. HANSEN
President,
Georg ia Institute of Technology,
At lanta, G a .
T. Y. Na
Professor of Mechanical Engineering,
The University of Michigan,
Dearborn Campus,
Dearborn, Mich.
Jet Pump Cavitation
Mixing-throat cavitation, in a liquid jet pump results from high jet velocities, low suc-
tion (NPSH) pressure, or low discharge pressure. Incipient cavitation at the jet
boundary has no effect on jet pump efficiency, but under severe conditions it spreads to
the walls. A limiting flow condition results which is independent of discharge pressure.
Efficiency deteriorates rapidly and the pump head-flow characteristics can no longer be
predicted by conventional theory.
Eight correlation parameters (1937-1968) and their interrelations are examined.
A Cavitation Index
volumetric flow ratio
ML = QSL/QN< cavitation-limited flow
ratio
Afmep = maximum efficiency flow ratio
Mop = operating (design) flow ratio,
(usually set equal to Afmep)
(.Continued on next page)
Journal of Basic Engineering S E P T E M B E R 1 9 7 0 / 4 8 3 Copyright © 1970 by ASME
EXPERIMENTAL RESULTS
o P0-S.3lpsia
x P0 =l3.90p$ia r 30
20 P~
Fig. 1 Effect of cavitation-Hmited flow on agreement between experi-
ment and theory [5]
resembles choked compressible flow, since Qs fails to respond^to
Pd once the cavitation limit is reached.
The design of liquid jet pumps for noncavitating operation is
well in hand; both analytical and empirical expressions have been
developed which accurately predict performance in terms of area
ratio R, primary flow rate, and applied operating pressures [5, 11,
15, 16], But as shown by Fig. 1, the predicted performance
curve is applicable only up to ML. Clearly, a means of pre-
dicting the value of ML is critically important to the jet pump
designer, since operation must be at, M < ML.
The purpose of this paper is three-fold. A summary will be
made of the various works on jet pump cavitation and the rela-
tions of these cavitation parameters. Experimental data will be
compared in terms of a recommended cavitation index. Results
of an experimental study on the cavitation of jet pumps for three
.ffi-value jet pumps will be presented. Finally, a limiting-flow
prediction equation {ML) based on the selected cavitation index
is examined, and the utility of the relation in the design and de-
velopment of jet pumps is discussed.
In one of the earliest (1937) comprehensive treatments of jet
pumps, Gosline and O'Brien [11] provided data and insight con-
cerning jet pump cavitation. They showed that a plot of Q,L2
versus P 0 — Pv or net positive suction head, NPSH, formed a
straight line, validating the theory that the onset of QsL is a re-
sult of reaching vapor pressure in the throat entry. Data were
limited however, and a correlation of data for different R values
was not achieved.
Rouse [1] studied the cavitation occurring at the boundary of
a high-velocity water jet penetrating into a tank of water. He
developed a cavitation index which correlated incipient cavitation
(sonic detection) for various absolute pressures, vapor pressures,
and jet velocities. This index . P„) = Y = XC(P„ - Pv). (27)
Thus X„ is simply the slope of the Y versus (P0 — P,,) correlation
of limited-flow data, e.g., the constant slope for the oil pump
tests, Fig. 3, is X„ = 0.68. A modified form which is particularly
useful when nozzle efficiencies are unknown is
M, (nh)' (28)
where
p{ - P„ = zix + x",).
The effect of nozzle efficiency is not known. However, Ki is of
the order of 0.01 in well designed nozzles and equations (26) and
(28) will give virtually identical results.
Sanger's Parameter, CO [9]. Sanger introduced another important
parameter defined by
2gJ
m^'^m (1 + KJ
(29)
Experimental data taken by Sanger showed good agreement with
equation (29) as did Bonnington's data [2], (Both co and
Bonnington's o~B are plotted as curved-line functions of VJV„;
w differs by the inclusion of P„.) Sanger's data provide needed
information on low area-ratio jet pumps, including the effect of
the spacing between nozzle exit and entrance to mixing tube.
Hansen and Na Parameter [4]. In testing a class of commercial
jet pumps Hansen and Na observed that for a given jet pump
configuration cavitation usually occurred at relatively constant
values of the secondary flow, Q„. One might, therefore, postulate
that the limiting value of Qs is obtained when the local pressure at
the entrance to the mixing tube reached the vapor pressure of the
fluid. Equation (24) then gives
Q,L = A,
2ge (P„ - P . )
1 + K, J 2gc .1 + K3 NPSH
(30)
NPSH
II
2(/c
1 + K2.
Hansen and Na thus defined a cavitation parameter as
NPSH
Ilk
igc
(31)
The significance of this parameter is that the parameter is con-
stant for a given jet pump configuration. Data taken by Hansen
and Na [4] showed that a constant value of
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