A DANGEROUS BLADE VIBRATION PHENOMENON DUE TO UNSTEADY FLOW IN CENTRIFUGAL COMPRESSORS

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS y t345 E. 47th St., New York, N.Y GT-265 ^v The Society shll not be responsible for sttements or opinions dvnced in ^J ppers or discussion t meetings of the Society or of its Divisions or Sections, or printed in its publictions. Discussion is printed only if the pper is published in n ASME Journl. Ppers re vilble from ASME for 15 months fter the meeting. Copyright 1993 by ASME Printed in U.S.A. A DANGEROUS BLADE VIBRATION PHENOMENON DUE TO UNSTEADY FLOW IN CENTRIFUGAL COMPRESSORS D. Jin, H. Hsemnn, U. Hupt, nd M. Rutenberg Institute of Turbomchinery University of Hnnover Hnnover, Germny ABSTRACT Dngerous blde excittion cused by unstedy flow in high pressure/high mss flow compressor running in low mss flow region hs been investigted. Experiments were crried out for compressors with two different types of vned diffusers. Blde vibrtion ws mesured with strin gges while simultneous unstedy pressure ws mesured with fst response dynmic trnsducers. All mesured results were nlysed in detil so tht n in-depth understnding of blde excittion mechnism cn be obtined. Firstly, the compressor with stright-chnnel vne diffuser t reduced rottionl speed of 12, 3 rpm in n unstble opertion region ws considered. The nlysis of blde vibrtion nd unstedy pressure showed n unusul excittion phenomenon. Besides strong blde vibrtion frequency component ner the first blde mode frequency excited by the rotting stll cells existed nother dngerous resonnce excittion with first blde mode component which dominted the blde vibrtion spectrum. A detiled pressure signl nlysis indicted tht this blde vibrtion ws excited by brod bnd pressure fluctution due to strong reverse flow occurring simultneously with the rotting stll. Further reducing the compressor mss flow to the opertion point shortly before surge, the rotting stll ws significntly wekened while the reverse flow kept its intensity until surge occurred. In this opertion region bldes suffered throughout violent excittion of resonnce becuse of the strong reverse flow. These blde excittion phenomen were lso found in the next experiment for the compressor with cmbered vne diffuser t higher rottionl speeds of n red = 13,5 nd 14, rpm. The mximum strin vlues of blde vibrtion were obtined to quntitvely estimte the dnger of blde vibrtion cused by this excittion. INTRODUCTION Rotting stll s n unstedy phenomenon occurring in the prt lod compressor opertion zone ws considered to ply n importnt role in the blde excittion due to its periodic occurrence (Hupt et l. 1986; Hgiwr et l. 1987; Ishihr nd Funkw 198). Recent reserch results showed tht not only the min frequency components of rotting stll cn be the cuse of dngerous blde vibrtion, but lso the circumferentilly symmetric mplitude profile of rotting stll pressure pttern hd strong dditionl excittion on bldes (Jin et l. 1992). The blde vibrtion stress excited by the doubled frequency component of rotting stll due to its non- sinusoidl pressure profile hd reched such high level tht it hd lredy exceeded the llowble stress limits of the blde.(hsemnn et l. 1991). The experimentl results in the pper presented blde excittion phenomenon with dngerous vibrtion stress in (4 V Fig. I Centrifugl compressor test fcility for blde vibrtion investigtion Presented t the Interntionl Gs Turbine nd Aeroengine Congress nd Exposition Cincinnti, Ohio My 24-27, 1993 Downloded From: on 1/11/218 Terms of Use:

2 sl impeller vned diffuser in gge loction of semiconductor trnsducers: 1) x = -1 mm 2) x/s =.1 3) x/s =.55 4) x/s = 1. 5) throt of diffuser vnes 6) diffuser exit 7) 57 circumferentilly locted Fig. 2 Cross-sectionl view of the test compressor with the indiction to the loction of strin gges nd pressure trnsducers high lod nd high speed centrifugl compressor with vned diffusers. Blde vibrtion signls showed n unusully strong excittion in n unstble opertion region of the compressor. An intermittent rotting stll with three rotting cells ginst impeller rottion ws determined by phse nlysis of the pressure signls obtined from the pressure trnsducers circumferentilly locted in the throts of the diffuser vnes. This rotting stll pressure pttern strongly excited the bldes with frequency component of fb = 3(fs + frs) = 3 f 5+ fp ner the first blde mode. Here fs stnds for the rel impeller rottionl frequency; frs for the rottionl frequency of the stll cells; f p for the frequency of the rotting stll pressure pttern obtined by high frequency pressure trnsducers nd fp = 3 frs. In ddition, the dngerous resonnce of blde excittion with first blde mode component dominted the blde vibrtion spectr. The observtions nd the nlyses of the flow nd the blde vibrtion behvior presented in this pper imed to chieve better understnding of the internl flow structure nd the blde excittion mechnism s well s their physicl essences. COMPRESSOR TEST FACILITY The mesurements of flow nd blde vibrtion were mde for single stge centrifugl compressor test rig operted in n open loop s shown in Fig. I nd 2. The compressor ws driven by 135 kw DC-motor coupled to ger box. The mximum vlues of the pressure rtio nd the mss flow rte of the compressor were 7t = S nd mred = 1 kg/s respectively. Flow entered the compressor in the xil direction through short suction pipe from settling chmber nd pssed through impeller nd long rdil diffuser. The diffuser configurtion cn be chnged to operte s either vneless diffuser or s vned diffuser with 19 vnes (see Fig. 2). The flow downstrem ws collected in n nnulr collecting chmber nd then left the compressor by n outlet tube which ws tngentil to the mchine. The unshrouded impeller of the test compressor used in this experiment is shown in Fig. 3. It hs chrcteristiclly thin bldes with pproximtely 1 mm in thickness t the inlet tip nd 4 mm t the outlet. The impeller hs n outer dimeter of 4 mm nd 28 rdil ended bldes with every second blde cut bck t the inlet. The impeller is mde of luminium lloy. Becuse of the blde thickness error cused by blde cutting the still first blde nturl frequencies re from 782 to 823 Hz (Hupt 1984). Fig. 3 Impeller used in the blde vibrtion investigtion NOMENCLATURE f frequency Subscripts m cell number of rotting stll m mss flow rte 4 diffuser inlet n rottionl speed 6 diffuser exit p pressure b blde s blde length t blde tip p pressure oscilltion t time red reduced to reference condotion x distnce from impeller inlet t blde tip rs rotting stll blde strin s shft, impeller p phse-trnsfer ngle between two signls tot totl physicl ngle between two trnsducers X rdius rtio it pressure rtio blde stress 2 Downloded From: on 1/11/218 Terms of Use:

3 The experiment ws performed for two different vned diffusers, stright chnnel vne diffuser nd cmbered vne diffuser respectively. The former is shown in Fig. 4(), which hs 19 stright wedge vnes. The vne leding edge is locted t rdius rtio A 4 of 1.15 nd extends to rdius rtio A6 of The wedge ngle is 9 57' nd is so set tht the ngle between the pressure side surfce nd the tngent to the leding edge circle is 17 48'. The diffuser width is 23.8 mm. The ltter, the cmbered vne diffuser is shown in Fig. 4(b). The diffuser width is 21.6 mm. The vne leding edge is locted t the rdius rtio A 4 of 1.15 nd extends to the A6 of 1.5. The vne inlet ngle is 17. d T 2, ,5 i rpm rpm m MEASURING SYSTEM The semiconductor strin gges locted in the zone of blde inlet shown in Fig. 2 were used to mesure blde vibrtion. The fundmentl clcultions (Hupt et l ) nd ltter tests (Hupt et l ) indicted tht this is the zone where the mximum stresses occur for the vibrtion in first nd second blde modes. These modes were indeed the min modes excited when the compressor opertes in the unstedy flow region. The strin gges on different bldes with loctions shown in Fig. 2 nd 3 were connected by thin wires to 8- chnnel- FM-telemetry trnsmitter mounted on the bore of the hollow shft t the impeller inlet. This trnsmitter is the rotting prt of the telemetry system described in detil by Hupt et l. (1985). The blde vibrtion signls trnsmitted by this system with trigger were recorded on seprte mgnetic tpe. One of these blde signls ws recorded simultneously with the different unstedy pressure signls on the 14-chnnel tpe recorder. Flow mesurements were mde by mens of severl high frequency response semiconductor pressure trnsducers to determine the time dependent chrcteristics of unstedy flow. These trnsducers were mounted on the compressor shroud long the flow pth from the impeller inlet to the diffuser vne exit. The propgtion of rotting stll pressure pttern in the meridionl direction ws exmined. The structure of the rotting stll pressure pttern ws determined by the phse nlysis of the pressure signls obtined from ,5 2,5 3.5 kg/s 5.5 reduced mss flow rte m r,d three rotting cells o three nd four cells simultneously two rotting cells o two nd four cells simultneously x stedy flow opertion Fig. 5 Unstedy flow mesurement results in the compressor mp with stright chnnel vne diffuser showing the chrcteristics of the rotting stll pressure pttern the pressure trnsducers circumferentilly locted in the throts of the diffuser vnes. A schemtic disply of the tnsducer positions is shown in Fig. 2. RESULTS AND DISCUSSIONS -Compressor with stright chnnel vne diffuser The investigtion of pressure nd blde vibrtion behvior ws crried out long constnt speed line. For ech speed line severl operting points were tken before nd during the existence of flow oscilltions. The flow oscilltions behvior chrcterised by rotting stll were determined nd plotted in Fig. 5. The flow chrcteristics for ll operting points t different speed lines were described by Hupt et l. (1988). In this pper very interesting flow phenomenon s well s its effect on blde vibrtion occuring t the constnt speed of nred = 12, 3 rpm will be reported. () stright chnnel vne diffuser (b) cmbered vne diffuser Fig. 4 Shpe of the different types of diffuser vnes used in the blde vibrtion experiments At the compressor operting point of rotting speed of nred = 12, 3 rpm nd mss flow rte rhred = 2.99 kg/s, the pressure signls obtined from pressure trnsducers nd the blde vibrtion signl from strin gges were unstble. The time period of the unstble pressure oscilltion signls from the impeller inlet to the diffuser exit nd the blde vibrtion signl tken simultneously were plotted in Fig. 6 (A). An intense unstble periodic pressure oscilltion nd violent blde vibrtion cn be observed. The period of the unstble periodic pressure oscilltions ws bout.5 second nd the time intervls between two unstble pressure oscilltion periods were bout 1 second. The strongest periodic pressure oscilltions occurred t xis =.55 nd the diffuser inlet. No significnt periodic pressure oscilltion cn be 3 Downloded From: on 1/11/218 Terms of Use:

4 7, A N C.) Ce found t the impeller inlet nd the diffuser exit. An evident blde vibrtion with mximum strin vlue of 1.43 mm/m cn be determined during this period of time. Figure 6 (B) provides nother view of this pressure signls where the periodic oscilltion components were filtered nd the pressure scle ws mplified. This picture shows n evident sustined rise in pressure level t the impeller inlet nd drop in diffuser region when the periodic pressure oscilltions pper. These chnges in pressur level cn be explined s tht the compressed ir with high pressure nd high energy flows long full circumference of the shroud from the diffuser region to the impeller inlet. The pressure vlue t the diffuser exit drops even more shrply becuse of the compressed ir leks to the impeller inlet nd through compressor outlet tube. When the reverse flow stops, the rised pressure t the impeller inlet drops to the vlue which is lower thn tht before occurrence of the pressure oscilco W E y ' c E.o E. " '> ie x=-1 mm xis =.1 x/s=.55 x/s=1, diff, throt. diff, exit Cu E on E B ^ ce d > N ry x= -1 mm x/s =.1 ii 2 x/s=,55 d N ry =; x/s=1, U f- diff. throt,25 sec,75.25,5 sec 1, Fig. 6 Blde vibrtion nd pressure chrcteristics during intermittent rotting stll t nred = 12, 3 rpm mr ed = 2.99 kg/s A o w E 1 v o E o J x=-1 mm x/s=.1 ^' x/s=.55 N 4 x/s=1. diff. throt dill. exit N B C w E^ E o > N H x=-1mm x/s=.1 x/s=.55 x/s=1. diff. throt n 1. sec sec 3, Fig. 7 Blde vibrtion nd pressure chrcteristics during continuous rotting stll t n red= 12, 3 rpm ltion. Then, the pressure in the compressor recover to the norml level. In the recovery period bldes re wekly excited nd the pressure chnge profile is similr to tht during surge, which ws reported by Hupt et l. (1987 ) nd Jin et l. (1992b). However, the mgnitude of the pressure chnges here is smller thn in tht cse. Further reducing the compressor mss flow rte the pressure oscilltion period ws extended while the time intervl between two pressure oscilltion periods ws shortened, until the sustined pressure oscilltion ws formed. At this opertion point the prmeters of compressor chrcteristics cn not be determined becuse of the intense pressure oscilltion. Figure 7 (A) shows the pressure signls, two long periodic pressure oscilltion periods of 4.S second with short time intervl bout.3 second in between. The violent blde vibrtion excited by this pressure oscilltion ws lso plotted. In order to investigte the chnge in pressure vlue of this periodic pressure oscilltion the component of the pressure signls were lso filtered nd plotted in Fig. 7 (B). The pressure signls t the time intervl between two periodic pressure oscilltion periods show pressure behvior in norml flow. The pressure vlues t the impeller inlet re lower thn tht during periodic pressure oscilltions. In diffuser, on the other hnd, the pressure vlues re higher. This cn be further indiction tht the continuous periodic pressure oscilltions were ccompnied by reverse flow mentioned bove. 4 Downloded From: on 1/11/218 Terms of Use:

5 Q. c} 1 L U N n C -5 1 N L -15 d _,7nr 9 18 deg. 36 physicl ngle fp = 38 Hz Phse nlyses of the signl frequency component t different circumferentil trnsducer loction significnt mplitude of the pressure oscilltion with brod bnd chrcteristics cn be observed. The mximum vlue of this mplitude t the impeller inlet is 3 mbr. The brod bnd behvior of the pressure spectr represents the chrcteristics of reverse flow in compressor, which ws described by Hupt et l. (1987b). Bldes cn be excited by the pressure oscilltion with brod bnd chrcteristics, especilly, t the impeller inlet, where bldes re sensitive to excittion. Frequency nlyses were lso performed for blde vibrtion signls. The blde vibrtion spectrum with verge vlue is shown in Fig. 9. A significnt mplitude of the discrete frequency component of 664 Hz ws excited by the rottionl cells of the rotting stll pressure pttern (fb = 3 f3 + fp = 3 x = 662 Hz). Here f s stnds for the impeller rottionl frequency nd fp for the mesured pressure oscilltion frequency of the rotting stll. The excittion with frequency components of 856 Hz (first blde mode) showing brod bnd chrcteristics is the response of the pressure oscilltion to the reverse flow. The discrete frequency components of 832 Hz (4 fs) re possibly excited by the circumferentilly symmetric sttic pressure distribution with four periodic wves, which re induced by the interction between the blde vibrtion nd the circumferentilly symmetric reverse flow. This complex excittion mechnism ws described by Hupt et l. (1989). 5 mbr E 3, Hz 5 f. 28 Hz Pressure spectr t different meridionl loctions long the flow pth Fig. 8 Pressure signl nlyses during continuous rotting stll t n red = 12, 3 rpm Detiled frequency nlyses were performed to comprehend the pressure chrcteristics nd the blde excittion mechnism. The pressure spectr with verge vlue obtined from the impeller inlet to the diffuser exit during continuous periodic pressure oscilltion re shown in Fig. 8. The discrete frequency components t 38 Hz disply the periodic pressure oscilltions. By phse nlysis of the pressure signls obtined from dditionl pressure trnsducers set t different circumferentil positions in the throts of diffuser vnes (8 = 57 nd 17 ) rotting stll pressure pttern with three rotting cells directed ginst the impeller rottion ws determined (see Fig. 8). The rottionl frequency of n individul cell is 13 Hz (38/3) Hz. The frequency components of 292 Hz t the impeller inlet re cused by the pressure difference between the pressure nd suction side of the blde. This vlue is equl to the product of the long blde number nd the shft rottionl frequency. Additionlly, lift. exit An oil injection experiment ws conducted to confirm the reverse flow mentioned bove. Oil ws injected in the throt flow chnnel ner the impeller exit (x/s =.8) through hole in the shroud wll, when the compressor ws operted t nred = 12, 3 rpm nd during the occurrence of the continuous rotting stll pressure pttern. The results of this experiment is given in Fig. 1. The coloured dye in the picture represents the reverse flow conditions ner the shroud. It is demonstrted by this experiment tht the reverse flow extends s fr s loction next to the impeller inlet, s shown in this picture, before the shroud wll ws removed. - The mss flow of the compressor ws further slowly reduced to surge. Shortly before the occurrence of the surge W c.3 4f, (832 Hz) r mm/m 3f5.f (664 Hz) C18.n,12,6 (856 Hz ) Hz 1 f5 =28 Hz fp =38Hz Fig. 9 Frequency nlysis of the blde vibrtion signl during continuous rotting stll t nred = 12, 3 rpm 5 Downloded From: on 1/11/218 Terms of Use:

6 1 ore E E c E J > N N " x=-1mm x/s=,1 n x/s=,55._ o x/s=1, diff. throut,5 1. sec 2..5 i norml ilv Fig. 1 Results of oil injection experiment t nred = 12, 3 rpm during the continuous three cell rotting stll very interesting flow phenomenon ppered. The pressure nd blde vibrtion signls t this time re shown in Fig. 11 (A). The pressure oscilltions chrcterizing rotting stll pressure pttern re few in mount nd fr prt. Pressure pulses t the impeller inlet with period from.25 to.3 second nd slight mplitude cn be observed. However, the blde vibrtion signl displys n intense unstble vibrtion strin. The mximum vibrtion strin is the sme s tht during the occurrence of the rotting stll pressure pttern. The pressure nd blde vibrtion signls obtined t the compressor operting point before the occurrence of the rotting stll pressure pttern re shown in the right picture in Fig. 11 (A) to compre the flow behvior nd the blde excittion. The picture in Fig. 11 (B) shows the blde vibrtion strin nd pressure signl gin. The high pressure frequency components were filtered nd the pressure scle ws mplified. Compring the norml flow shown in the right picture in Fig. 11(B) the pressure level for the compressor operting point shortly before surge t the impeller inlet is higher nd t the diffuser exit is lower. This pressure level chnge chrcterizes the reverse flow from the diffuser region to the impeller inlet. The pressure spectr with verge vlue from the impeller inlet to the diffuser exit re shown in Fig. 12, which show significnt brod bnd chrcteristics of the pressure signls. Furthermore, the frequency spectr with the verge vlue of the pressure signls mesured t the position 1 mm before the impeller inlet re shown in Fig. 13 which displys the process of the pressure behvior chnging with mss flow rte reduction t the rottionl speed of n red = 12,3 rpm. At the compressor operting point of lrge mss flow rte, for exmple thred = 3.87 nd 3.48 kg/s, the pressure spectr show obvious mplitudes of the discrete frequency component of 292 Hz which re cused by the blde pressure difference between the pressure nd the suction side while the pressure oscilltion mplitude showing B i3 " $ (ii NI 3U N o W E^lL H ' c ^ D > V x=-1mm x/s=,1 x/s:q55 x/s=1, diff. throt ", II.5 1, sec 2..5 Fig. 11 Blde vibrtion nd pressure chrcteristics t nred = 12, 3 rpm nd t the opertion point shortly before surge brod bnd chrcteristics is not significnt. The discrete frequency component (38 Hz) t thred = kg/s demonstrtes the intermittent rotting stll pressure pttern. Menwhile the pressure spectrum with brod bnd chrcteristics indictes the reverse flow ccompnied by the pressure oscilltion of the rotting stll pressure pttern. During the occurrence of the continuous rotting stll pressure pttern 3 mbr fp (3 8 Hz ) 14. f, (292 Hz ) diff, exit dill. throt E 18 ox/s =1, 12 x/s=,55 '" 6 /s,1 x=-1 mm Hz 5 f,=28 Hz Fig. 12 Pressure spectr t different meridionl loctions long the flow pth t n red = 12, 3 rpm nd t the opertion point shortly before surge 6 Downloded From: on 1/11/218 Terms of Use:

7 d 1 j mb ul C, n,3 4 f5 ( 832 Hz ) mm/m,18,12 3f5+f (664 Hz),6 14 f, (292 Hz) 2 3,48 kg/s 3.87 kg/s Hz 5 f,=28 Hz trnsducer loction: x = -1 mm Hz -^-...r before surge no-rotting stll) (continuous rotting stll) 2.99 kg/s Fig. 13 Pressure spectr t n red = 12, 3 rpm with mss flow rte reduction the mplitude of the discrete frequency component t 38 Hz nd the pressure oscilltion with brod bnd chrcteristics re remrkbly incresed. However, the mplitude of the discrete frequency component demonstrting the pressure difference between the blde pressure nd the suction side is wekened, since the reverse flow with turbulent behvior destroys the estblishment of the pressure difference (Hupt et l. 1987c). The pressure spectrum t the operting point shortly before surge displys significnt mplitude of the pressure oscilltion with brod bnd chrcteristics, whose mximum vlue region extend to the lower frequency rnge. Here, the frequency component mplitude displying rotting stll pressure pttern is submerged nd the frequency component mplitude showing pressure difference in blde chnnel is very wek. The blde vibrtion signl shortly before surge ws lso nlysed. The blde vibrtion signl spectrum with verge vlue ws plotted in Fig. 14. The discrete frequency component with smll mplitude of 664 Hz ws excited by the rotting stll pressure pttern (fb = 3 f 5 + fp = 3 x = 662 Hz). The significnt mplitude of the discrete frequency component t 832 Hz (4 fs) ws excited by circumferentilly symmetric sttic pressure profile with four periodic fs = 28 Hz fp = 38 Hz ( 856 Hz) 1 Fig. 14 Frequency nlysis of the blde vibrtion signl t nred = 12, 3 rpm nd t the opertion point shortly before surge wves induced by the interction between blde vibrtion nd the circumferentilly symmetric reverse flow (Hupt et l. 1989). The frequency component t 856 Hz (first blde mode) with brod bnd chrcteristics demonstrtes the response of the reverse flow. -Compressor with cmbered vne diffuser At the rottionl speed nred = 13,5 nd 14, rpm the pressure signls disply the intense pressure oscilltions. The phse nlysis defined tht it is cused by the rotting stll with three cells mentioned bove. The blde vibrtion nd pressure signls t nred = 13,5 rpm nd th red = 3.42 kg/s re shown in Fig. 16. The periodic pressure oscilltions with frequency vlue of 2 Hz were defined s the rotting stll pressure pttern. The pressure level chnges t the , 1,8 I Similr investigtions were performed for the compressor with cmbered vne diffuser. The compressor operting points displying pressure oscilltion chrcteristics of rotting stll pressure pttern were plotted on the compressor mp (Fig. 15). At the compressor operting points ner surge t the rottionl speed of nred = 12,3 nd 12,9 rpm rotting stll pressure pttern with three rotting cells directed ginst the impeller rottion cn be determined. However, the pressure oscilltion mplitudes of this pressure pttern re not significnt nd the blde vibrtion strins excited by this pressure oscilltion re lso very wek. On the rottionl line of n red = 14, 5 rpm t the point before surge significnt periodic pressure oscilltion cn be observed, which ws defined s rotting stll pressure pttern with two rotting cells directed ginst the impeller direction (Jin et l. 1992). 2, 1,7 1,6 irj 145 r pm 14rprn 135 rpm 129 rpm 123 rpm r n, ed reduced mss flow rte m r. two rotting cells three rotting cells x stedy flow pttern Fig. 15 Unstedy flow mesurement results in the compressor mp with cmbered vne diffuser showing the chrcteristics of the rotting stll pressure pttern 7 Downloded From: on 1/11/218 Terms of Use:

8 TI co, o ^ > N^ x =-1mm c. x/s=.1 1 x/s=,55 o x/s=1. d F dill. throut to , sec 6. cmbered vne diffuser Fig. 16 Blde vibrtion nd pressure chrcteristics during intermetent rotting stll t fl red = 13, 5 rpm ri1 red = 3.42 kg/s impeller inlet nd in the diffuser region mentioned bove re not esy to be identified. However, the very rough pressure signls with chotic pressure oscilltions t the position of 1 mm before the impeller inlet nd t the diffuser exit during the occurrence of the rotting stll pressure pttern cn be observed. The pressure nd blde vibrtion signl nlyses were performed for this cse. The pressure spectr t the compressor operting point during the occurrence of continuous rotting stll pressure pttern, which is directly followed by surge, were plotted in Fig. 17. It cn be found tht the pressure oscilltions with significnt brod bnd chrcteristics dominted the pressure spectr. The mximum mplitude vlue t the impeller inlet reches 5 mbr in brod frequency rnge. This brod bnd pressure chrcteristics cused by reverse flow cn be min motivtion of blde vibrtion. The verge vlues of blde vibrtion frequency spectr with reduction of mss flow rte re shown in Fig. 18. At the operting point of tred = 3.42 kg/s the blde vibrtion spectrum due to the intermittent rotting stll shows significnt frequency component. The discrete frequency compo- y c.2 N mm^.12 `o.8.4 3f,.1, (7 Hz) G f, (9 Hz) I. blde mode (84 Hz) II. blde mode (172 Hz) 3.62 kg/s 3,79 kg/s Hz 25 f: =226 Hz fp=2hz cmbered vne diffuser shortly before surge 3,1.2 kg/s Fig. 18 Frequency nlyses of the blde vibrtion signls t nred = 13, 5 rpm with mss flow rte reduction nent with the vlue of 7 Hz is the response of the excittion cused by the rotting stll pressure pttern (fb = 3 f 3 + fp = 3 x = 698 Hz). The frequency components of 9 Hz re excited by the circumferentilly symmetric sttic pressure profile with four periodic wves (fb = 4 f5 = 94 Hz)(Hupt et l. 1989). However, the brod bnd excittion with the mximum vlue t both 84 Hz (first blde mode) nd 172 Hz (second blde mode) domintes the frequency spectr t the operting point with intermittent rotting stll pressure pttern. Here it must be pyed ttention tht the nturl frequency of the mesured blde in this cse is lower thn tht for the experiment of the compressor with stright chnnel vne diffuser. At the operting point with continuous rotting stll pressure pttern this excittion is even stronger. At the rottionl speed of nred = 14, rpm only the intermittent rotting stll pressure pttern with three cells directed ginst the impeller rottion, which is immeditely followed by surge, cn be determined. The blde vibrtion nd pressure signls during the rotting stll were plotted in o W E o o E o f Z > VI N rx=-1mm x/s =,1 3 C D mbr E uni 6 d Hz 5 ff. throut 1. i U) d Q. UQ In x /s =,55 x/s=1, diff. throut f226 Hz f5 =2Hz cmbered vne diffuser,25,5 sec 1. cmbered vne diffuser Fig. 17 Pressure spectr t different meridionl loctions long the flow pth t nred = 13, 5 rpm during continuous rotting stll Fig. 19 Blde vibrtion nd pressure chrcteristics during intermittent rotting stll pressure pttern t nred = 14, rpm 8 Downloded From: on 1/11/218 Terms of Use:

9 1 Li, ( 93 Hz ) I. blde mode ( 84 Hz. blde mode (172 Hz).2 ifs. f, without mm/m (72 HZ) rotting stll with rotting stll Hz 25 tregwncy f fe x13hz f235 Hz cmbered vne diffuser Fig. 2 Frequency nlyses of the blde vibrtion signls t nred = 14, rpm Fig. 19. The pressure signls disply strong periodic pressure oscilltion with frequency of 13 Hz in the impeller chnnel s well s significnt chotic pressure oscilltion with n evident pressure level chnge t the impeller inlet nd the diffuser exit. This pressure behvior is similr to tht described for the cse of which indictes strong reverse flow from the diffuser exit to the impeller inlet ccompnied by the intermittent rotting stll pressure pttern. The frequency nlysis of the blde vibrtion signls performed for the time periods with nd without rotting stll pressure pttern respectively. In the time period with rotting stll pressure pttern in Fig. 2 the frequency component t 72 Hz is excited by the rotting stll cell (fb = 3 fs + fp = 3 x = 718 Hz). The brod bnd excittion with both frequency vlue of 84 (first blde mode) nd 172 Hz (second blde mode) is excited by the reverse flow. The spectrum t the time period without rotting stll shows no significnt frequency component mplitude excited by rotting stll nd reverse flow. The frequency component of 93 Hz is excited by the circumferentilly symmetric sttic pressure profile (fb = 4 fs = 94 Hz). The mximum strins of the blde vibrtion excited by the pressure oscilltion ptterns mentioned bove nd tht Tble 1. Summry of unstedy flow nd blde vibrtion dt rotor mss mesured cell of cell mx. blde mx. blde speed flow rte pressure rotting reltive strin strin frequency stll speed luring surge diffuser nred rh fp m nrs/n r e diffuser with rpm kg/s Hz - X mm/m mm/m continuous R.S stright 12, wedge vnes before surge mbered 13, vne diffuser 14, before surge N/mm2 in in F113 C C!.n r D±.5 C o G^ C ) o W Q Mteril: Al Cu Mg 2 Dt for mbient temperture nd 5 x 1 cycles N/mm2 5 sttic stress slt mx. sttic blde stress Fig. 21 Goodmn-digrm for luminium lloy showing llowble lternting bending stress vlues during surge re shown in Tble 1. The mximum blde vibrtion strins in the compressor with stright chnnel diffuser t the rottionl speed of 12, 3 rpm re very high. During the intermittent nd continuous rotting stll the mximum blde strin vlues re 1.43 nd 1.3 mm/m, respectively. At the compressor operting point shortly before surge, when the rotting stll wekens, the mximum blde strin excited simply by the reverse flow cn lso rech 1.34 mm/m. The mximum blde strin during surge t this rottionl speed is lso 1.34 mm/m. (Jin et l ). The dnger of this blde vibrtion becomes evident, if the mteril dt of the impeller is considered. According to the Goodmn-digrm for the type of luminum lloy of the impeller shown in Fig. 21 the degree of dnger cn be evluted by rough estimtion. The strin vlue of Emx = 1.43 mm/m represents blde stress of = 11 N/mm2 for this mteril. The digrm shows tht even for low vlue of sttic lod on the blde due to the centrifugl force, the considered vibrtion represents cse ner the llowed stress limit nd sfety mrgin. The mximum vlues of blde strins in the compressor with cmbered vne diffuser re lso significnt. In the cse of rottionl speed nred = 13, 5 rpm the mximum blde strin is.89 mm/m becuse of the slightly wek rotting stll nd reverse flow. At the rottionl speed nred = 14, the mximum blde strin due to the intermittent rotting stll pressure pttern nd the strong reverse flow is 1.34 mm/m which is the sme s tht during surge. CONCLUSIONS The rotting stll pressure ptterns with three cells directed ginst impeller rottion re llwys ccompnied by the reverse flow from the diffuser exit to the impeller inlet. This rotting stll pressure pttern cn develop into simple reverse flow when compressor mss flow rte is further reduced to the point shortly before surge. The reverse flow is chrcterised by brod bnd behvior of the pressure frequency spectr in the compressor, pressure rises t the impeller inlet nd pressure drops in the diffuser region. 9 Downloded From: on 1/11/218 Terms of Use:

10 I Besides the excittion of the blde vibrtion cused by rotting stll pressure ptten ner the blde resonnce frequency, the bldes re lso intensively excited by the reverse flow with brod bnd chrcteristics. The blde vibrtion strin cn rech such vlue tht it is ner the llowed vlues of the impeller mteril. The reverse flow without rotting stll pressure pttern cn lso excite dngerous blde vibrtion. When the engine order frequency is locted ner the blde mode, the bldes cn be excited by the circumferentilly symmetric sttic pressure profile with periodic wves which is induced by the interction between the blde vibrtion nd the circumferentilly symmetric reverse flow. The blde vibrtion strin due to these unstedy flow phenomen mentioned bove cn rech such high level tht it is ner the llowble vlues of the impeller mteril. ACKNOWLEDGEMENT The reserch described in this pper ws funded by the Germny Reserch Assocition (DFG). The uthors re grteful to DFG for their support. The dvice of Mrs. X. Hung of Wshington Stte University is grtefully cknowledged. The uthors would lso like to thnk Mr. P. Tnneberg for his contribution in running the tests nd Ms. X. Yu for her drwing work in this pper. REFERENCES Hgiwr, N., Yoneym, M., Ishii, H., Mtsuur, Y. nd Kshiwbr, Y., 1987, "An Anlysis of Blde Vibrtion of n Axil Flow Compressor during Rotting Stll", JSME-Pper, No. 87-TOKYO-IGTC-92. Proceedings of the 1987 Int. Gs Turbine Congress, Vol. III, pp , Jpn. Hsemnn, U., Hupt, U., Jin, D., Seidel, U., Chen, J. nd Rutenberg, M., 1991, "Rotting Stll Flow nd Dngerous Blde Excittion of Centrifugl Compressor Impeller, Prt II: Cse Study of Blde Filure", ASME-Pper, No. 91-GT-13, 36th ASME Int. Gs Turbine nd Aeroengines Congress, Orlndo, USA. Hupt, U., 1984, "Untersuchung des Schufelschwingungsverhltens hochbelsteter Rdilverdichterlufrder" Disserttion Universiti t Hnnover. Hupt, U., Bmmert, K. nd Rutenberg, M., , " Blde Vibrtion on Centrifugl Compressors -Fundmentl Considertions nd Initil Mesurements", ASME-Pper, No. 85-GT-92, ASME Int. Gs Turbine Congress, Houston, Texs, USA. Hupt, U., Abdel-Hmid, A. N., Kemmer, N. nd Rutenberg, M., 1986, "Excittion of Blde Vibrtion by Flow Instbility in Centrifugl Compressors", ASME-Pper, No. 86-GT-283, 31st Int. Gs Turbine Congress of the ASME, Diisseldorf, Germny. Hupt, U., Jin, D., Seidel, U. nd Rutenberg, M. 1987, "On the Mechnism of Blde Excittion due to Surge on Centrifugl Compressors", JSME-Pper, 87-Tokyo-IGTC- 23, Int. Gs Turbine Congress, vol. II, Tokyo, Jpn. Hupt, U., Rutenberg, M. nd Abdel-Hmid, A. N., 1987b, "Blde Excittion by Brod Bnd Pressure Fluctutions in Centrifugl Compressor", ASME-Pper, No.87-GT-17, 32nd Int. Gs Turbine Congress of the ASME, Anheim, Cliforni, USA. Trnsctions of the ASME, Journl of Turbomchinery, Vol. 11 (Jn. 1988), pp Hupt, U., Chen, Y.N. nd Rutenberg, M., 1987, " On the Nture of Rotting Stll in Centrifugl Compressor nd Vned Diffusers, Prt I: Detection of Reverse Flow Behvior", JSME-Pper, 87-TOKYO-IGTC-22, Int. Gs Turbine Congress. Vol. II Tokyo Jpn. Hupt, U., Seidel, U., Abdel-Hmid, A. N. nd Rutenberg, M., 1988, "Unstedy Flow in Centrifugl Compressor with Different Types of Vned Diffusers", ASME-Pper, No. 88-GT-22, 33rd ASME Int. Gs Turbine nd Aeroengine Congress, Amsterdm, the Netherlnds, Trnsctions of the ASME, Journl of Turbomchinery, Vol. 11, No. 3 (July 1988), pp Hupt, U., Jin, D. nd Rutenberg, M., 1989, "On the Mechnism of Dngerous Blde Vibrtion due to Blde Flow Interctions on Centrifugl Compressors", ASME Pper, No. 89-GT-291, 34th ASME Int. Gs Turbine nd Aeroengine Congress, Toronto, Ontrio, Cnd. Ishihr, K. nd Funkw, M., 198, "Experimentl Investigtion on the Vibrtion of Blde due to Rotting Stll", Bulletin of the JSME, vol 23, No. 177, pp , Jpn. Jin, D., Hupt, U., Hsemnn, H. nd Rutenberg, M., 1992, "Blde Excittion by Circumferentilly Asymmetric Rotting Stll in Centrifugl Compressors", ASME- Pper, No. 92-GT-148, 37th Int. Gs Turbine nd Aeroengine Congress of the ASME, Cologne, Germny. Jin, D., Hupt, U., Hsemnn, H. nd Rutenberg, M., 1992b, "Excittion of Blde Vibrtion due to Surge of Centrifugl Compressors", ASME-Pper, No. 92-GT-149, 37th Int. Gs Turbine nd Aeroengine Congress of the ASME, Cologne, Germny. Hupt, U., Bmmert, K. nd Rutenberg, M., 1985b, " Blde Vibrtion on Centrifugl Compressor -Blde Response to Different Excittion Conditions", ASME-Pper, No. 85-GT-93, ASME Int. Gs Turbine Congress, Houston, Texs, USA. 1 Downloded From: on 1/11/218 Terms of Use: