Much speculation has surrounded the possible unsteady hydrodynamic forces which could be responsible for the excitation of whirl instabilities in turbomachines.  However there exist very few measurements of these forces which would permit one to evaluate the merits of the existing fluid mechanical analyses.  In keeping with the informal nature of this workshop we will present details of a proposed test program for the measurement of the unsteady forces on centrifugal impellers caused by either (i) azimuthal asymmetry in the volute geometry or (ii) an externally imposed whirl motion of the impeller.  In the second case the forces resulting from the imposed whirl motions with frequencies ranging from zero to synchronous will be measured by means of a force balance upon which the impeller is mounted.  This work is presently being carried out under contract with the NASA George Marshall Space Flight Center, Huntsville, Alabama (Contract NAS 8-33108)

Acosta, A. J.

Brennen, C. E.

Caughey, T. K.

English

Caltech Authors - Main

A TEST PROGRAX TO MEASURE FLUID MECHANICAL 
WHIRL-EXCITATION FORCES I N  CENTRIFUGAL PUMPS 
C.E.  Brennen, A . J .  Acosta and T.K. Caughey 
C a l i f o r n i a  I n s  t i  cure  of Technology 
Pasadena, Cal i furn3 a 91125 
Euch specu la t ion  has surrounded t h e  p o s s i b l e  unsteady hydrodynamic f o r c e s  
which could be r e spons ib l e  f o r  t h e  e x c i t a t i o n  of w h i r l  i n s t a b i l i t i e s  i n  turbo- 
machines. However t h e r e  e x i s t  very few measurements of t hese  fo rces  which 
would permit  one t o  e v a l u a t e  t h e  m e r i t s  of t h e  e x i s t i n g  f l u i d  mechanical m a l -  
yses .  I n  keeping wi th  t h e  informal  n a t u r e  of t h i s  workshop we w i l l  p resent  
d e t a i l s  of a proposed t e s t  program f o r  t h e  measurement of t h e  unsteady f o r c e s  
on c e n t r i f u g a l  impe l l e r s  caused by e i t h e r  ( i )  azimuthal  asymmetry i n  t h e  vol-  
u t e  geometry o r  ( i i )  an e x t e r n a l l y  imposed w h i r l  motion of t h e  impel le r .  I n  
the second case  t h e  f o r c e s  r e s u l t i n g  from the imposcd w h i r l  motions wi th  f r e -  
quencies ranging from ze ro  t o  synchronous w i l l  b e  measured by means of a f o r c e  
ba lance  upon which t h e  impel le r  i s  mounted. This  work i s  present1.y being car -  
r i e d  orst under c o n t r a c t  wi th  t h e  NASA Ceurge Marshal l  Space F l i g h t  Center ,  
H u n t s v i l l e ,  Alabama (Contract  NAS 8-33108). 
INTRODUCTION 
In r ecen t  y e a r s  It has  been i n c r e a s i n g l y  recognized that hydrodynamic 
cross-coupli.ng f o r c e s  can cause s e r i o u s  r o t o r  dynamic problems i n  high speed 
turbomachines. Such problems have been experienced n o t  only i n  steam t u r b i n e s  
( R e f s . l , 2 ) ,  bu t  a l s o  i n  l a r g e  cumpressvrs (Refs. 3 , 4 )  and i n  high speed pumps 
(Ref. 5 ) .  Though o t h e r  i n s ~ a b i l i t y  mechanisms such a s  i n t e r n a l  damping o r  
non- i so t ropic  s h a f t  s t i f f n e s s  r o t o r  i n e r t i a  can be f a i r l y  r e a d i l y  cha rac t e r i z -  
ed t h e  same cannot be  s a i d  of t h e  hydrodynamic cross-coupling fo rces .  Various 
hydrodynamic flows have been i d e n t i f i e d  as p o s s i b l e  c o n t r i b u t o r s  t o  t hese  de- 
s t a b i l i z i n g  f o r c e s ,  Eear ings  and p a r t i c u l a r l y  s e a l s  can c l e a r l y  p lay  such a 
r o l e  a s  t h e  papers  i n  t h i s  volume a t t e s t .  But much l e s s  i s  known about t h e  
p o t e n t i a l  f o r  d e s t a b i l i z i n g  f o r c e s  a r i s i n g  from t h e  flows a s soc i a t ed  with t h e  
jmpel ler  and d i f f u s e r  of a  compressor o r  pump; t h i s  b r i e f  paper w i l l  be con- 
f i ned  t o  t h i s  i s s u e .  
The flow a s s o c i a t e d  wick t h e  i m p e l l e r / d i f f u s e r  i s  s i g n i f i c a n t l y  d i f f e r -  
en t  from t h a t  i n  a s e a l  o r  bea r ing  i n  t h a t  t h e  geometry of t h e  vo lu t e  can 
cause c h i s  flow t o  b e  s i g n i f i c a n t l y  non-axisymmetric. The work of Domm and 
Hergt (Ref. 6)  h a s  shown t h a t  t h i s  could g ive  r i s e  t o  a l a t e r a l  f o r c e  on t h e  
impe l l e r  even i n  t h e  absence of an o r b i t i n g  motion of t h e  s h a f t .  They der ived  
magnitudes f o r  t h i s  s t a t i o n a r y  l a t e r a l  f o r c e  based on a source  v o r t e x  model 
f o r  the impel le r .  I n  an  a c t u a l  impe l l e r  t h i s  l a t e r a l  f o r c e  may a l s o  con ta in  
un unsteady component a t  t h e  bl.ade passage frequency though the magnitude of 
t h i s  unsteady f o r c e  and the r o l e  i t  might p l a y  i n  the r o t o r  dy~arn ics  has no t  
yet been determined. More r e a l i s t i c  i t t ipeller models would be requi red  fo r  t h i s  
purpose. 
Recerlrly Colding-Jorpensen ( R e f .  7 .  see a l s o  t h i s  vulume) has exterideZ 
t h e  work of Domm and Hergt.  Iniposing an o r b i t i n g  motion un t h e  suu rce lvu r t ex  
model of t h e  impel.ler,  he analyzed t h e  r e s u l t i n g  r a d i a l  and t a n g e n t i a l  f o r c e s  
on the "impeller" and found tha t  p o s i t i v e  o r  w h i r l  e x c i t a t i o n  t a n g e n t i a l  f o r c e s  
d i d  indeed r e s u l t  from such an a n a l y s i s  and t h a t  the magnitudes of t h e s e  f o r c e s  
w e r e  s u f f i c i e n t  t o  have played a s i g n i f i c a n t  r o l e  i n  t h e  Space  S h u t t l e  high 
pres su re  hydrogen pump i n s t a b i l i t y  (Ref. 5 ) .  However the main ques t ion  here is  
t h e  v a l i d i t y  of t h e  s i m p l e  source/vortex model used f o r  t h e  impel le r :  f u r t h e r  
analyses wi th  aure r e a l i s t i e  i n p c l l e r  models w i l l  be require2  i n  o r d e r  t o  deter-  
mine t h e  l i m i t s  of v a l i d i t y  of such analyses .  O n e  u se fu l  intermediate modcl 
s h i c h  is s t i l l  f a i r l y  slnlple is  t h e  a c t u a t o r  d i s k  which has been suggested by 
t he  work of Chamieh and A c u s t u  (Ref .  8 ,9 ) .  
Other ana lyses  have sought d e s t a b i l i z i n g  t a n g e n t i a l  f o r c e s  i n  t h e  absence 
of v o l u t e  asynnnetries. Here w e  have two p o s s i b l e  mechanisms which have been 
suggested.  I n  an addenda t o  his paper on whirl-excitation forces i n  s e a l s ,  
Alford (1965) suggested t h a t  t h e  same mechanism would occur  i n  the t i p  c l e a r -  
ance  Ilows of steam t u r b i n e s  and o t h e r s  have f u r t h e r  ex f r apo la t ed  t h i s  t o  the 
t i p  clearance f lows i n  any turbomachine. The suppos i t i on  i s  t h a t  a l o c a l i z e d  
r educ t ion  i n  chc t i p  clearance would cause a l o c a l i z e d  decrease i n  the  torque  
shea r  stress w i t h  the r e v e r s e  occurr ing  on t h e  o t h e r  end of t h c  diameter. This 
would lead  t o  a p o s i t i v e  o r  wh i r l - exc i t i ng  t angen t i a l  fo rce .  
Secondly, Thompson (Ref. 3) has posed t h e  ques t ion  of whether such a 
force would arise due t o  t h e  pe r tu rba t ions  imposed on t h e  main f l o w  through the 
impe l l e r ,  Assuming q u a s i - s t a t i c  periormance f o r  each b lade  passage he us& a 
d e t a i l e d  flow analysis program t o  assess the d i s t r i b u t i o n  of f o r c e s  on t h e  
b l ades  of an impel le r  performing wh i r l i ng  motions. P o s i t i v e  e x c i t a t i o n  d id  
indeed occur  with some i m p e l l e r  des igns .  It is  not  however c l e a r  whether t h e  
q u a s i - s t a t i c  assumption i s  v a l i d  i n  t h e s e  circumstances.  
Ulrimately any review of t h e  e x i s t i n g  s t a t e  of knowledge such as we have 
given above reveals  a s ingular  lack of  hard experimental  d a t a  w i t h  which t o  
assess the  m e r i t s  of t h e  vari-ous analyses.  T h e  primary purpase of t h i s  paper 
i s  t o  stress the  facL that  much remains t o  be done be fo re  the  status of whirl- 
e x c i t a t i o n  fo rces  on impe l l e r s  can be  p u t  on a f i rm  foundat ion.  W e  a r e  pre-  
s e n t l y  embarking on an experimental  program whose i n t e n t  i s  t o  measure t h e s e  
f o r c e s  and t h e  remainder of t h i s  paper is  devuted t o  a b r ie f  d e s c r i p t i o n  of 
t h i s  f a c i l i t y .  
ROTOR FORCE TEST FACILITY 
The o b j e c t i v e  of the  f a c i l i t y  is  t o  impose know o r b i t i n g  motions of f re-  
quency Si on the basic  r o t a r y  motion of a number of t y p i c a l  c e n t r i f u g a l  pump 
impe l l e r s  ( r o t a t i n g  frequency, 3) and t o  measure t h e  r a d i a l  and t a n g e n t i a l  
f o r c e s  r e s u l t i n g  from this motiun. The co re  of the experiment i s  shown i n  

7ig. l .The flow e n t e r s  the c e n t r i f u g a l  impel le r (6) f rom t h e  i n l e t  connect ion(3)  
and i n l e t  b e l l ( 4 ) .  ~ o l u t e s ( 2 ) o E  va r ious  geometries w i l l  b e  used i n  measure- 
ments of l a t e r a l  fo rce  due t o  v o l u t e  asymmetry: t hey  a r e  contained i n  t h e  pump 
housing (1) so  t h a t  they may be  of l i gh twe igh t  cons t ruc t ion .  The i c p e l l e r  (5) 
i s  mounted d i r e c t l y  on a n  i n t e r n a l  f o r c e  balance ( 6 ) .  The main s h a f t  (10) 
r o t a t e s  i n  a double bea r ing  system (7,8,11) designed s o  t h a t  r o t a t i o n  of t h e  
sprocket  ( 9 )  causes t h e  o r b i t i n g  motion. I n  a d d i t i o n  t o  t h e  r o t a t i n g  i n t e r n a l  
ba lance  a back-up f o r c e  measuring sys tem(l2 ,13)externa l  t o  t h e  e c c e n t r i c  d r i v e  
system w i l l  a l s o  be used. Figure 2 ,  an o v e r a l l  schematic of t h e  mechanical 
system, shows t h e  main d r i v e  system comprised of a 20HP v a r i a b l e  speed motor, 
a  gear  box and a f l e x i b l e  s h a f t  (necessary t o  acconunodate t h e  o r b i t i n g  motion).  
This  w i l l  be capable of speeds up t o  35000 rpm (u  -t 6OHz.) F igure  2 a l s o  shvws 
t h e  e c c e n t r i c  d r i v e  motor (2HP) which can genera te  o r b i t  f r equenc ie s ,  R, 
from 0 t o  60 Hz. 
Pas t  experience w i t h  unsteady flow measurements has  emphasized t h e  need 
f o r  r igorous  dynamic c o n t r o l  of t h e  experiments.  I n  t h e  p re sen t  f a c i l i t y  a  
proxinlity probe senses  t h e  r o t a t i n g  frequency, w: t h i s  s i g n a l  is  then used 
t o  produce d i g i t a l  s i g n a l s  w i t h  f requencies  Mw/N, w/N, (N-M)u/N and 
(N+M)u/N where M and N are s e l e c t e d  i n t e g e r s  between 0 and 100 (MzN). 
The s i g n a l  a t  P ~ / N  is  then  used t o  d r i v e  t h e  e c c e n t r i c  motor such t h a t  
R= k ) / N ;  a  feedback c o n t r o l  system f i t t e d  t o  t h i s  motor ensures  a c c u r a t e  phase- 
lack ing .  I n  t h i s  way measurements can be  made over a  wide range of R / u  from 
zero  t o  un i ty .  
Pas t  experience has  a l s o  ind ica t ed  t h a t  t h e  unsteady flow measurements 
can be q u i r e  noisy  and t h a t  accu ra t e  r e s u l t s  r e q u i r e  c r o s s - c o r r e l a t i o n  of 
measurement and d r i v i n g  s i g n a l  over  many cyc le s .  I n  t h e  p re sen t  ca se  the  
measuremenrs may inc lude  (i) six-components of force measurement from t h e  
i n t e r n a l  balance o r  s e v e r a l  cumpments of f o r c e  from t h e  e x t e r n a l  ba lance  ( i i )  
a x i a l  and r a d i a l  displacement  of t h e  impe l l e r  from fou r  f i b re -op t i c  d i sp l ace -  
ment probes (iii) pres su re  t ransducer  measurements from l o c a t i o n s  bo th  up- 
s t ream and downstream of t h e  impe l l e r  a s  we l l  a s  w i t h i n  t h e  impel le r -volu te  
flow (iv) unsteady flow r a t e  measurement us ing  e x i s t i n g  l a s e r  doppler  ve loc i -  
meters  o r  e lectro-magnet ic  flow meters .  A s i x t e e n  channel d i g i t a l  s i g n a l  
processor  i s  used f o r  c ros s -co r re l a r ion  of t h e s e  measurements w i th  any of t h e  
b a s i c  f requencies  ( w , M f * r / N , w / N , ( ~ - M ) w / N  and (N+M)u/N) i n  o rde r  t o  e x t r a c t  
t h e  components of f o r c e  a t  any of t h e s e  fundamental f requencies  
o r  h ighe r  harmonics t h e r e o f ;  t h i s  c ros s -co r re l a t ion  can cover  up t o  10,000 
cyc le s  of t h e  fundamental frequency. 
The m o s t  c r i t i c a l  component i n  t h e  system i s  probably r h e  i n r e r n a l  b a l -  
ance and i ts  design i s  a s  y e t  unproven. One must ensure  high n a t u r a l  f r e -  
quencies  (>500Hz.) and y e t  maintaill  s u f f i c i e n t  s e n s i t i v i t y .  The p re sen t  can- 
d i d a t e  design c o n s i s t s  of fou r  p o s t s  p a r a l l e l  wi th  t h e  a x i s  which a r e  mono- 
l i t h i c  with two s o l i d  end p l a t e s ,  one a t t ached  t o  t h e  s h a f t  and t h e  o t h e r  a t -  
tached t o  t h e  impel le r .  These pos t s  a r e  s t rain-gaged i n  such a  way as t o  
e x t r a c t  a l l  s i x  fo rce  componenLs with s e n s i t i v i t i e s  of t h e  o r d e r  of mV/lb. 
The hydrau l i c  system i n t o  which t h i s  mechanical system i s  be ing  installed 
was previously u t i l i z e d  f o r  measurements of t h e  unsteady hydrau l i c  performance 
of c a v i t a t i n g  (and non-cavi tat ing)  ax ia l  flow pumps ( s e e ,  f o r  example Refs. 

1 0 1 1 )  S p e c i f i c a l l y  t h i s  involved f i n d i n g  t h e  r e l a t i o n s  between t h e  unsteady 
p re s su res  and flow r a t e s  a t  i n l e t  and a t  d i scharge  a s  a func t ion  of t h e  f r e -  
quency of t h e  unsteady components of t h e  flow and of t h e  mean flow condi t ions .  
A schematic of t h i s  system, known a s  t h e  Dynamic Pump Tes t  F a c i l i t y  (DPTF), i s  
included a s  Fig. 3 .  The mechanical system of t h e  Rotor Force Tes t  F a c i l i t y  
(RFTF) descr ibed  above i s  being i n s t a l l e d  i n  t h e  lower lef t -hand corner  i n  
SIREN VALYE 
PHASE-LOCK 
DRIVE- 
'SILENT'THROTTLE WWNSTREAM 
OOWNSTREAM 
ELECTROMAGNETIC 
FLOW METER 
OOWNSTREAM L O V  
MEASUREMENT SECTION 
OOWNSTREAM FLOW 
SMOOTHING SECTION 
TRANSPARENT HOUSING 
b IMPELLER 
PHom MULTIPLIER a ups UPSTREAM FLOW SMOOTHING SECTION 
- UPSTREAM ELECTRO- 
MAGNETIC FLOW METER 
Figure  3 .  Schematic planview of t h e  Dynamic Pump Tes t  F a c i l i t y  
p l ace  of t h e  impe l l e r /vo lu t e  system depic ted  i n  t h e  Fig.  3 .  The DPTF included 
two flow f l u c t u a t o r s  ( s i r e n  va lves)  w i th  phase-lock d r i v e  systems s i m i l a r  t o  
t h a t  descr ibed  above f o r  t h e  new e c c e n t r i c  d r i v e  system a s  w e l l  a s  instrumenta-  
t i o n  f o r  measurement of t h e  sma l l  o s c i l l a t o r y  p e r t u r b a t i o n s  i n  p re s su re  and 
flow r a t e .  I n  t h e  context  of t h e  p re sen t  experiment t h e s e  w i l l  aga in  be u t i -  
l i z e d  a t  a l a t e r  s t a g e  t o  i n v e s t i g a t e  whether flow f l u c t u a t i o n s  a r e  a s s o c i a t e d  
w i t h  impel le r  wh i r l  o r  whether they  can induce impe l l e r  wh i r l .  From a g l o b a l  
dynamic po in t  of view i t  i s  p o s s i b l e  t h a t  t h e  e n t i r e  system of f l u c t u a t i o n s  
(mechanical and hydraul ic )  may have t o  be represented  by a g loba l  t r a n s f e r  
func t ion .  
CONCLUDING REMARKS 
Though b e r e f t  of r e s u l t s  t h i s  paper i s  intended t o  h i g h l i g h t  t h e  l a c k  of 
experimental  evidence f o r  wh i r l - exc i t i ng  f l u i d  f o r c e s  a r i s i n g  from t h e  flow 
through turbomachines. We hope t h a t  t h e  experimental  f a c i l i t y  descr ibed  
above w i l l  be  va luab le  i n  provid ing  such evidence. 
REFERENCES 
Alford J .S . ,  1965. P r o t e c t i n g  turbomachinery from se l f - exc i t ed  r o t o r  
w h i r l .  J.  Eng. Power, Vol. 87A, pp. 333-344. 
Pollman, E.,  Schwerdtfeger,  H. and Termuehlen, H . ,  1978. Flow exc i t ed  
v i b r a t i o n s  i n  h igh  p re s su re  t u r b i n e s  (steam w h i r l )  . J. Eng. Power, Vol. 
100, pp.  219-228. 
Thompson, W.E., 1978. F lu id  dynamic e x c i t a t i o n  of c e n t r i f u g a l  compressor 
r o t o r  v i b r a t i o n s .  J. F lu ids  Engineering,  Vol.. 100, No. 1, pp.  73-79. 
Doyle, H.E., 1980. Problems wi th  rotodynamic i n s t a b i l i t i e s  i n  high 
performance turbomachinery. Proc. Workshop on Rotordynamic I n s t a b i l i t y  
Problems i n  High Performance Turbomachinery, May 1980, Texas A & M Univ. 
Ek, M . C . ,  1978. Solu t ions  of t h e  subsynchronous w h i r l  problem i n  t h e  
h igh  p re s su re  hydrogen turbomachinery of t h e  Space S h u t t l e  Main Engine. 
AIAA/SAE 14 th  J o i n t  Propulsion Conference, Las Vegas, J u l y  1978. 
Domm, H . ,  and Hergt ,  P . ,  1970. Radia l  f o r c e s  on impel le r  of v o l u t e  
ca s ing  pumps, Flow Research on Blading (L.S. Dzung, e d . ) ,  E l s e v i e r ,  
pp.  305-321. 
Colding-Jorgensen, J . ,  1979. The e f f e c t  of f l u i d  fo rces  on r o t o r  s t a b i l -  
i t y  of c e n t r i f u g a l  compressors and pumps, Ph.D. Thes is ,  Technical  Univ. 
of  Denmark. 
Chamieh, D. and Acosta,  A . J . ,  1979. Dynamic f o r c e s  on a wh i r l i ng  c e n t r i f  
uga l  r o t o r .  Proc. 6 t h  Conf. on F lu id  Machinery, Akademiai Kiado, 
Budapest, Hungary. 
Chamieh, D . ,  Acosta,  A . J . ,  Brennen, C.E. and Caughey, T . K . ,  1980. A 
b r i e f  n o t e  on t h e  i n t e r a c t i o n  of a n  a c t u a t o r  cascade wi th  a s i n g u l a r i t y .  
Unpublished manuscript.  
Ng, S.L. and Brennen, C . ,  1978. Experiments on t h e  dynamic behavior  of 
c a v i t a t i n g  pumps. J.  F lu ids  Eng., Vol. 100, pp. 166-176. 
Brennen, C.E., Meissner,  C . ,  Lo, E.Y. and Hoffman, G.W. ,  1980. Sca l e  
e f f e c t s  i n  t h e  dynamic t r a n s f e r  func t ions  f o r  c a v i t a t i n g  inducers .  To 
be presented  a t  ASME Winter Annual Meeting, Chicago, 1980. 


A Test Program to Measure Fluid Mechanical Whirl-Excitation Forces in Centrifugal Pumps

A TEST PROGRAX TO MEASURE FLUID MECHANICAL 
WHIRL-EXCITATION FORCES I N  CENTRIFUGAL PUMPS 
C.E.  Brennen, A . J .  Acosta and T.K. Caughey 
C a l i f o r n i a  I n s  t i  cure  of Technology 
Pasadena, Cal i furn3 a 91125 
Euch specu la t ion  has surrounded t h e  p o s s i b l e  unsteady hydrodynamic f o r c e s  
which could be r e spons ib l e  f o r  t h e  e x c i t a t i o n  of w h i r l  i n s t a b i l i t i e s  i n  turbo- 
machines. However t h e r e  e x i s t  very few measurements of t hese  fo rces  which 
would permit  one t o  e v a l u a t e  t h e  m e r i t s  of t h e  e x i s t i n g  f l u i d  mechanical m a l -  
yses .  I n  keeping wi th  t h e  informal  n a t u r e  of t h i s  workshop we w i l l  p resent  
d e t a i l s  of a proposed t e s t  program f o r  t h e  measurement of t h e  unsteady f o r c e s  
on c e n t r i f u g a l  impe l l e r s  caused by e i t h e r  ( i )  azimuthal  asymmetry i n  t h e  vol-  
u t e  geometry o r  ( i i )  an e x t e r n a l l y  imposed w h i r l  motion of t h e  impel le r .  I n  
the second case  t h e  f o r c e s  r e s u l t i n g  from the imposcd w h i r l  motions wi th  f r e -  
quencies ranging from ze ro  t o  synchronous w i l l  b e  measured by means of a f o r c e  
ba lance  upon which t h e  impel le r  i s  mounted. This  work i s  present1.y being car -  
r i e d  orst under c o n t r a c t  wi th  t h e  NASA Ceurge Marshal l  Space F l i g h t  Center ,  
H u n t s v i l l e ,  Alabama (Contract  NAS 8-33108). 
INTRODUCTION 
In r ecen t  y e a r s  It has  been i n c r e a s i n g l y  recognized that hydrodynamic 
cross-coupli.ng f o r c e s  can cause s e r i o u s  r o t o r  dynamic problems i n  high speed 
turbomachines. Such problems have been experienced n o t  only i n  steam t u r b i n e s  
( R e f s . l , 2 ) ,  bu t  a l s o  i n  l a r g e  cumpressvrs (Refs. 3 , 4 )  and i n  high speed pumps 
(Ref. 5 ) .  Though o t h e r  i n s ~ a b i l i t y  mechanisms such a s  i n t e r n a l  damping o r  
non- i so t ropic  s h a f t  s t i f f n e s s  r o t o r  i n e r t i a  can be f a i r l y  r e a d i l y  cha rac t e r i z -  
ed t h e  same cannot be  s a i d  of t h e  hydrodynamic cross-coupling fo rces .  Various 
hydrodynamic flows have been i d e n t i f i e d  as p o s s i b l e  c o n t r i b u t o r s  t o  t hese  de- 
s t a b i l i z i n g  f o r c e s ,  Eear ings  and p a r t i c u l a r l y  s e a l s  can c l e a r l y  p lay  such a 
r o l e  a s  t h e  papers  i n  t h i s  volume a t t e s t .  But much l e s s  i s  known about t h e  
p o t e n t i a l  f o r  d e s t a b i l i z i n g  f o r c e s  a r i s i n g  from t h e  flows a s soc i a t ed  with t h e  
jmpel ler  and d i f f u s e r  of a  compressor o r  pump; t h i s  b r i e f  paper w i l l  be con- 
f i ned  t o  t h i s  i s s u e .  
The flow a s s o c i a t e d  wick t h e  i m p e l l e r / d i f f u s e r  i s  s i g n i f i c a n t l y  d i f f e r -  
en t  from t h a t  i n  a s e a l  o r  bea r ing  i n  t h a t  t h e  geometry of t h e  vo lu t e  can 
cause c h i s  flow t o  b e  s i g n i f i c a n t l y  non-axisymmetric. The work of Domm and 
Hergt (Ref. 6)  h a s  shown t h a t  t h i s  could g ive  r i s e  t o  a l a t e r a l  f o r c e  on t h e  
impe l l e r  even i n  t h e  absence of an o r b i t i n g  motion of t h e  s h a f t .  They der ived  
magnitudes f o r  t h i s  s t a t i o n a r y  l a t e r a l  f o r c e  based on a source  v o r t e x  model 
f o r  the impel le r .  I n  an  a c t u a l  impe l l e r  t h i s  l a t e r a l  f o r c e  may a l s o  con ta in  
un unsteady component a t  t h e  bl.ade passage frequency though the magnitude of 
t h i s  unsteady f o r c e  and the r o l e  i t  might p l a y  i n  the r o t o r  dy~arn ics  has no t  
yet been determined. More r e a l i s t i c  i t t ipeller models would be requi red  fo r  t h i s  
purpose. 
Recerlrly Colding-Jorpensen ( R e f .  7 .  see a l s o  t h i s  vulume) has exterideZ 
t h e  work of Domm and Hergt.  Iniposing an o r b i t i n g  motion un t h e  suu rce lvu r t ex  
model of t h e  impel.ler,  he analyzed t h e  r e s u l t i n g  r a d i a l  and t a n g e n t i a l  f o r c e s  
on the "impeller" and found tha t  p o s i t i v e  o r  w h i r l  e x c i t a t i o n  t a n g e n t i a l  f o r c e s  
d i d  indeed r e s u l t  from such an a n a l y s i s  and t h a t  the magnitudes of t h e s e  f o r c e s  
w e r e  s u f f i c i e n t  t o  have played a s i g n i f i c a n t  r o l e  i n  t h e  Space  S h u t t l e  high 
pres su re  hydrogen pump i n s t a b i l i t y  (Ref. 5 ) .  However the main ques t ion  here is  
t h e  v a l i d i t y  of t h e  s i m p l e  source/vortex model used f o r  t h e  impel le r :  f u r t h e r  
analyses wi th  aure r e a l i s t i e  i n p c l l e r  models w i l l  be require2  i n  o r d e r  t o  deter-  
mine t h e  l i m i t s  of v a l i d i t y  of such analyses .  O n e  u se fu l  intermediate modcl 
s h i c h  is s t i l l  f a i r l y  slnlple is  t h e  a c t u a t o r  d i s k  which has been suggested by 
t he  work of Chamieh and A c u s t u  (Ref .  8 ,9 ) .  
Other ana lyses  have sought d e s t a b i l i z i n g  t a n g e n t i a l  f o r c e s  i n  t h e  absence 
of v o l u t e  asynnnetries. Here w e  have two p o s s i b l e  mechanisms which have been 
suggested.  I n  an addenda t o  his paper on whirl-excitation forces i n  s e a l s ,  
Alford (1965) suggested t h a t  t h e  same mechanism would occur  i n  the t i p  c l e a r -  
ance  Ilows of steam t u r b i n e s  and o t h e r s  have f u r t h e r  ex f r apo la t ed  t h i s  t o  the 
t i p  clearance f lows i n  any turbomachine. The suppos i t i on  i s  t h a t  a l o c a l i z e d  
r educ t ion  i n  chc t i p  clearance would cause a l o c a l i z e d  decrease i n  the  torque  
shea r  stress w i t h  the r e v e r s e  occurr ing  on t h e  o t h e r  end of t h c  diameter. This 
would lead  t o  a p o s i t i v e  o r  wh i r l - exc i t i ng  t angen t i a l  fo rce .  
Secondly, Thompson (Ref. 3) has posed t h e  ques t ion  of whether such a 
force would arise due t o  t h e  pe r tu rba t ions  imposed on t h e  main f l o w  through the 
impe l l e r ,  Assuming q u a s i - s t a t i c  periormance f o r  each b lade  passage he us& a 
d e t a i l e d  flow analysis program t o  assess the d i s t r i b u t i o n  of f o r c e s  on t h e  
b l ades  of an impel le r  performing wh i r l i ng  motions. P o s i t i v e  e x c i t a t i o n  d id  
indeed occur  with some i m p e l l e r  des igns .  It is  not  however c l e a r  whether t h e  
q u a s i - s t a t i c  assumption i s  v a l i d  i n  t h e s e  circumstances.  
Ulrimately any review of t h e  e x i s t i n g  s t a t e  of knowledge such as we have 
given above reveals  a s ingular  lack of  hard experimental  d a t a  w i t h  which t o  
assess the  m e r i t s  of t h e  vari-ous analyses.  T h e  primary purpase of t h i s  paper 
i s  t o  stress the  facL that  much remains t o  be done be fo re  the  status of whirl- 
e x c i t a t i o n  fo rces  on impe l l e r s  can be  p u t  on a f i rm  foundat ion.  W e  a r e  pre-  
s e n t l y  embarking on an experimental  program whose i n t e n t  i s  t o  measure t h e s e  
f o r c e s  and t h e  remainder of t h i s  paper is  devuted t o  a b r ie f  d e s c r i p t i o n  of 
t h i s  f a c i l i t y .  
ROTOR FORCE TEST FACILITY 
The o b j e c t i v e  of the  f a c i l i t y  is  t o  impose know o r b i t i n g  motions of f re-  
quency Si on the basic  r o t a r y  motion of a number of t y p i c a l  c e n t r i f u g a l  pump 
impe l l e r s  ( r o t a t i n g  frequency, 3) and t o  measure t h e  r a d i a l  and t a n g e n t i a l  
f o r c e s  r e s u l t i n g  from this motiun. The co re  of the experiment i s  shown i n  

7ig. l .The flow e n t e r s  the c e n t r i f u g a l  impel le r (6) f rom t h e  i n l e t  connect ion(3)  
and i n l e t  b e l l ( 4 ) .  ~ o l u t e s ( 2 ) o E  va r ious  geometries w i l l  b e  used i n  measure- 
ments of l a t e r a l  fo rce  due t o  v o l u t e  asymmetry: t hey  a r e  contained i n  t h e  pump 
housing (1) so  t h a t  they may be  of l i gh twe igh t  cons t ruc t ion .  The i c p e l l e r  (5) 
i s  mounted d i r e c t l y  on a n  i n t e r n a l  f o r c e  balance ( 6 ) .  The main s h a f t  (10) 
r o t a t e s  i n  a double bea r ing  system (7,8,11) designed s o  t h a t  r o t a t i o n  of t h e  
sprocket  ( 9 )  causes t h e  o r b i t i n g  motion. I n  a d d i t i o n  t o  t h e  r o t a t i n g  i n t e r n a l  
ba lance  a back-up f o r c e  measuring sys tem(l2 ,13)externa l  t o  t h e  e c c e n t r i c  d r i v e  
system w i l l  a l s o  be used. Figure 2 ,  an o v e r a l l  schematic of t h e  mechanical 
system, shows t h e  main d r i v e  system comprised of a 20HP v a r i a b l e  speed motor, 
a  gear  box and a f l e x i b l e  s h a f t  (necessary t o  acconunodate t h e  o r b i t i n g  motion).  
This  w i l l  be capable of speeds up t o  35000 rpm (u  -t 6OHz.) F igure  2 a l s o  shvws 
t h e  e c c e n t r i c  d r i v e  motor (2HP) which can genera te  o r b i t  f r equenc ie s ,  R, 
from 0 t o  60 Hz. 
Pas t  experience w i t h  unsteady flow measurements has  emphasized t h e  need 
f o r  r igorous  dynamic c o n t r o l  of t h e  experiments.  I n  t h e  p re sen t  f a c i l i t y  a  
proxinlity probe senses  t h e  r o t a t i n g  frequency, w: t h i s  s i g n a l  is  then used 
t o  produce d i g i t a l  s i g n a l s  w i t h  f requencies  Mw/N, w/N, (N-M)u/N and 
(N+M)u/N where M and N are s e l e c t e d  i n t e g e r s  between 0 and 100 (MzN). 
The s i g n a l  a t  P ~ / N  is  then  used t o  d r i v e  t h e  e c c e n t r i c  motor such t h a t  
R= k ) / N ;  a  feedback c o n t r o l  system f i t t e d  t o  t h i s  motor ensures  a c c u r a t e  phase- 
lack ing .  I n  t h i s  way measurements can be  made over a  wide range of R / u  from 
zero  t o  un i ty .  
Pas t  experience has  a l s o  ind ica t ed  t h a t  t h e  unsteady flow measurements 
can be q u i r e  noisy  and t h a t  accu ra t e  r e s u l t s  r e q u i r e  c r o s s - c o r r e l a t i o n  of 
measurement and d r i v i n g  s i g n a l  over  many cyc le s .  I n  t h e  p re sen t  ca se  the  
measuremenrs may inc lude  (i) six-components of force measurement from t h e  
i n t e r n a l  balance o r  s e v e r a l  cumpments of f o r c e  from t h e  e x t e r n a l  ba lance  ( i i )  
a x i a l  and r a d i a l  displacement  of t h e  impe l l e r  from fou r  f i b re -op t i c  d i sp l ace -  
ment probes (iii) pres su re  t ransducer  measurements from l o c a t i o n s  bo th  up- 
s t ream and downstream of t h e  impe l l e r  a s  we l l  a s  w i t h i n  t h e  impel le r -volu te  
flow (iv) unsteady flow r a t e  measurement us ing  e x i s t i n g  l a s e r  doppler  ve loc i -  
meters  o r  e lectro-magnet ic  flow meters .  A s i x t e e n  channel d i g i t a l  s i g n a l  
processor  i s  used f o r  c ros s -co r re l a r ion  of t h e s e  measurements w i th  any of t h e  
b a s i c  f requencies  ( w , M f * r / N , w / N , ( ~ - M ) w / N  and (N+M)u/N) i n  o rde r  t o  e x t r a c t  
t h e  components of f o r c e  a t  any of t h e s e  fundamental f requencies  
o r  h ighe r  harmonics t h e r e o f ;  t h i s  c ros s -co r re l a t ion  can cover  up t o  10,000 
cyc le s  of t h e  fundamental frequency. 
The m o s t  c r i t i c a l  component i n  t h e  system i s  probably r h e  i n r e r n a l  b a l -  
ance and i ts  design i s  a s  y e t  unproven. One must ensure  high n a t u r a l  f r e -  
quencies  (>500Hz.) and y e t  maintaill  s u f f i c i e n t  s e n s i t i v i t y .  The p re sen t  can- 
d i d a t e  design c o n s i s t s  of fou r  p o s t s  p a r a l l e l  wi th  t h e  a x i s  which a r e  mono- 
l i t h i c  with two s o l i d  end p l a t e s ,  one a t t ached  t o  t h e  s h a f t  and t h e  o t h e r  a t -  
tached t o  t h e  impel le r .  These pos t s  a r e  s t rain-gaged i n  such a  way as t o  
e x t r a c t  a l l  s i x  fo rce  componenLs with s e n s i t i v i t i e s  of t h e  o r d e r  of mV/lb. 
The hydrau l i c  system i n t o  which t h i s  mechanical system i s  be ing  installed 
was previously u t i l i z e d  f o r  measurements of t h e  unsteady hydrau l i c  performance 
of c a v i t a t i n g  (and non-cavi tat ing)  ax ia l  flow pumps ( s e e ,  f o r  example Refs. 

1 0 1 1 )  S p e c i f i c a l l y  t h i s  involved f i n d i n g  t h e  r e l a t i o n s  between t h e  unsteady 
p re s su res  and flow r a t e s  a t  i n l e t  and a t  d i scharge  a s  a func t ion  of t h e  f r e -  
quency of t h e  unsteady components of t h e  flow and of t h e  mean flow condi t ions .  
A schematic of t h i s  system, known a s  t h e  Dynamic Pump Tes t  F a c i l i t y  (DPTF), i s  
included a s  Fig. 3 .  The mechanical system of t h e  Rotor Force Tes t  F a c i l i t y  
(RFTF) descr ibed  above i s  being i n s t a l l e d  i n  t h e  lower lef t -hand corner  i n  
SIREN VALYE 
PHASE-LOCK 
DRIVE- 
'SILENT'THROTTLE WWNSTREAM 
OOWNSTREAM 
ELECTROMAGNETIC 
FLOW METER 
OOWNSTREAM L O V  
MEASUREMENT SECTION 
OOWNSTREAM FLOW 
SMOOTHING SECTION 
TRANSPARENT HOUSING 
b IMPELLER 
PHom MULTIPLIER a ups UPSTREAM FLOW 
SMOOTHING SECTION - UPSTREAM ELECTRO- 
MAGNETIC FLOW METER 
Figure  3 .  Schematic planview of t h e  Dynamic Pump Tes t  F a c i l i t y  
p l ace  of t h e  impe l l e r /vo lu t e  system depic ted  i n  t h e  Fig.  3 .  The DPTF included 
two flow f l u c t u a t o r s  ( s i r e n  va lves)  w i th  phase-lock d r i v e  systems s i m i l a r  t o  
t h a t  descr ibed  above f o r  t h e  new e c c e n t r i c  d r i v e  system a s  w e l l  a s  instrumenta-  
t i o n  f o r  measurement of t h e  sma l l  o s c i l l a t o r y  p e r t u r b a t i o n s  i n  p re s su re  and 
flow r a t e .  I n  t h e  context  of t h e  p re sen t  experiment t h e s e  w i l l  aga in  be u t i -  
l i z e d  a t  a l a t e r  s t a g e  t o  i n v e s t i g a t e  whether flow f l u c t u a t i o n s  a r e  a s s o c i a t e d  
w i t h  impel le r  wh i r l  o r  whether they  can induce impe l l e r  wh i r l .  From a g l o b a l  
dynamic po in t  of view i t  i s  p o s s i b l e  t h a t  t h e  e n t i r e  system of f l u c t u a t i o n s  
(mechanical and hydraul ic )  may have t o  be represented  by a g loba l  t r a n s f e r  
func t ion .  
CONCLUDING REMARKS 
Though b e r e f t  of r e s u l t s  t h i s  paper i s  intended t o  h i g h l i g h t  t h e  l a c k  of 
experimental  evidence f o r  wh i r l - exc i t i ng  f l u i d  f o r c e s  a r i s i n g  from t h e  flow 
through turbomachines. We hope t h a t  t h e  experimental  f a c i l i t y  descr ibed  
above w i l l  be  va luab le  i n  provid ing  such evidence. 
REFERENCES 
Alford J .S . ,  1965. P r o t e c t i n g  turbomachinery from se l f - exc i t ed  r o t o r  
w h i r l .  J.  Eng. Power, Vol. 87A, pp. 333-344. 
Pollman, E.,  Schwerdtfeger,  H. and Termuehlen, H . ,  1978. Flow exc i t ed  
v i b r a t i o n s  i n  h igh  p re s su re  t u r b i n e s  (steam w h i r l )  . J. Eng. Power, Vol. 
100, pp.  219-228. 
Thompson, W.E., 1978. F lu id  dynamic e x c i t a t i o n  of c e n t r i f u g a l  compressor 
r o t o r  v i b r a t i o n s .  J. F lu ids  Engineering,  Vol.. 100, No. 1, pp.  73-79. 
Doyle, H.E., 1980. Problems wi th  rotodynamic i n s t a b i l i t i e s  i n  high 
performance turbomachinery. Proc. Workshop on Rotordynamic I n s t a b i l i t y  
Problems i n  High Performance Turbomachinery, May 1980, Texas A & M Univ. 
Ek, M . C . ,  1978. Solu t ions  of t h e  subsynchronous w h i r l  problem i n  t h e  
h igh  p re s su re  hydrogen turbomachinery of t h e  Space S h u t t l e  Main Engine. 
AIAA/SAE 14 th  J o i n t  Propulsion Conference, Las Vegas, J u l y  1978. 
Domm, H . ,  and Hergt ,  P . ,  1970. Radia l  f o r c e s  on impel le r  of v o l u t e  
ca s ing  pumps, Flow Research on Blading (L.S. Dzung, e d . ) ,  E l s e v i e r ,  
pp.  305-321. 
Colding-Jorgensen, J . ,  1979. The e f f e c t  of f l u i d  fo rces  on r o t o r  s t a b i l -  
i t y  of c e n t r i f u g a l  compressors and pumps, Ph.D. Thes is ,  Technical  Univ. 
of  Denmark. 
Chamieh, D. and Acosta,  A . J . ,  1979. Dynamic f o r c e s  on a wh i r l i ng  c e n t r i f  
uga l  r o t o r .  Proc. 6 t h  Conf. on F lu id  Machinery, Akademiai Kiado, 
Budapest, Hungary. 
Chamieh, D . ,  Acosta,  A . J . ,  Brennen, C.E. and Caughey, T . K . ,  1980. A 
b r i e f  n o t e  on t h e  i n t e r a c t i o n  of a n  a c t u a t o r  cascade wi th  a s i n g u l a r i t y .  
Unpublished manuscript.  
Ng, S.L. and Brennen, C . ,  1978. Experiments on t h e  dynamic behavior  of 
c a v i t a t i n g  pumps. J.  F lu ids  Eng., Vol. 100, pp. 166-176. 
Brennen, C.E., Meissner,  C . ,  Lo, E.Y. and Hoffman, G.W. ,  1980. Sca l e  
e f f e c t s  i n  t h e  dynamic t r a n s f e r  func t ions  f o r  c a v i t a t i n g  inducers .  To 
be presented  a t  ASME Winter Annual Meeting, Chicago, 1980. 


The details of a test program for the measurement of the unsteady forces on centrifugal impellers are discussed. Various hydrodynamic flows are identified as possible contributors to these destabilizing forces

NASA Technical Reports Server

A TEST PROGRAM TO MEASURE FLUID MECHANICAL
WHIRL-EXCITATION FORCES IN CENTRIFUGAL PL_fPS
C.E. Brennen, A.J. Acosta and T.K. Caughey
California Institute of Technology
Pasadena, California 91125
ABSTRACT
Much speculation has surrounded the possible unsteady hydrodynamic forces
which could be responsible for the excitation of whirl instabilities in turbo-
machines. However there exist very few measurements of these forces which
would permit one to evaluate the merits of the existing fluid mechanical anal-
yses. In keeping with the informal nature of this workshop we will present
details of a proposed test program for the measurement of the unsteady forces
on centrifugal impellers caused by either (i) azimuthal asymmetry in the vol-
ute geometry or (ii) an externally imposed whirl motion of the impeller. In
the second case the forces resulting from the imposed whirl motions with fre-
quencies ranging from zero to synchronous will be measured by means of a force
balance upon which the impeller is mounted. This work is presently being car-
ried out under contract with the NASA George Marshall Space Flight Center,
Huntsville, Alabama (Contract NAS 8-33108).
INTRODUCTION
In recent years it has been increasingly recognized that hydrodynamic
cross-coupling forces can cause serious rotor dynamic problems in high speed
turbomachines. Such problems have been experienced not only in steam turbines
(Refs.l,2), but also in large compressors (Refs. 3,4) and in high speed pumps
(Ref. 5). Though other instability mechanisms such as internal damping or
non-isotropic shaft stiffness rotor inertia can be fairly readily characteriz-
ed the same cannot be said of the hydrodynamic cross-coupling forces. Various
hydrodynamic flows have been identified as possible contributors to these de-
stabilizing forces. Bearings and particularly seals can clearly play such a
role as the papers in this volume attest. But much less is known about the
potential for destabilizing forces arising from the flows associated with the
impeller and diffuser of a compressor or pump; this brief paper will be con-
fined to this issue.
The flow associated with the impeller/diffuser is significantly differ-
ent from that in a seal or bearing in that the geometry of the volute can
cause this flow to be significantly non-axisymmetric. The work of Domm and
Hergt (Ref. 6) has shown that this could give rise to a lateral force on the
impeller even in the absence of an orbiting motion of the shaft. They derived
magnitudes for this stationary lateral force based on a source vortex model
for the impeller. In an actual impeller this lateral force may also contain
229
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an unsteady componentat the blade passage frequency though the magnitude of
this unsteady force and the role it might play in the rotor dynamics has not
yet been determined. More realistic impeller models would be required for this
purpose.
Recently Colding-Jorgensen (Ref. 7, see also this volume) has extended
the work of Dommand Hergt. Imposing an orbiting motion on the source/vortex
model of the impeller, he analyzed the resulting radial and tangential forces
on the "impeller" and found that positive or whirl excitation tangential forces
did indeed result from such an analysis and that the magnitudes of these forces
were sufficient to have played a significant role in the Space Shuttle high
pressure hydrogen pumpinstability (Ref. 5). Howeverthe main question here is
the validity of the simple source/vortex model used for the impeller: further
analyses with more realistic impeller models will be required in order to deter-
mine the limits of validity of such analyses. Oneuseful intermediate model
which is still fairly simple is the actuator disk which has been suggested by
the work of Chamiehand Acosta (Ref. 8,9).
Other analyses have sought destabilizing tangential forces in the absence
of volute asymmetries. Here we have two possible mechanismswhich have been
suggested. In an addenda to his paper on whirl-excitation forces in seals,
Alford (1965) suggested that the samemechanismwould occur in the tip clear-
ance flows of steam turbines and others have further extrapolated this to the
tip clearance flows in any turbomachine. The supposition is that a localized
reduction in the tip clearance would cause a localized decrease in the torque
shear stress with the reverse occurrin_ on the other end of the diameter. This
would lead to a positive or whirl-exciting tangential force.
Secondly, Thompson(Ref. 3) has posed the question of whether such a
force would arise due to the perturbations imposedon the main flow through the
impeller. Assuming quasi-static performance for each blade passagehe used a
detailed flow analysis program to assess the distribution of forces on the
blades of an impeller performing whirling motions. Positive excitation did
indeed occur with someimpeller designs. It is not however clear whether the
quasi-static assumption is valid in these circumstances.
Ultimately any review of the existing state of knowledge such as we have
given above reveals a singular lack of hard experimental data with which to
assess the merits of the various analyses. The primary purpose of this paper
is to stress the fact that muchremains to be done before the status of whirl-
excitation forces on impellers can be put on a firm foundation. Weare pre-
sently embarking on an experimental program whose intent is to measurethese
forces and the remainder of this paper is devoted to a brief description of
this facility.
ROTOR FORCE TEST FACILITY
The objective of the facility is to impose known orbiting motions of fre-
quency _ on the basic rotary motion of a number of typical centrifugal pump
impellers (rotating frequency, _) and to measure the radial and tangential
forces resulting from this motion. The core of the experiment is shown in
230
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i•
_.,,_, _i¸¸¸¸¸¸¸¸_...._ _5-_
0
4-1
0
4-1
0
>
.,.-I
.1-1
4-1
,-4
0
>
-P-t
-,-4
4-1
.p.+
0
231
Fig.l.The flow enters the centrifugal impeller(6)from the inlet connection(3)
and inlet bell(4). Volutes(2)of various geometries will be used in measure-
ments of lateral force due to volute asymmetry: they are contained in the pump
housing (i) so that they may be of lightweight construction. The impeller (5)
is mounted directly on an internal force balance (6). The main shaft (i0)
rotates in a double bearin_ system (7,8,11) designed so that rotation of the
sprocket (9) causes the orbiting motion. In addition to the rotating internal
balance a back-up force measuring system(12,13)external to the eccentric drive
system will also be used. Figure 2, an overall schematic of the mechanical
system, shows the main drive system comprised of a 20HP variable speed motor,
a gear box and a flexible shaft (necessary to accommodate the orbiting motion).
This will be capable of speeds up to 35000 rpm (_J _ 60Hz.) Figure 2 also shows
the eccentric drive motor (2HP) which can _enerate orbit frequencies, _,
from 0 to 60 Hz.
Past experience with unsteady flow measurements has emphasized the need
for rigorous dynamic control of the experiments. In the present facility a
proximity probe senses the rotating frequency, w; this signal is then used
to produce digital signals with frequencies M_/N, u/N, (N-M)_/N and
(N+M)_/N where M and N are selected integers between 0 and I00 (MmN).
The signal at _/N is then used to drive the eccentric motor such that
_= _%/N; a feedback control system fitted to this motor ensures accurate phase-
locking. In this way measurements can be made over a wide range of _/_ from
zero to unity.
Past experience has also indicated that the unsteady flow measurements
can be quite noisy and that accurate results require cross-correlation of
measurement and driving signal over many cycles. In the present case the
measurements may include (i) six-components of force measurement from the
internal balance or several components of force from the external balance (ii)
axial and radial d_splacement of the impeller from four fibre-optic displace-
ment probes (iii) pressure transducer measurements from locations both up-
stream and downstream of the impeller as well as within the impeller-volute
flow (iv) unsteady flow rate measurement using existing laser doppler veloci-
meters or electro-magnetic flow meters. A sixteen channel digital signal
processor is used for cross-correlation of these measurements with any of the
basic frequencies (_,M_/N,_/N,(N-M)_/N and (N+M)_/N) in order to extract
the components of force at any of these fundamental frequencies
or higher harmonics thereof; this cross-correlation can cover up to i0,000
cycles of the fundamental frequency.
The most critical component in the system is probably the internal bal-
ance and its design is as yet unproven. One must ensure high natural fre-
quencies (>500Hz.) and yet maintain sufficient sensitivity. The present can-
didate design consists of four posts parallel with the axis which are mono-
lithic with two solid end plates, one attached to the shaft and the other at-
tached to the impeller. These posts are strain-gaged in such a way as to
extract all six force components with sensitivities of the order of mV/ib.
The hydraulic system into which this mechanical system is being installed
was previously utilized for measurements of the unsteady hydraulic performance
of cavitating (and non-cavitating) axial flow pumps (see, for example Refs.
232
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i
F
L _
I
J ]
!
P
O.A
I
I
L ........ •
._
0
0
0
0
0
0
0
u_
0
0
0
tU
0
0
0
0
0
I:L
0
0
U_
c,.l
0
n
.r-I
233
i0,Ii). Specifically this involved finding the relations between the unsteady
pressures and flow rates at inlet and at discharge as a function of the fre-
quency of the unsteady components of the flow and of the mean flow conditions.
A schematic of this system, known as the Dynamic Pump Test Facility (DPTF), is
included as Fig. 3. The mechanical system of the Rotor Force Test Facility
(RFTF) described above is being installed in the lower left-hand corner in
DOWNSTREAM --'_ELECTROMAGNETIC
FLOW METER
DOWNSTREAM LDV
MEASUREMENT SECTION
DOWNSTREAM FLOW
SMOOTHING SECTION ---_
5,_N
UPSTREAM FLOW
SMOOTHING SECTION
UPSTREAM ELECTRO-
MAGNETIC FLOW METER
Figure 3. Schematic planview of the Dynamic Pump Test Facility
place of the impeller/volute system depicted in the Fig. 3. The DPTF included
two flow fluctuators (siren valves) with phase-lock drive systems similar to
that described above for the new eccentric drive system as well as instrumenta-
tion for measurement of the small oscillatory perturbations in pressure and
flow rate. In the context of the present experiment these will again be uti-
lized at a later sta_e to investigate whether flow fluctuations are associated
with impeller whirl or whether they can induce impeller whirl. From a global
dynamic point of view it is possible that the entire system of fluctuations
(mechanical and hydraulic) may have to be represented by a global transfer
function.
CONCLUDING REMARKS
Though bereft of results this paper is intended to highlight the lack of
experimental evidence for whirl-exciting fluid forces arising from the flow
through turbomachines. We hope that the experimental facility described
above will be valuable in providing such evidence.
234
lo
.
.
,
.
.
.
.
.
I0.
ii.
REFERENCES
Alford J.S., 1965. Protecting turbomachinery from self-excited rotor
whirl. J. Eng. Power, Vol. 87A, pp. 333-344.
Pollman, E., Schwerdtfeger, H. and Termuehlen, H., 1978. Flow excited
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Thompson, W.E., 1978. Fluid dynamic excitation of centrifugal compressor
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Doyle, H.E., 1980. Problems with rotodynamic instabilities in high
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235



A test program to measure fluid mechanical whirl-excitation forces in centrifugal pumps

A b r i e f n o t e on t h e i n t e r a c t i o n of a n a c t u a t o r cascade wi th a s i n g u l a r i t y .

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Radia l f o r c e s on impel le r of v o l u t e ca s ing pumps,

Sca l e e f f e c t s i n t h e dynamic t r a n s f e r func t ions f o r c a v i t a t i n g inducers . To be presented a t ASME Winter Annual Meeting,

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A Test Program to Measure Fluid Mechanical Whirl-Excitation Forces in Centrifugal Pumps

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