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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York, N.Y GT-171 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is printed only If the paper is published in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not falling within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (OCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction shout be addressed to the ASME Technical PubEshing Department. Copyright by ASME All Rights Reserved Printed in U.SA EXPERIMENTAL INVESTIGATION ON ROTATING STALL IN A CENTRIFUGAL BLOWER WITH TWO AND FOUR STAGES AND VANELESS DIFFUSERS Gianmario L. Amulfi Diego Micheli Universita di Udine Universita di Trieste Dipartimento di Dipartimento di Energetica Energetica e Macchine Trieste, Italy Udine, Italy Piero Pinamonti Universita di Udine Dipartimento di Energetica e Macchine Udine, Italy , BEAK ABSTRACT The paper presents the results of an experimental investigation on a multistage centrifugal blower, during rotating stall. The test plant allows to change the turboinachine characteristics; in this research the blower has been tested in two different configurations: two-stage and four-stage, with vaneless diffusers. The unsteady flow field inside the blower has been measured by means of hot-wire anemometers. Three single hot-wire probes have been utilised to measure the development of the rotating stall, while a crossed hot-wire probe has been utilised to obtain the instantaneous flow field behind the impellers. The measurements have been done at different flow rate values, including stall inception. NOMENCLATURE absolute velocity impeller outlet diameter th mass flow rate shaft power specific work a absolute flow angle related to tangential direction overall efficiency ne polytropic efficiency power coefficient = P p 4 ro air density (inlet conditions) (4) flow coefficient = p' 1 Co4 173 pressure coefficient = W (0-2 D-2 impeller angular velocity INTRODUCTION Turbomachinay unsteady flow phenomena (stall and surge) have been studied by researchers and industrial designers for a long time, since these phenomena cause shaft vibrations and lower performance at low flow conditions, limiting the compressor's stable operating range. So a better knowledge of stall and surge features is very important to increase efficiency and reliability of gas turbine engines and industrial compressors. Moreover, in recent years the study of stall and surge inception processes have been proved to be very important and several passive and active control systems have been studied to reduce flow instabilities. A theoretical approach of the rotating stall is important to anticipate the main characteristics of this phenomenon such as inception conditions, stall cell number and cell speed propagation. It is of particular importance to know the value of the flow coefficient at which the instability starts and possibly its connection with the flow angle at the impeller inlet/outlet. The first stall theories have been developed during the 1950s s, and are known as "small perturbations theories". All these theories are based on the study of the small perturbations which are caused by the stall and which are superimposed on the stationary flow (Emmons et al. 1955, Stenning and Kriebel 1958, Acton 1960). A second group of theoretical approaches based on a vortex model proposes a flow ideal model which comprises vortices that are caused by rotating stall (Kriebel, Fabri and Siestnnick in Pampreen 1993). Almost all these theories have been developed for linear cascades and are therefore applicable at best to axial turbomachines; only Acton's theory has been developed for centrifugal compressors. Subsequently the Jansen theory (Jansen 1964) has been developed, based on the study of the rotating stall behaviour in centrifugal compressor vaneless diffusers. This theory proposes that the periodic flow oscillations are the result of the interaction between the three-dimensional botmdary layer on diffuser walls and the undisturbed flow. More recent studies must also be recalled (Cumpsty and Cat 1982, Moore 1984a and b, Frigne and Van Den Braembussche 1985, Moore 1989X these are based on the flow pattern in the blade-toblade plane. All these theoretical approaches should enable us to calculate the rotating stall main characteristics, applying in some cases a very Presented at the International Gas Turbine and Aeroengine Congress & Exhibition Birmingham, UK June -13, 1996

2 simple methodology. On the other band it must be pointed out that the effective applicability is often restricted, especially for centrifugal turbomachinery, because these theories have been developed for particular cases or because it is impossible to know all the information necessary for their correct application. Several experimental programmes have also been carried out to enable a better understanding of centrifugal compressor instabilities. These experimental works have been carried out almost only on single-stage turbomachines, measuring the pressure distributions on impeller and diffuser walls (Rodgers 1977, Kammer and Rautenberg 1986, Mizuki and Oosawa 1992, Abdefliamid et al. 1979, Hunziker and Gyarmathy 1993), or more often measuring the instantaneous velocity intensities by hot-wire anemometers, with single-wire probes located behind the impellers and in the diffusers (Jansen 1964, Frigne and Van Den Bramnbussche 1984, Kinoshita and Senoo 1985, Kammer and Rautenberg 1986, Mizuki and Oosawa 1992, Aariga et al. 1987), at times utilising multichannel hot-wire anemometers with two-wire or three-wire probes to measure the instantaneous velocity vectors (Watanabe et al. 1994). These experimental procedures have been developed considerably in the last ten years, due to the improvement of the instrumentation and the development of the acquisition/analysis techniques. With these tools it has been possible to analyse more accurately the unstable flow characteristics in a particular turbomachine, both as stall time evolution, and as phenomenon development in the different turbomachine portions. In this context, the paper presents the experimental results obtained on a modular centrifugal blower, in both two-stage and fourstage configurations, with vaneless diffusers. Detailed experiments have been carried out in the unstable operating range, measuring the performance, the pressure distributions at diffuser outlet, and the instantaneous velocities at the first impeller inlet and within all the diffusers, by using hot-wire anemometers with single-wire and K. wire probes. These measurements show us the general characteristics of the rotating stall phenomenon, that occurs at the low flow-rates, as well as its evolution in all the turbomachine components. Finally it must be noted that previous measurements have been carried out on the same blower, in the four-stage configuration, in stable flow conditions, by the same authors (Amulfi et al. 1995). EXPERIMENTAL PROCEDURE Test Rig Experiments were carried out at the Laboratory of the Dipartimento di Energetica - University of Trieste on the modular centrifugal blower plant (Fig. 1). In this investigation both the two- and four-stage blower configurations were used. All the stages are identical geometrically, the shrouded impellers have 16 backswept blades (inlet angle 35 0 and l outlet angle 67 related to tangential direction, inlet diameter 160 nun, outlet diameter 465 nun, inlet width 23 mm, outlet width 8 nun); the vaneless diffusers have parallel straight walls (inlet diameter 467 mm, outlet diameter 570 mm, axial width mm); the stages are connected by vaned return channels with eight curved blades. The turbomachine was driven by a DC motor connected to a speed increasing gear. The blower operated in an open circuit the air flow entered through a radial inlet pipe. Mass flow rate was Fig. 1 Schematic of experimental rig: 1) blower, 2) DCmotor, 3) speed increasing gear, 4) throttle valve, 5) orifice flow meter, a)b)c) pressure transducers, d) load cell, e) magnetic pick-up, f) g) h) thermocouples, i) hotwire probes. controlled by a motorized throttle valve in the discharge pipe, opening and closing operations being slow enough to distinguish inception of unsteady phenomena. Pressures were measured by variable inductance transducers, temperature by type K-thermocouples, torque by a load cell, rotational speed by a magnetic pick-up indicator, mass flow rate by an orifice flow meter in the inlet duct and velocities by constant temperature hot wire anemometers. Operating Conditions The blower characteristics for the two- and four-stage configurations are shown in Fig. 2a and 2b respectively. The tests were carried out at three rotational speeds (2000, 3000 and 4000 rpm) for both configurations; the design speed is 3000 rpm. The overall efficiency q includes gear mechanical losses and lubricating power. Falls in pressure coefficient iv suggest stall inception at about = 33 in the two stage configuration and p = 30 to 40 (increasing with speed) in the four stage one. In Fig. 3 the polytropic efficiency is shown for both configurations, at the rotational speed of 3000 rpm. 2

3 r n iegzegraspa,jas\ P e rpm o e3000 rpm rpm So e e2000 rpm t, e3000 rpm 0. o.4000 rpm cc Fig. 2a Overall performance map of the two-stage blower. Fig. 2b Overall performance map of the four-stage blower. This paper reports the experimental results obtained with two kinds of measurements: the first one deals with stall inception during control valve closing, the second one with developed rotating stall at fixed valve position relating to three unsteady (9 = 1, 2, 3) and one steady flow coefficient (cp = 4). All these measurements were performed at the three above-mentioned rotational speeds p o e two stage 3-8 four stage cc Fig. 3 Polytropic efficiency at 3000 rpm. Velocity Measuring Technique Flow measurement was performed by constant temperature hotwire Dantec anemometers, both with single (55P11) and X-wires (55P62) straight miniature probes. The sensors were tungsten wires, of 5 pa diameter and 1.25 mm length. Three hot-wire units (55M01) with standard bridge (55M) were used and a low-pass filter at khz was applied to cut high frequency noise: the base frequency was 800 Hz (blade passing frequency at 3000 rpm). The hot-wire instantaneous voltage signals were sent to an analog/digital converter board. The phase reference was taken by the pick-up device through a time base that provided the trigger signal. The data acquisition system (HP6900) was controlled by a HP9000 computer (Fig. 1). Calibration and volt/velocity conversion Were performed as described by the authors (Amulfi et al., 1993). A stationary hot-wire technique was used, which involves three single-wire probes (Fig. 4a). The first single-wire probe was radially located within the diffusers, at several different distances from the machine axis, and was used to measure the intensity of the velocity vector and the flow decay downstream of the impeller in the diffusers of all the stages. The second h-w probe, located at the diffuser exit and at a 60 circumferential. distance from the first one, led to the finding of the number of stall cells. They both had their Wire oriented parallel with the machine axis, i.e. normal to absolute velocity, if the axial velocity is neglected owing to the geometry of the diffusers. The third one was placed at the first stage impeller inlet, with the wire tangential to peripheral velocity, i.e. approximately normal to velocity 3

4 60 pressure transducer 1 location a Hot-wire probes impeller outlet distance from impeller [mm] single wire probe thermocouple pressure transducer X wire probe t single wire Ii 5 probe diffuser exit 1st stage inlet Fig. 4a Schematic of the single hot-wire probes measuring points. Fig. 4b Schematic of the X hot-wire probes measuring points (two-stage vector, and was used to eventually detect the instability in the blower suction. Furthermore an X-wire probe, axially placed at the outlet of the first and the last impeller (Fig. 4b), was used to measure twodimensional flow (direction and intensity of velocity), adopting a fixed orientation technique. During these measurements, the probe orientation was accurately set to minimize the aerodynamic disturbances caused by the tips of the wire supports.. The sampling rate was 9 readings/revolution for over 9 revolutions to measure stall inception, 64 readings/revolution for 128 revolutions to measure stall propagation speed, 256 readings/revolution for 32 revolutions to measure the number of stall cells and 640 readings/revolution for Over 12 revolutions to measure two-dimensional [low. These values were chosen in order to maximize resolution without losing physical sense. EXPERIMENTAL RESULTS Stall inception measurements These measurements had the aim of studying how the machine changes its operating conditions from stability to instability during a progressive flow reduction. The motorized throttle valve that controlled the mass flow rate was initially completely open, and the rotational speed set at the selected value. The valve was then slowly closed over a period of about 60 seconds, without any regulation of angular velocity, whose maximum reduction was of about % of the initial value. Two single wire probes were located in the diffusers of two different stages, at a distance of 50 mm from impeller exit, and a third one was placed at the first stage impeller inlet The signal of the pressure transducer connected to the orifice flow meter was also measured. Two-stage configuration. Figure 5 shows the signals of the three probes at the speed of 3000 rpm. A sudden increase of the amplitude of the oscillations due to stall inception can be noted in the signals of the stages at about the 320th revolution; it occurs at the inlet also, as it will be pointed out afterwards, but the poor quality of the inlet signal, probably due to a local distortion of the flow field corresponding to the probe, makes it difficult to "separate" that oscillation from noise. Another sudden increase can be noted when mass flow rate falls to zero at about the 530th revolution. The frequency of stall was revealed by means of FFT analysis. Figure 6 shows the signal spectra obtained when analysing two strings of 24 data extracted respectively before and after the stall inception, as a function of the frequency made dimensionless in respect to the base rotation frequency of the impeller. It can be observed that the stall is revealed by a peak at a frequency equal to one half of the base frequency approximately, that can be observed in the second spectrum only. An oscillation at the same frequency was also detected analysing data in the zero flow region. Figure 7 shows the trend of the signal amplitude corresponding to the stall frequency obtained analysing the data by means of a "window" which had 24 points, moving along the data string with a step of nine points. The two curves, related to the diffusers of the first and second stage, show the absence of the stall oscillation until the 4

5 02 00 I I I I I I I I I F --- inlet stagel ii! i i I I I ii i I I I I ii I II 5 e 0.08 ii i I o ootf. f stagei 02f f stage IS "1"1"."1"1"1"1"."1"4"1"1" dirt. stage, III! IIIIII MI IIIIII --- alif. stage2 IIIIIIIIII revolutions Fig. 5 Signals of the hot-wire probes during stall Inception (3000 rpm, two-stage 0 a 0.04 C , 320th revolution, and tend to an amplitude of almost constant value, that is higher for the second stage. Figure 8 is an enlargement of Fig. 5 and shows in detail the stall inception in the diffusers; the curves prove that the rotating stall started instantaneously throughout the machine, and that the oscillations were completely developed after a few revolutions. In particular it can be seen that this transition was of six to seven revolutions in the second stage. The described inception mechanism was qualitatively similar to that of the "progressive amplitude precursor described by Chen at al. (1994) for a single stage centrifugal compressor, but in that case the maximum amplitude was recorded after 270 revolutions, and a stationary oscillation was finally detected instead of a rotating one. Similar results were obtained with the measurements at the speed of 2000 and 4000 rpm revolutions Fig. 7 Amplitude of the oscillation of the h-w probe signals at the stall frequency (the revolution number in the abscissa indicates the ending point of the 24 points 'window' of the FFT analysis, moving along the data string rpm, two stage Four-stage configuration. In this configuration the stall revealed itself with an oscillation at approximately the same frequency found in the two-stage blower, but the inception mechanism appeared to be clearly influenced by the higher number of stages. In Fig. 9 the signals detected in the diffusers of the first, second and fourth stage during stall inception at the speed of 3000 rpm are reported. It can be seen that stall is less evident in the first stage than in the others, showing a shape similar to that recorded in the twostage configuration. In the second stage the inception is abrupt and 5.00 VtAtri tke4&*fi'frvtfint\a LIVA4h1\00 1( _.47, 0.04 rev.: Frequency / rps Fig. 6 Signal spectra of the probe located in the diffuser of the second stage; upper: before stall inception; lower: after stall inception (3000 rpm, two-stage Chf f. stagel kw**444144pionnamawav Olf f stage revolutions Fig. 8 Signal of the hot-wire probes located In the diffusers: detail of the stall inception zone (3000 rpm, twostage 5

6 5.00 Co co Co :00 CO CO k04)'sikt 1* 1 1\1A0WAN --- cliff. stagel it cliff. stage2 1 I ivihi I --- diff. stage4 I I I I MikkkRiMPfrilitiff+64dotyit ) IN \ erk revolutions i i SO Fig. 9 Signal of the hot-wire probes located in the diffusers: detail of the stall inception zone (3000 rpm, fourstage occurs at the 329th revolution, with a delay of about 15 revolutions, while the fourth stage presents a progressive increase of the amplitude that begins at the 320th revolution, with a modest delay of about six revolutions, and takes about 12 revolutions to reach the final value. Comparing this figure with Fig. 8 it can be observed that the second stage in the two stage configuration has a behaviour similar to that of the fourth stage: it must be noted that they are both the last stages in the respective configurations and therefore they are affected by the geometry of the delivery duct The similarity of behaviour of these two stages is confirmed by the spectra of Fig. when compared with these of Fig. 6: the fourth stage has a spectrum similar to that of the second in the previous configuration, while the second stage in the actual configuration is gra0.04 g i E Ta I.122o. E 63 0 rev.: diff.stage2. rev.: dill, stage 4 _ , Frequency/ ups Frequency rps Fig. 11 Velocity spectra in the diffuser of the second stage, at various distances from the impeller exit and at various mass flow rates (3000 rpm, two-stage characterized by a spectrum with a higher value of the main peak, at a frequency value 0.5, and a second harmonic of non negligible amplitude. Constant flow measurements Measurement were done to characterize stall features as the number of cells and the development of the perturbation through the diffuser and the stages. The tests were performed locking the throttle valve at differing positions to obtain the prearranged flow rate values. ig. Signal spectra after the stall inception of the robes located In the diffuser of the second stage (upper) nd of the fourth stage (lower) (3000 rpm, four-stage on figuration). Two-stage configuration. Figure 11 shows the spectra of velocity recorded at 3000 rpm in the second stage, at various distances from the impeller exit and at the four different flow rates. The peaks at the frequency value 16 are related to the blade passing 6

7 Mom- Suction. stags 1 ' 3000 rpm PHI -8. 0O f a; Plik-21.0tra rt a > Fr; Fig. 12 Velocity spectra In the diffuser of the first stage, at various distances from the impeller exit and at two mass flow rates rpm, two-stage frequency, at stationary flow condition (9 = 4) they are the only significant peaks while at stall conditions they are less pronounced and the contribution of harmonics of frequency value from 14 to 18 becomes significant, showing a more complex shape of the blade to blade flow distribution. In all cases at a distance of 50 mm from the rotor, corresponding to the diffuser exit, the blade paccage had no more influence on the flow field configuration. At p 5 3 the stall is revealed by the peaks at the frequency value about 0.5 and its higher order harmonics. The amplitude of the latter ones decreases as flow decreases so that at p = 1, at a distance of 30 mm from the impeller exit, stall oscillation becomes quite sinusoidal. Where the contribution of higher order harmonics is significant, the amplitude of the main peak, at frequency 0.5, decreases in the first part of the diffuser, near the impeller, while it is practically constant at p = 1. The same flow configuration was observed in the first stage at the minimum flow rate (9 = 1). At the intermediate flow rates the spathe of Fig. 12 were obtained: at p = 2 stall. amplitude shows a minimum located in the middle of the diffuser while at p = 3 the amplitude is very small at the impeller exit and regularly increases throughout the diffuser. It could be supposed that at p = 3 the flow in the first impeller was almost stable but that it became unstable in the diffuser, Note that the average diffuser inlet flow angle, calculated from continuity and slip factor correlations, was in the first stage equal to 9.5 ; this value was in good agreement with that of the critical flow angle for reverse flow obtained by means of the diagrams of Senoo and Kinoshita (1977), and approximately equal to 9. Note also that Senoo and Kinoshita (1977) suggested that a reverse flow occurs near the inlet for a narrow diffuser, as it seems to happen in this case (Fig 12) at cp = 3. Moreover, Fig. 8 showed that the stall inception in the first diffuser was not abrupt, according to the diffuser stall features described by Pampreen (1993). Then the oscillations moved from the diffuser through the return vane channel and caused the stall in the second impeller. It could be confirmed, according to Japikse 12 Frequenc-y/rps Fig. 13 Velocity spectra in the inlet of the first stage, at various mass flow rates (3000 rpm, four-stage (1992), by the quite abrupt stall inception seen in Fig. 8 for the second stage, by the higher order harmonics shown in Fig. 11, and also by the decrease of the pulsations in the diffuser near the impeller exit (Fig. 11), as shown by Pampreen (1993) with reference to an impeller stall. The rotating stall then developed along the diffuser with an amplitude that was nearly equal with radius, as reported by Pampreen (1993), in relation to stall in vaneless diffusers. At p = 2 rotating stall occurred in the first impeller also, but pulsations decreased in the diffuser and the flow seemed to become nearly stable; then the flow angle of the wall - streamline probably became identical to zero, producing a reverse flow and therefore a rotating stall again., The stall in the first impeller was also confirmed by the oscillations detected at the first stage inlet, as can be seen in Fig. 13. A peak can be noted at the frequency value 0.5, even if the amplitude is low compared with that obtained at the impeller exit. The spectrum reported at cp = 3 shows that the impeller flow was unstable also in this case, even if the perturbation was yen/ small at the impeller exit. Measurements made with two probes located in the diffusers at a 60 circumferential distance showed that there was only one stall cell in both the stages, moving with a speed that was equal to the percent of the angular speed of the rotors; this result was also confirmed varying the mass flow rate and the rotational speed. Four-stage configuration. The main features of rotating stall were also the same in the four stage configuration. In particular a single cell was always detected and its rotational speed was again about one half (more exactly ) of that of the impeller. Figure 14 shows the velocity spectra in the diffusers of the four stages when cp = 2, limited to the low frequencies characterizing rotating stall. The first stage shows a trend of the pealc at the frequency value 0.5 similar to that indicated in Fig. 12 for the two stage configuration, with a minimum located in an intermediate 7

8 1 Stn. 1. PHI 20 ElIZ:=NM 1.5 Stag. 1/2 ph rpm 13 tz ugs: MISS!!; ,1r S! I 2 Frequarcy INIMMOIMer 0 WEIMINer flln illflaull 1imammorlammammma. 1 requency rp PF11 20 Stage. P11.1= S LI s 4404, Stage 1/2 phi rpm II r lallineempr Fs! mm mm,.. ytnaimumm am. Am mamma mamma Fneqw rcy I $93 sti a rnkiteney I us MMONI olomootonmor 3 Nc a.s 4 Stage 2/2 phi =.201 xeee rpm Fig. 14 Velocity spectra in the diffusers at various distances from the impeller exit ((p = 2, 3000 rpm, four-stage Stags 2/2 ph rpm position in the diffuser. All the other stages seemed to have a behaviour like that previously seen in Fig. 11. We must however note the large amplitude of the second and third harmonics of stall frequency in the fourth stage, near the impeller exit; the corresponding stall signal was quite complex, maybe as a consequence of the propagation of disturbances from the preceding stages. X probe measurements. A more detailed description of the unstable flow field at the diffuser inlet was obtained by means of the miniature X-wire probe, positioned axially in the middle of the diffuser at a distance of 3% of the outlet radius of the impeller. With this technique the instantaneous velocity vectors were obtained, after an accurate calibration. In Fig. 15 the trend of the absolute velocity C, made dimensionless in respect to its mean value C are shown for both the stages of the two stage configuration. The velocities are plotted for four consecutive revolutions comparing the situation at cp = 1 and p = 3. In the first stage at cp = 3 the blade to blade velocity distribution is almost regular, while at the minimum flow rate the flow instability is clearly shown. In the second stage the stall may be already observed atp = 3; at cp = 1 the velocity rises abruptly in the transition zone between the stall cell and the undisturbed blade channels, assuming a particular triangular shape, like the pressure signal shown by Abdelluunid and Bertrand (in Pampreen 1993). The same measurements carried out in the four-stage configuration gave similar results. Figure 16 shows for example the velocity traces obtained in the first and last stage at the minimum flow rate: they are almost equal to that shown in Fig. 15 for the twostage configuration. *1114hipliibli S 7 c.2 2 a a REVOUJTIONS Fig. 15 Traces of the absolute velocity, made dimensionless in respect to its mean value, In the inlet of the diffusers at two mass flow rates (3000 rpm, two-stage It is also interesting to analyze the trends of the absolute flow angle a. In Fig. 17 the values obtained in the fourth stage is shown for all the flow rates; only two revolutions are given to illustrate better the trends in the blade to blade plane. At = 4 the stable condition is observed with a regular increase of a from suction side to pressure side. At lower flow rates the flow configuration is a function of the position on the instability wave: in the area of the higher values of a the trends are equal to those observed in stable condition while in the area disturbed by the stall cell potence the trends are irregular, showing a shape opposite to the previous one in the area of minimum values. This is a return flow region, characterized by negative values of a, whose circumferential amplitude increases as flow rate decreases. The influence of the position on the instability wave can also be shown with a FFT analysis of the differing part of the signal, as illustrated in Fig. 18 that refers to the velocity in the second stage of the two-stage configuration at minimum flow rate (plotted in Fig. 15). The upper spectrum is relative to the stable area and presents a main peak at the blade passing frequency (fievips = 16); in the lower spectrum relative to the disturbed area, the peak isn't evident and none characterizing frequency is shown. 8

9 1; to 0 I Stage 4 phi rpm Stags 4 phi rpm * :g REVOLUTIONS Fig. 16 Traces of the absolute velocity, made dimensionless in respect to its mean value, in the Inlet of the diffusers (9 12 1, 3000 rpm, four-stage CONCLUSIONS The internal flow in a multistage centrifugal blower with vaneless diffusers has been analyzed during unstable operating conditions. The instantaneous velocities have been measured by hot wire anemometers with stationary probes, comparing two blower configurations (two-stage and four-stage) and three differing rotational speeds. At low flow rates the presence of rotating stall has been detected. It resulted in a discontinuity in the pressure rise curve and was characterized by single stall cell rotating with frequency close to one half of the impeller rotating frequency. These main stall characteristics were almost the same in both the blower configurations and in all the stages, for all the different operating conditions tested. rev.: > Frequency./ rps Fig. 18 Absolute velocity spectra in the diffuser of the second stage; upper: relative to the stable area; lower: relative to the disturbed area ((p n 1, 3000 rpm, twostage a Stage 4 phi rpm /of Stage 4 phi rpm amokoly REVOLUTIONS Fig. 17 Traces of the absolute flow angle (related to the tangential direction), in the diffuser of the fourth stage at various mass flow rates (3000 rpm, four-stage The stall inception happens almost at the same moment in all the blower parts in the two-stage configuration, while in the four-stage one a short delay among the stages was observed. In every single stage the amplitude of the oscillation was completely developed in a few revolutions, and in some cases the inception was abrupt. The presence of the oscillation also in the suction of the first stage was always detected. The velocity detailed measurements, carried out in all the vaneless diffusers, showed the evolution of the phenomenon, characterized by a flow field quite different in the various stages and for the several flow rate conditions. One can observe that the onset of stall depended on the interaction of the impellers with the diffusers, but the main characteristics of the phenomenon lead us to conclude that the blower was working with an "impeller rotating stall". Finally, the instantaneous measurements using an X-wire probe at outlet of the impellers have shown the velocity vector modification when the flow is influenced by the stall cell and its circumferential development. 9

10 ACKNOWLEDGMENTS The authors would like to thank the MURST (Italian Research Ministry 7 40% fund) for the financial support granted to this research. REFERENCES Abdelhamid, AN., Colava], W.H., Barrows, J.F., 1979, "Experimental Investigation of Unsteady Phenomena in Vaneless Radial Diffusers", Trans. AMIE, Journal of Engineering for Power, vol. 1, March, pp Acton, 0., 1960, "Experimental and Theoretical Study of Rotating Stall in a Centrifugal Compressor", Atti della Accademia delle Scienze di Torino, Italy, vol. 95 (in Italian). Ariga, I., Masada, S., Ookita, A., 1987, "Inducer Stall in a Centrifugal Compressor within Inlet Distortion", Trans. ASME, Journal of Turbomachinmy, vol. 9, January, pp Armin, Gt., Michell, D., Pinamonti, P., 1993, "Velocity and Turbulence Measurements in a Centrifugal Turbomaclaine by Hot- Wire Anemometer", Proc. If MisMac Congr., Firenze, Italy, pp (in Italian). Arnulfl, G.L., Michell, D., Pinamonti, P., 1995, "Velocity Measurements Downstream of the Impellers in a Multistage Centrifugal Blower', Trans. ASME, Journal of Turbomachinery, vol. 117, October, pp Chen, J., Hasemann, H., Shi, L., Rautenberg, M., 1994, "Stall Inception Behaviour in a Centrifugal Compressor", ASME Paper 94- GT-159. Curripsty, NA., Greitzer, E., 1982, "A Simple Model for Compressor Stall Cell Propagation", Trans. ASME, Journal of Engineering for Power, vol. 4, January, pp Emmons, H., Pearson, C., Grant, H., 1955, "Compressor Surge and Stall Propagation", Trans. ASME, vol. 77, May, pp Frigne, P., Van Den Braembussche, R., 1984, "Distinction between Different Types of Impeller and Diffuser Rotating Stall in a Centrifugal Compressor with Vaneless Diffuser", Trans. ASME, Journal of Engineering for Gas Turbine and Power, vol.6, April, pp Frigne, P., Van Den Braembussche, It, 1985, "A Theoretical Model for Rotating Stall in the Diffuser of a Centrifugal Compressor", Trans. ASME, Journal of Engineering for Gas Turbine and Power, vol. 7, April, pp Hunziker, R., Gyannathy, G., 1993, "The Operational Stability of a Centrifugal Compressor and its Dependence on the Characteristics of the Subcomponents", ASME Paper 93-GT-284. Jansen, W., 1964, "Rotating Stall in a Radial Vaneless Diffuser", Trans. ASM:E, Journal of Basic Engineering, vol. 86, December, pp Japikse, D., 1992, "Centrifugal Compressor Design and Performance", Concepts Eli Inc., Vermont, USA. Kammer, N., Rautenberg, M., 1986, "A Distinction between Different Types of Stall in Centrifugal Compressor Stage", Trans. ASME, Journal of Engineering for Gas Turbine and Power, vol. 8, January, pp Kinoshita, Y., Senoo, Y., 1985, "Rotating Stall Induced in Vaneless Diffusers of Very Low Specific Speed Centrifugal Blowers", Trans. ASME, Journal of Engineering for Gas Turbine and Power, vol. 7, April, pp Mizuki, S., Oosawa, Y., 1992, "Unsteady Flow within Centrifugal Compressor Channels under Rotating Stall and Surge", Trans. ASME, Journal of Turbomachinery, vol. 114, April, pp Moore, F.K., 1984a, "A Theory of Rotating Stall of Multistage Axial Compressors: part I - Small Disturbances", Trans. ASME, Journal of Engineering for Power, vol. 6, April, pp Moore, F.K., 1984b, "A Theory of Rotating Stall of Multistage Axial Compressors. part II - Finite Disturbances", Trans. ASIvE, Journal of Engineering for Power, vol. 6, April, pp Moore, F.K., 1989, "Weak Rotating Flow Disturbances in a Centrifugal Compressor with a Vaneless Diffuser", Trans. AWE, Journal of Turbornachinery, vol. Ill, October, pp Pampreen, R.C., 1993, "Compressor Surge and Stall", Concepts Ell Inc., Vermont USA. Rodgers, C., 1977, "Impeller Stalling as Influenced by Diffusion Limitations", Trans. ASME, Journal of Fluids Engineering, vol. 99, March, pp Senoo, Y., Kinoshita, Y., 1977, "Influence of Inlet Flow Conditions and Geometries of Centrifugal Vaneless Diffusers on Critical Flow Angle for Reverse Flow", Trans. AS/I& Journal of Fluids Engineering, vol. 99, March, pp Stenning, A., Kriebel, A., 1958, "Stall Propagation in a Cascade of Airfoils", Trans. ASME, vol. 80, May, pp Watanabe, IL, Konomi, S., Ariga, I., 1994, "Transient Process of Rotating Stall in Radial Vaneless Diffusers", ASME Paper 94-GT- 161.