Previous experiments conducted in the Rotor Force Test Facility at the California Institute of Technology have thoroughly examined the effect of leakage flows on the rotordynamic forces on a centrifugal pump impeller undergoing a prescribed circular whirl.  These leakage flows have been shown to contribute substantially to the total fluid induced forces acting on a pump.  However, to date nothing is known of the flow field in the leakage annulus of shrouded centrifugal pumps.  No attempt has been made to qualitatively or quantitatively examine the velocity field in the leakage annulus. Hence the test objective of this experiment is to acquire fluid velocity data for a geometry representative of the leakage annulus of a shrouded centrifugal pump while the rotor is whirling using laser velocimetry.  Tests are performed over a range of whirl ratios and a flowrate typical of Space Shuttle Turbopump designs.  In addition to a qualitive study of the flow field, the velocity data can be used to anchor flow models

Acosta, A. J.

Brennen, C. E.

Caughey, T. K.

Ferguson, T. V.

Lee, G. A.

Sivo, J. M.

English

Caltech Authors - Main

FED-Vol. 191, Laser Anemometry - 1994: 
Advances and Applications 
ASME 1994 
LASER VELOCIMETER MEASUREMENTS IN THE LEAKAGE ANNULUS 
OF A WHIRLING SHROUDED CENTRIFUGAL PUMP 
J. M. Sivo, A. J. Acosta, C. E. Brennen, and T. K. Caughey 
Division of Engineering and Applied Science 
California Institute of Technology 
Pasadena, California 
T. V. Ferguson and G. A. Lee 
Rocketdyne Division 
Rockwell International Corporation 
Canoga Park, California 
ABSTRACT 
Previous experiments conducted in the Rotor Force 
Test Facility at  the California Institute of Technology 
have thoroughly examined the effect of leakage flows on 
the rotordynamic forces on a centrifugal pump impeller 
undergoing a prescribed circular whirl. These leakage 
flows have been shown to contribute substantially to the 
total fluid induced forces acting on a pump. However, 
to date nothing is known of the flow field in the leakage 
annulus of shrouded centrifugal pumps. No attempt 
has been made to qualitatively or quantitatively exam- 
ine the velocity field in the leakage annulus. Hence the 
test objective of this experiment is to aquire fluid ve- 
locity data for a geometry representative of the leakage 
annulus of a shrouded centrifugal pump while the rotor 
is whirling using laser velocimetry. Tests are performed 
over a range of whirl ratios and a flowrate typical of 
Space Shuttle Turbopump designs. In addition to a 
qualitative study of the flow field, the velocity data can 
be used to anchor flow models. 
NOMENCLATURE 
d Location across leakage gap measured 
normal from impeller shroud 
H Clearance between impeller shroud and 
housing 
Q Volumetric leakage flow rate 
E Eccentricity of impeller's circular whirl orbit 
w Main shaft radian frequency 
R Whirl radian frequency 
INTRODUCTION 
Previous experimental and analytical results have 
shown that discharge-to-suction leakage flows in the an- 
nulus of a shrouded centrifugal pump contribute sub- 
stantially to the fluid induced rotordynamic forces (Ad- 
kins, 1988). Experiments conducted in the Rotor Force 
Test Facility (RFTF) at  Caltech on an impeller un- 
dergoing a prescribed whirl have indicated that the 
leakage flow contribution to the normal and tangential 
forces can be as much as 70% and 30% of the total, re- 
spectively (Jery, 1986). Recent experiments at Caltech 
have examined the rotordynamic consequences of leak- 
age flows and have shown that the rotordynamic forces 
are functions not only of the whirl ratio but also of 
the leakage flow rate and the impeller shroud to pump 
housing clearance. The forces were found to be in- 
versely proportional to the clearance and a region of 
forward subsynchronous whirl was found for which the 
average tangential force was destabilizing. This region 
decreased with flow coefficient (Guinzburg, 1992a). In 
recent experimental work, the present Caltech authors 
demonstrated that when the swirl velocity within the 
leakage path is reduced by the introduction of ribs or 
swirl brakes on the housing, then a substantial decrease 
in both the destabilizing normal and tangential forces 
could be achieved (Sivo, 1993).The motivation for the 
present research is that no previous experiments have 
examined the flow field within the leakage annulus. No  
measurements have been made of the destabilizing swirl 
or tangential velocity or demonstrated the complex na- 
ture of the flow. Analytical models of such flows use 
bulk average velocities to examine the rotordynamic 
forces. Such bulk flow models may be seriously flawed 
when it comes to estimating the meridional and tangen- 
tial velocities. It is hoped that this present research will 
serve as preliminary insight into the complex nature of 
leakage flows. 
TEST APPARATUS 
The present experiments were conducted in the Ro- 
tor Force Test Facility (RFTF) at Caltech and were 
sponsored by Itocketdyne. The leakage flow test sec- 
tion of the facility is shown in Figure (1). 
The working fluid is water. The main components 
of the test section apparatus consist of a solid or dummy 
impeller (or rotating shroud) made of acrylic, a housing 
(or stationary shroud) instrumented for pressure mea- 
surements, a rotating dynamometer (or internal force 
balance), an eccentric whirl mechanism (not shown) 
and a leakage exit seal ring. The solid acrylic impeller 
is used so that backscatter to the velocimeter is mini- 
mized. The surface of the stationary housing has been 
modified to accommodate an optical flat glass window 
to view the fluid in the annulus. The leakage flow an- 
nulus between the impeller and housing is inclined at 
4 5 O  to the axis of rotation. The nominal clearance be- 
tween the solid impeller and the housing can be varied 
by axial adjustment of the housing. The flow through 
the leakage path is generated by an auxiliary pump. 
The solid impeller is mounted on a spindel attached to 
the rotating dynamometer connected to a data acqui- 
sition system which permits measurements of the ro- 
tordynamic force matrix. Jery, 1986 and Franz, 1989 
describe the operation of the dynamometer. The eccen- 
tric drive mechanism imposes a circular whirl orbit on 
the basic main shaft rotation. The radius of the circular 
whirl orbit (or eccentricity) can be varied. The seal ring 
at the leakage exit models a wear ring. The clearance 
between the seal ring and impeller face is adjustable. 
The test approach makes use of the Rocketdyne Laser 
2-focus Velocirneter to aquire fluid velocities across the 
shroud cavity. A pseudo-time history is obtained by 
using the velocimeter's multi-windowing synchronizer 
capability. This time history allows correlation of the 
velocity data to the rotor whirl. 
A Polytec model 4000 laser-2-focus (L2F) ve- 
locirneter was used to conduct the testing. This two- 
spot or transit type velocimeter employed a 1 watt ar- 
gon ion laser in an optical head with a 450 rnrn focal 
length lens. During testing the laser was operated at 
a lower power level resulting in about 125 mW in each 
spot. The L2F acquired data in a coaxial backscatter 
mode which was a requirement given the physical con- 
straints of the tester hardware. The electronic signal 
processing sub-section of the velocimeter included an 
FIGURE 1. LEAKAGE FLOW TEST SECTION 
analog time-to-pulse-height converter with a multichan- 
nel analyzer for display and accumulation of the data. 
Final reduction of the data was performed on a PC uti- 
lizing data reduction software from Polytec. Velocity 
and flow angles calculated from the time-of-flight and 
angular distributions, respectively, were output. 
The L2F velocimeter in conjunction with an elec- 
tronic synchronizer tagged the velocity and flow angle 
data relative to the whirl frequency of the impeller. 
Phase lock signals from the motor drive electronics were 
input to a strobe light. The rotating tester was viewed 
under the strobe light to determine the orbital position 
of the rotor when the whirl signal was generated. Six- 
teen circumferential zones or windows were generated 
by the synchronizer. Each window represents 22.5 de- 
grees of rotation of the rotor during its whirl period. 
Therefore each window represents a pseudo-time his- 
tory of the annulus flow as the orbital eccentricity of 
the impeller alternately squeezes and restores the nom- 
inal dimensions of the flow passage. The velocity and 
flow angle in each of these 16 windows was determined 
by the L2F. 
The velocimeter was setup so the beam from the 
optical head were normal to the optical access window 
and concurrently to the surface of the impeller. Mea- 
surements were made in the plane normal to the prop- 
agation of the beam. The stop spot rotated about the 
start spot in this plane in order to explore for the di- 
rection of the flow vector. 
The axial length of the measurement volumes was 
determined by the steep intensity gradient along the 
beam axis. The measurement volume length can be de- 
fined as the space bounded by the points where the in- 
tensity in the beam was one-fourth of the maximum in- 
tensity at the waist, or in other words, where the beam 
radius has doubled. Using this definition, the length of 
the measurement volume was calculated to be 0.76 mm 
(0.030 inch). 
The data were acquired through a rectangular- 
shaped window approximately 2.5 inches in length, 0.5 
inches in height and 0.5 inches thick. The optical head 
(and hence the measurement volumes) was aligned with 
the test hardware by monitoring the speckle pattern 
reflected from the metal boundaries in the test hard- 
ware and the window frame. The accuracy of the 
speckle alignment data was validated by measurements 
made with depth micrometers. In addition, measure- 
ments were performed to assure the optical head was 
oriented normal to the optical access window so that 
the data collected across the annulus was in the same 
plane. Other measurements assured the head tracked 
level along the length of the window and that the rela- 
tive angle between the zero angle position of the spots 
and the test hardware was quantified. 
To improve the signal-to-noise ratio of the ve- 
locimeter measurements, the fluid was supplementally 
seeded with silver coated hollow glass micro-spheres. 
The mean particle size was 14 microns and the parti- 
cle density was 1.4 gm/cm3. A small quantity of the 
dry powdered spheres were mixed with distilled water. 
About 40-100 cc of this slurry was injected with a sy- 
ringe into a pressure port located downstream of the 
test article. The particles, typically injected just before 
data collection at each measurement station, would cir- 
culate for about one hour in the closed loop test rig 
before settling out. 
The L2F excels at measuring flows characterized by 
high velocity and low turbulence particularly in turbo- 
machinery. Its performance efficiency in low velocity, 
high turbulence flows exemplified by this testing was 
not as good. A sufficient quantity of data was required 
for good statistical analysis of the time- of-flight and 
angular distributions in each of the 16 windows. Con- 
sequently, long run times were necessary. The infor- 
mation in Table 1 illustrates some of the parameters of 
the test. Overall, the number of data samples in each 
of the 16 windows was sufficient to yield an accuracy 
of at least +/ - 2% in velocity and generally closer to 
+/ - 1%. The flow angle data is accurate to about 
+/ - 1 degree. 
such as whirl frequency ratio, axial location, and leak- 
age annulus depth. For all tests a nominal annulus 
clearance H of 0.1072 in, a whirl eccentricity E of 0.Olin 
and a leakage exit face seal clearance of 0.02 in were 
maintained. Tests were conducted at a shaft speed w of 
2000 RPM and a leakage flow rate Q of 10 GPM. The 
above flow approximates the range of typical leakage 
rates for the new Space Shuttle Alternate Turbopump 
(ATP) presently being developed. Whirl ratios fl/w of 
0.2, 0.3, 0.4 were tested. Velocity measurements were 
made at three meridional locations along the leakage 
path: Near Inlet (.6054 in), Near Midpoint (1.4054 in), 
Near Discharge (2.4054 in). The leakage path length 
along the impeller is 2.74 in. At each of these three 
stations, measurements were made at 5 depth locations 
across the bakage gap. Measuring from the solid im- 
peller these positions were 0.0178 in (16.7%), 0.0357 in 
(33.4%), 0.0536 in (50%), 0.0715 in (66.7%), and 0.0893 
in (83.3%). 
The above test matrix is summarized in Table 2. 
Table 1. Parameters of the Tests 
Table 2. Velocimeter Measurements 
RPM Q(GPM) O/w d(in) 
TEST MATRIX 
This experiment is designed to measure the veloc- 
ity of simulated leakage flows for different parameters 
RESULTS FOR MEASUREMENTS OF MERlDlONAL 
VELOCITY 
In the following results not all gap locations are 
reported. This is due to the low signal to noise ratio 
for gap locations near the impeller shroud surface. 
Figure (2) shows plots of the absolute meridional 
velocity a t  a whirl ratio of 0.2 for the three meridional 
locations respectively. Looking at  the Near Inlet figure 
it can be seen that reverse flow occurs for the 33.4% gap 
point between the whirl orbit angles of 110' and 260°. 
Further away from the impeller across the gap there is 
no reverse flow. Looking at the Near Mid-point figure 
reverse flow occurs again for the 33.4% gap point, but 
only for a smaller range of whirl orbit angles of 120° 
to 220°. Once again further away from the impeller 
across the gap there is no reverse flow. Looking at the 
Near Discharge figure, there is no evidence of reverse 
flow. Hence from the above it appears that there is 
recirculation near the surface of the impeller between 
the inlet to the leakage path and the midpoint. 
Figure (3) shows similar plots except for a whirl 
ratio of 0.3. The Near Inlet figure shows that the 16.7% 
and 33.4% gap points are in reverse flow at  all whirl 
orbit locations. For the Near Mid-point figure there is 
reverse flow between 120° and 200'. From the Near 
Discharge figure there is no evidence of reverse flow. 
Hence, once again it appears that there is a recirculation 
zone near the surface of the impeller between the inlet 
to the leakage path and the midpoint. 
Figure (4) shows similiar plots except for a whirl ra- 
tio of 0.4. There is no evidence of recirculation, except 
in the Near Mid-point figure where there is recirculation 
for the 33.4% gap point at  150'. 
As a general observation of all the above plots it 
can be seen that the meridional velocity is largest fur- 
thest from the rotating impeller and is quite large at 
the 83.3% gap point, which is closest to the stationary 
casing. 
RESULTS FOR MEASUREMENTS OF TANGENTIAL 
VELOCITY 
Figures (5) through (7) show the absolute tangen- 
tial velocity for the three whirl ratios tested and five 
gap locations. A few general observations can be made. 
Near Inlet plots show the velocity profiles spread out 
with the largest tangential velocity nearest the impeller, 
as expected. Also except for the 0.4 whirl case, near dis- 
charge velocity profiles bunch up close to the impeller. 
It is as if a bulk tangential velocity is set up. 
CONCLUSIONS 
Laser velocimeter measurements have been made 
in the leakage annulus of a whirling shrouded impeller. 
Results clearly show regions of recirculation which di- 
minish at  higher whirl ratios. These recirculation re- 
gions are dominant near the entrance to the leakage 
path. In some cases these recirculation regions exist 
entirely around the impeller. 
ACKNOWLEDGEMENTS 
The assistance provided by Caltech graduate stu- 
dent Qiao Lin with the experimentai program is appre- 
ciated. We would also like to thank Mike Yandell of 
Rocketdyne for the test section design. 
REFERENCES 
Adkins, D. R. and Brennen, C. E., 1988, Analysis of 
Hydrodynamic Radial Forces on Centrifugal Pump Im- 
pellers, ASME J. Fluids Eng., Vol. 110, No. 1, pp. 
20-28. 
Franz, R. J., 1989, Experimental Investigation of the 
Effect of Cavitation on the Rotordynamic Forces on 
a Whirling Centrifugal Pump Impeller, Ph.D. Thesis, 
California Institute of Technology. 
Guinzburg, A., 1992a, Rotordynamic Forces Generated 
By Discharge-To-Suction Leakage Flows in Centrifugal 
Pumps, Ph.D. Thesis, California Institute of Technol- 
ow. 
Jery, B., 1986, Experimental Study of Unsteady Hydro- 
dynamic Force Matrices on Whirling Centrifugal Pump 
Impellers, Ph.D. Thesis, California Institute of Tech- 
nology. 
Sivo, J. M., Acosta, A. J., Brennen, C. E. and Caughey, 
T. K., 1993, The Influence of Swirl Brakes on the Ro- 
tordynamic Forces Generated by Discharge-To-Suction 
Leakage Flows in Centrifugal Pumps, presented at the 
ASME Second Pumping Machinery Symposium, Wash- 
ington, D. C., June 20-24, 1993. 
Zhuang , F . , 1989, Experimental Investigation of the 
Hydrodynamic Forces on the Shroud of a Centrifugal 
Pump Impeller, E249.9, Division of Engineering and 
Applied Science, California Institute of Technology. 
Meridional Velocity 
LEGEND: Percentage across gap 
16.7% Cl 33.4% El 50% 7 66.7% 0 83.3% 
Near 
Inlet 
Mid-Point 
FIGURE 2. 
Location in Whirl Orbit (deg) 
1 15 FIGURE 3. 
- Meridional Velocity Tangential Velocity 
Near 
Inlet 
Mid-Point 
FIGURE 4. 
Location in Whirl Orbit (deg) 
116 AGURE 5. 
Near 
Discharge 
Tangential Velocity 
-- 
22 
21 
20 
19 Near 
18 
Inlet 
17 
16 
15 
0 60 120 180 240 300 360 
27 
26 
25 
24 
23 Mid-Point 
22 
21 
20 
19 
0 60 120 180 240 300 360 
Location in Whirl Orbit (deg) 
Near 
Discharge 
FIGURE 6. 117 FIGURE 7. 


Laser Velocimeter Measurements in the Leakage Annulus of a Whirling Shrouded Centrifugal Pump

1992a, Rotordynamic Forces Generated By Discharge-To-Suction Leakage Flows in Centrifugal Pumps,

Adkins, D. R. and Brennen, C. E., 1988, Analysis of Hydrodynamic Radial Forces on Centrifugal Pump Impellers, ASME J. Fluids Eng., Vol. 110, No. 1, pp. 20-28.

Analysis of Hydrodynamic Radial Forces on Centrifugal Pump Impellers,

Experimental Investigation of the Effect of Cavitation on the Rotordynamic Forces on a Whirling Centrifugal Pump Impeller,

Experimental Investigation of the Hydrodynamic Forces on the Shroud of a Centrifugal Pump Impeller, E249.9, Division of Engineering and Applied Science, California Institute of Technology. Meridional Velocity LEGEND: Percentage across gap 16.7% Cl 33.4%

Experimental Study of Unsteady Hydrodynamic Force Matrices on Whirling Centrifugal Pump Impellers,

Franz, R. J., 1989, Experimental Investigation of the Effect of Cavitation on the Rotordynamic Forces on a Whirling Centrifugal Pump Impeller, Ph.D. Thesis, California Institute of Technology.

Guinzburg, A., 1992a, Rotordynamic Forces Generated By Discharge-To-Suction Leakage Flows in Centrifugal Pumps, Ph.D. Thesis, California Institute of Technolow.

Jery, B., 1986, Experimental Study of Unsteady Hydrodynamic Force Matrices on Whirling Centrifugal Pump Impellers, Ph.D. Thesis, California Institute of Technology.

The Influence of Swirl Brakes on the Rotordynamic Forces Generated by Discharge-To-Suction Leakage Flows

https://authors.library.caltech.edu/150/1/SIV145.pdf

Laser Velocimeter Measurements in the Leakage Annulus of a Whirling Shrouded Centrifugal Pump

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