Backward and forward subsynchronous instability was observed in a flexible model test rotor under the influence of swirl flow in a straight-through labyrinth packing. The packing pressure drop at the onset of instability was then measured for a range of operating speeds, clearances and inlet swirl conditions. The trend in these measurements for forward swirl and forward instability is generally consistent with the short packing rotor force formulations of Benchert and Wachter. Diverging clearances were also destabilizing and had a forward orbit with forward swirl and a backward orbit with reverse swirl. A larger, stiff rotor model system is now being assembled which will permit testing steam turbine-type straight-through and hi-lo labyrinth packings. With calibrated and adjustable bearings in this new apparatus, direct measure of the net destabilizing force generated by the packings can be made

Miller, E. H.

Vohr, J. H.

English

NASA Technical Reports Server

PBELIMII'UUXY IXVESTIGATION OF LABYRINTH PACKING PRESSURE DROPS 
AT ONSET OF SWIRL-INDUCED ROTOR INSTABILITY 
E. H, Miller and J. B. Vohr 
General Electric Company 
Schenectady, New York 12345 
Backward and forward subsynchronous instabi 1 ity has been 
observed in a flexible model test rotor under the influence of swirl 
flow in a straight-through labyrinth packing. The packing pressure 
drop at the onset of instability was then measured for a range of 
operating speeds, clearances and inlet swirl conditions. 
The trend in these measurements for forward swirl and forward 
instability is generally consistent with t short packing rotor 
force formulations of Benchert and Wachter 171 ,
Diverging clearances were also destabilizing and had a forward 
A larger, stiff rotor model system is now being assembled which 
orbit with forward swirl and a backward orbit with reverse swirl. 
will permit testing steam turbine-type straight-through and hi-lo 
labyrinth packings. With calibrated and adjustable bearings in this 
new apparatus, we expect to be able to directly measure the net 
destabil izing force generated by the packings. 
I NTRODUCTI ON 
Destabilizing packing force measurement obtained by integrating 
circumferential and axia pre s e distributions have been reported 
by several i n v e s t i g a t o r s ~ l ~ 2 , ~ ~ ~ ~  and servT5tg Fadfbrate various 
analytical p ing force prediction methods 9 3 9 . One 
measurements on a rotor system and in separating the destabilizing 
cross coupled direct force from the viscous damping forces. 
To demonstrate the importance of inlet swirl in the generation 
of destabilizing packing forces, a rotor packing model with very 
simple instrumentation was built and tested in our laboratory. With 
inlet swirl velocities greater than packing surface speed and in the 
same direction, the rotor became unstable at its first critical 
speed. Shaft rotation was reversed and the rotor again became 
unstable with about the same pressure drop across the packing. In 
this case, however, the subsynchronous orbit continued rotating in 
the direction of inlet swirl opposing the direction of shaft 
rotat ion. 
investigator M Wright has reported success in making direct force 
We then modified the packing model and with additional 
instrumentation, carried out a limited test program to measure the 
pressure and flow conditions which would cause instability with 
281 
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alternate packing designs. These tests have the virtue that both 
direct and damping forces are physically integrated by the rotor. 
They have the disadvantage that, with the bearings and 
instrumentation used in this program, we could not quantify the 
level of the destabilizing force nor could we separate the direct 
from the damping forces to make comparisons with published data. 
Discussion of Initial Test Results 
Figure 1 shows the assembled rotor/labyrinth facility. The two 
By opening both 
flexible air hoses are independently valved to permit either forward 
or backward swirl without changing shaft rotation. 
valves together the inlet velocities are essentially cancelled and a 
test condition of negligible swirl exists. While the preliminary 
tests had used a single admission with reversed motor rotation to 
get backward swirl, the two admission modification greatly 
simplified the test procedure and analysis. 
Figures 2 and 3 show the rotor and labyrinth components of this 
facility. Air is admitted to the central plenum through the 
tangential inlets. Pitot tubes installed in the plenum measured the 
swirl velocity. Two static pressure taps are also located in the 
inlet plenum. 
strong rotational speed dependency. As shown by Figure 4, we did 
not find any. 
dynamic characteristics over the speed range so that we can learn if 
the apparent larger packing forces at low speed is a real effect. 
A typical vibration spectrum at the onset of instability is 
displayed on Figure 5 for a backward swirl. Filtered synchronous 
and subsynchronous orbits were simultaneously displayed and forward 
and backward orbits were easily distinguished. Figure 6 shows this 
display for the typical backward swirl condition. 
The first series of tests were to determine if there was a 
In future test programs we will measure the bearing 
Figure 7 shows some particularly interesting data. Both forward 
and reverse admission valves were fully opened to the shop air 
supply. The plenum chamber built up to 15 psia and the rotor 
remained 88stable18 by previous criteria. When the instrument gain 
was increased by lox, a non-synchronous orbit was observed at about 
15 Hertz, sometimes rotating in the synchronous direction sometimes 
against. 
The initial tests were run at .010 radial clearance on all the 
teeth. The packing casing was then disassembled without disturbing 
the rotor and bearings, and the clearances were enlarged in a 
diverging geometry from .0108' at the packing inlet to .020 mils at 
the exit. The third and last test of the clearance series was with 
the clearances further enlarged to a cylindrical .020" clearance. 
The data for the six test conditions are show on Figure 8. Since 
there has been much conjecture about the "stabilizing" effect of 
diverging clearance, special attention was given to the diverging 
test condition. In addition to the usual visual examination of 
orbits on the CRT, analysis of the signal confirmed that the 
282 
subsynchronous orbit with diverging clearance packings was in the 
same direction as inlet swirl and, in all respects, the divergitlg 
W 2 0  mil clearance case behaved 1 ike an intermediate clearance 
between 10 and 20 mils. 
The raw data is misleading in indicating a lower critical 
pressure, hence, a higher level of instability- as clearances are 
increased. This effect is related to our test model, since as 
clearances are increased the inlet velocity a1 o increases and this 
increases inlet swirl. Benchert and Wachter ' s f  1) short packing 
formula shows packing force increasing proportionally with inlet 
velocities from our pitot data and the referenced force formulation, 
we have corrected the raw trend line of Figure 7 to a constant inlet 
swirl condition. 
ure and inlet swirl energy. Using average tangential 
A further test was carried out to assess the effect of swirl, 
independent of clearance. The labyrinth packing casing was modified 
with temporary nozzle liners in the inlets and tests were repeated 
at the . O N  mil clearance but now with higher nozzle inlet 
velocities. The results of these tests are shown on Figure 9 and 
confirm the strong correlation between destabilizing force and inlet 
swirl velocity. 
P R O P O S E D  TEST PROGRAM 
As noted earlier, it was not possible to obtain a quantitative 
determination of destabilizing forces contributed by the packings 
because the flexible shaft and the unknown and asymmetrical cross 
coupling stiffness forces of the cylindrical bearings. In a future 
series of tests, this problem will be remedied by using a stiff test 
rotor supported on tilting pad bearings with adjustable, isotropic 
calibrated stiffness and damping characteristics. A picture of the 
test facility is shown.in Figure 10. 
This facility was originally designed for dynamic testing of 
bearings but is being modified by replacing the midspan test bearing 
with an appropriate labyrinth test seal and by installing the 
special isotropic support bearings. A schematic of the test rig is 
shown in Figure 11. Air is introduced into a central plenum, passes 
through the labyrinth and exhausts to atmosphere at either end. 
Various seal configurations and clearances will be tested. The air 
in the plenum will be introduced with controlled and measured swirl. 
The tilting pad support bearings are punted on springs having a 
stiffness value of approximately 2.3 x 10 lb/in. Two pads at 
each end of the rotor are oriented at 450 from vertical, resulting 
in isotropic stiffness and damping characteristics; i.e., vertical 
and horizontal stiffness are the same. Use of tilting pads 
eliminates any destabilizing cross coupling stiffness coefficients 
from the bearings. 
For such an isotropic system, the force balance at the threshold 
283 
of instabi 1 ity is: 
where Ks is the destabilizing cross coupling stiffness force 
generated by the test seal, Bb is the isotropic damping 
coefficient of the spring mounted support bearings, Bs is the 
damping coefficient of the seal and Y is the natural frequency of 
the rotor on its support bearings. Because the tilting pad bearings 
are mounted on springs whose stiffness is relatively low compared 
with the stiffness of the oil film between the pad and the rotor, 
the effective damping Bb of the oil film and support spring in 
series is quite low and may be varied by changing the vertical load 
imposed on the shaft as shown in Figure 12. Test procedures would 
establish a desired level of damping Bb in the test rig by 
imposing the appropriate load, bring the rotor up to the desired 
test speed, and then slowly increase the plenum pressure in the test 
seal up to the onset of whirl. The net destabilizing force from the 
seal, (Ks - Bs)P , will then be determined from the equation 
above. Damping of the support bearings, Bb, can be determined 
either by measuring the exponential decay of shaft vibration induced 
by striking the test rotor or by measuring the response of the rotor 
to a known unbalance. 
A recognized deficiency of the proposed testing is that it will 
not yield separate determination of Ks and Bs, but only a 
measurement of the net destabilizing effect of the seal. However, 
the tests will suffice to check the ability of various analyses to 
predict this overall destabilizing force permitting one to evaluate 
the usefulness of these analyses as design tools. 
ACKNOWLEDGMENT 
The authors acknowledge the substantial assistance of L. L. Bethel 
and S. I(. Tung in all phases of this rotor stability investigation. 
REFERENCES 
1.  
2. 
3 .  
4. 
5.  
6 .  
Benchert, H.;  and Wachter, J.: Flow Induced Spring Constants o f  
Labyrinth Seals, I. Mech. E., Sept. 1980. 
Leong, Y . ;  and Brown, R.: Circumferential Pressure Distribution 
in a Model Labyrinth Seal, NASA CP-2250, 7982, pp. 223-241. 
Childs, D.;  and Dressman, J.: Testing of Turbulent Seals for 
Rotordynamic Coefficients, NASA CP-2250, 1982, pp. 157-171. 
Greathead, S.; and Slocombe, M.:  Further Investigations into Load 
Dependent Low Frequency Vibration of the High Pressure Rotor on 
Large Turbo-Generators, I. Mech. E., Sept. 1980. 
Iwatsubo, T.; et al.: Flow Induced Force of Labyrinth Seal, NASA 
Murphy, 8; and Vance, J.: Labyrinth Seal Effects on Rotor Whirl 
Instability, Proceedings I. Mech. E., 1980. 
CP-2250, 1982, pp. 205-222. 
c 
284 
7. Kurohashi, M.;  et al.: Spring and Damping Coefficients of 
Labyrinth Seals, Proceedings I. Mech. E . ,  1980. 
8. Kostyuk, A.: Theoretical Analysis of Aerodynamic Forces in 
Labyrinth Glands o f  Turbomachines, Teploenergetika, 1972. 
9. Wright, D. V. :  Air Model Tests of Labyrinth Seal Forces on a 
Whirling Rotor, Eng. Power, Oct. 1978. 
285 
40.00 
Figure 2. - Schematic of rotor f o r  packing stability test. 
286 
Figure 3. - Schematic of l aby r in th  f o r  packing s t a b i l i t y  test. 
287 
8. 
5.- 
t; a ' * -  
a 
I 
-I c 3. 
2 
c, 
(Y u 
2. 
1. 
ai 
Figure 4 .  - Threshold pressure drop at various rotational speeds. 
- 
- 
- 
I I I 1 
288 
Figure 5. - Typical CCW admission vibration spectrum €or packing 
stability test; CW rotation, 2000 rpm; 4 psi; CCW inlet. 
289 
Figure 6. - Typical CCW admission orbits for packing stability test; 
CW rotation; 2000 rpm; 4 psi; CCW inlet. 
1OX G A I N  
Figure 7. - CW and CCW admission orbits for packing stability test; CW 
rotation; 2000 rpm; 15 psi; both inlets fully open. 
290 
8. 
5. 
(CI 
In '. a 
a 
-I < 3. 
E 
t 
M 
2. 
1. 
I 
8.00- 
5.08 
4.00 
3.88 
2. 08 
1.88 
- A  
_/--- _ -  
CORRECTED TO CONSTANT SWIRL _- -  
cc--- 
- 
- 
- 
- 
I I I I 
m a m 
AVERAGE CLEARANCE - MILS 
Figure 8. - Threshold pressure drop at various clearances. 1600 rpm. 
 
NOZZLE INLET AREA - SQ. I N .  
Figure 9. - Threshold pressure drop at alternate inlet areas. 1600 
rpm. 
291 
Figure 10. - Large-stiff-rotor labyrinth packing facility. 
292 
BEAR I NG SEAL LOADER 
4 48 FLOW i n .  /- 
I 1 1  
7 in. SHAFT i3; I % DRIVE ~ 0-3600 MOTOR RPN 1 
SIDE VIEW 
LOADER BEARING 
\ 
SUPPORT BEARING 
Figure 11. 
END VIEW I 
SUPPORi BEAR1 NG 
SPRING, K = 2.3 x l o 4  l b / i n .  
- Schematic of seal force test rig. 
293 
a n 
1- 
I I I I I I 
200  400 600 800 1000 1200 1400 
LOAD, LB 
Figure 12.  - Test r i g  damping a t  various bearing loads. 
294 


Preliminary investigation of labyrinth packing pressure drops at onset of swirl-induced rotor instability

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