THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y STALL INCEPTION BEHAVIOR IN A CENTRIFUGAL COMPRESSOR

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1 THE AMERICA SOCIETY OF MECHAICAL EGIEERS 345 E. 47th St., ew York,.Y 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. Papers are available from ASME for 15 months after the meeting. - Printed in U.S:A. Copyright 1994 by ASME 94-GT-159 STALL ICEPTIO BEHAVIOR I A CETRIFUGAL COMPRESSOR J. Chen Inst. of Engineering Thermophysics Chinese Academy of Sciences Beijing, China Li Shi Inst. of Engineering Thermophysics Chinese Academy of Sciences Beijing, China H. Hasemann Institute of Turbomachinery University of Hannover Hannover, Germany M. Rautenberg Institute of Turbomachinery University of Hannover Hannover, Germany ABSTRACT In studying the stall inception process, while most results were reported for axial compressors, the present paper investigates the stall inception behavior typified in a centrifugal compressor. The test was conducted with a radially-bladed impeller and in a speed range of rpm. Extensive pressure transducers were used to study the frequency characteristics of emerging stall waves. As a result, stall precursors were detected, all with clear mode seen from frequency analysis, but very much different by the behavior of their onset, existence and development. The first type, called the stable-amplitude precursor, exists in a time range of about impeller revolutions, with unpredictable and different frequencies from the fully developed stall. Such perturbation, once appeared, may grow to the full stall straightly, or may appear for several times intermittently before finally reaching the full stall, thus acting as a pre-precursor in the whole stall inception process. The second type is the progressive-amplitude precursor when the perturbation emerges as long as 270 impeller revolutions prior to and progressively develops into the full amplitude stall with no change of frequency during this process. The third type, which has been detected for the rotating stall with evident reverse flow symptom, is the precursive pressure increase accompanied with the stable- or progressive-amplitude perturbation, before the full stall establishes. The inception process is also examined for surge during the test of the same compressor, in which the existence of rotating stall in front of every surge cycle and the low frequency precursive wave before surge cycles is demonstrated. Finally, the blade passage frequencies for precursor pressure signals are further analysed to address the monitoring strategy during stall inception process. omenclature a amplitude ratio of precursor to full stall f frequency m number of stall cells m mass flow rate n rotational speed of impeller pressure blade length along tip T duration of precursor existence in impeller revolutions t time x distance from inducer inlet along blade tip blade angle X radius ratio r/r2 of diffuser it total pressure ratio Subscripts b blade p pressure oscillation s impeller shaft 2 impeller exit 3 diffuser inlet Abbreviation Pre. precursor RS rotating stall ITRODUCTIO The phenomenon of stall and surge has been a long-term problem of concern in turbomachinery industry for decades, but it is a new trend in the recent years that the research of this phenomenon has been revitalized and expanded to a great extent. Apart from the constant demand for better efficiency and reliability of the gas turbine engine and compressor installation, the motivation comes also from the ever-increasing interest in researching the unsteady nature of turbomachinery flow which is believed to be in a potential to lead to innovative performance of the machines. It is an interesting fact that can be witnessed in this trend that much new effort has been paid to the study of the inception process of stall. Along with the recent development in active controlling the flow instability in compressor and/or compression system (Epstein et al, 1986), there is a real question of how the instability behaves at its infant stage, so that the effect of controlling action can be realized efficiently. In fact, the ekperimental works in detecting the stall precursor such as the modal type perturbations (Garner et al, 1990 and Day, 1991) and the more sudden growth of small localised disturbance (Day, 1991) were all linked closely with the theme of active control. There are other works, however, which have more general aims towards understanding the flow mechanism of Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, etherlands June 13-16, 1994

2 instability through its inception process. Among them, the contributions of Jackson, 1986 and McDougall et al, 1989, the works of Inoue et al, 1990 and Hoenen et al, 1993, and the new result of Day et al, 1993 can be cited. Results of all the mentioned works have been made in a context of axial machines. This paper presents results of the investigation of stall inception process for centrifugal machine. Firstly, the test with a radially-bladed impeller, along with the measurement and data processing procedure, are described. Different types of stall precursors detected are then displayed by their time-traced pressure recordings. The behavior of these precursors are discussed by various means of analyses: the frequency spectra, the propagation characteristics, the time range of their existence with respect to the impeller revolution, the corresponding blade vibration excitation and the blade passage frequencies within these precursor signals. Finally, the inception process of surge is also demonstrated. EXPERIMETAL FACILITY AD MEASUREMET Centrifugal compressor test rig. The test rig shown in Fig. 1 is an open loop system in which the flow enters the compressor through an axial suction pipe from a settling chamber and leaves downstream in an annular collecting chamber ended with a tangential outlet tube. The compressor is driven by a 1350 kw DC-motor with a gear box coupled in-between providing a wide range of speed variation. The mass flow is throttled by two gate valves of different size located in a long piping system which furthers the outlet tube to the ambient atmosphere. This test rig has been continuously being used for various research programs on flow instability and its detailed description can be found in Haupt et al (1986) for example..' HA Fig. 1 Radial compressor test facility for unsteady flow investigation Tested impeller and diffuser. There are a number of impellers and diffusers tested in the past in this rig for their rotating stall and surge characteristics ( Haupt et al, 1987b, 1988). The one used in present study is an unshrouded radially-bladed impeller which has 20 blades, but with every second blade cut back at the inlet till x/s = 0.27 (Fig. 2). The impeller inlet diameter at the shroud is 280 mm and the inlet diameter ratio is At outlet, the impeller diameter is 400 nun and the blade width is 26 mm. The diffuser, not shown here, is of vaned type with the number of vanes equal to 19, the radius ratio 7,, 3 = 1.15 and the inlet diffuser angle Q3b = 27. Fig. 2 The tested unshrouded radially-bladed impeller Instrumentation. Extensive dynamic pressure transducers (Kulite XCQ-080) are used to record the pressure oscillation of rotating stall and surge. As many as ten transducers could be positioned at different locations along the flow path on the shroud wall, as shown in Fig. 3. At x/s = 0.7, two additional shroudmounted transducers at 58 and 125 circumferentially apart from the one indicated in Fig. 3 are used to thorough the phase relationship of pressure signals. Moreover, blade-mounted semiconductor strain gages with FM-telemetry transmitter are also used to determine the blade vibration characteristics and the number of rotating stall cells. All these sensors are actually arranged in different combination for different test cases often undertaken concurrently with another internal flow test program. The maximum number of sensors at a time is eleven as limited by the number of channels of the magnetic tape machine. The magnetic tape records all signals simultaneously and continuously, and is therefore very convenient for the analysis of stall inception process. In addition to the dynamic measurement, conventional pressure and temperature probes are used to determine the overall compressor performance. x.-5mm x/s = 0,05 0,15 0,27 0,4 0,55 0,7 0,8 0,95 r/rz i3o3 Fig. 3 Location of pressure transducers in the shroud wall of tested compressor Data processing, The frequency and phase analyses of the recorded pressure and blade vibration signals are mainly performed by the computer software although the HP 3582A dual channel spectrum analyzer is sometimes also used for a cross-check. Using the computer procedure seems more convenient for capturing the short duration signal of the stall precursor and for the repeated

3 analysis. The number of stall cells, for the fully developed stall as well as for the emerging stall, can be determined by considering the relation between the phase-transfer angle from signals of two circumferentially apart transducers and the physical angle from the geometric disposition of these two transducers. This number can also be determined by a combined frequency analysis of pressure and blade vibration signals, i.e. by the relation of blade excitation: m = ( f6 ± fp ) / fs where the positive and negative sign corresponds to stall cell rotating with and against the impeller rotation respectively. Details for determining the stall cell number by the above approaches can be found also in Haupt et al (1986). Test procedure. To study the stall inception process, the test has been conducted in a speed range of rpm. For every speed line, the compressor is first running into rotating stall detectable mainly by the on-line frequency analyzer. The compressor is then unthrottled back to an closest steady operating point before stall starts. From this point, the compressor is throttled again, step by step, with a simultaneous recording of all the dynamic signals onto the tape. The recording time usually lasts for about 30 seconds and is long enough for observing the inception process and for running the compressor till surge point. For safety reason, surge point was reached only for the speed lower than rpm. Obviously, the recorded inception process depends on the deepness and the speed of throttling. Therefore, supplementary tests with a constant throttling speed controlled by the smaller valve are performed to check the repeatability of the recorded signals of the stall inception. Compressor performance. Part of the compressor map, for the tested speed lines, is given here with indications of types of stall and numbers of stall cells ( Fig. 4). The positive and negative signs of the cell number are understood as the cell rotating with and against the impeller respectively. This result is typical for the phenomenon of flow instability happened in centrifugal compressors and suits very well the task of studying the stall inception behavior on ; E mojo because of the variety of flow instability covered for the speeds n > tested -- continuous and intermittent rotating stall, standing stall, x/s= 1,0 stall with evident reverse flow symptom, and surge as well ,10 1,80 0,90 0,60 0 1, ,50 6,00 7,50 9,00 rh kg/s rpm 1, rpm rpm 1,20 Hm3J r m T007pm A Steady flow points Closest point to RS and surge o Closest point to standing stall and surge Closest point to surge Fig. 4 Compressor map with indications of types of stall and numbers of stall cells Component performance with its different configurations is not given here, but left for further research. EXPERIMETAL RESULTS AD DISCUSSIOS Based on the time-traced pressure recordings for different stall occurrences on different speed lines, their precursive signals are carefully examined and grouped into the following three types. Stable-amplitude precursor. This is the most frequent type of stall precursors detected from the test results. A typical example at n = rpm is shown in Fig. 5 which contains the pressure and blade vibration signals in the time domain for the fully developed stall and its several precursors. The cell number for the full rotating stall is m = -3 in its front part (RS 1), but later changed to m = +4 (RS 2). Here, if we look at the full stall only, its pressure oscillation comes to light quite suddenly (see also the sudden increase of blade vibration signal on top of the Fig. 5), within the time of no more than two waves. But there are several precursive signals emerging long before the full stall as indicated by Pre. A, B and C in Fig. 5. These precursors show a similar wave form, with stable but much less amplitude, compared to the full stall. The mode of these precursive waves can be seen from the discrete frequency resolved in the pressure spectra in Fig. 6, with slightly different values for different precursors. Furthermore, results by means of the phase analysis between three circumferential transducers at x/s = 0.7 and the blade excitation analysis show a rotating nature of these precursive waves and give the number of stall cells as indicated in Fig. 6. The clear mode frequency in Fig. 6 also indicates that these cells are of longer length scale. The duration of the existence of these precursors are quite different, from about 20 impeller revolutions for precursor C, being the shortest, to about 90 impeller revolutions for precursor B, the longest. Pre. A Pre. B Pre C RS 1 RS 2 W E x/s0,9 a x/s=0,8 a. ^ x/5=0,7 a x/s=0,55 3 I x/s = 0,4 u - I x/s=0,3, I x1s=0,15 x/s = 0,05 r T Irf 0.5s Fig. 5 Pressure and blade strain signals in the time domain showing intermittent stall precursors nred = rpm An interesting question of such intermittent precursors is under what circumstance they appear. Unlike the example in Fig. 5 which shows no sign of any order of their emergence, the precursors in Fig. 7 for n = rpm appear in a more ordered sequence, with nearly equal time interval between each precursor (A, B, C and D). It is quite plausible that the precursor needs some o accumulate certain strength for its onset, and the Fig. 7 is 3

4 perhaps the case when all the influential conditions for every precursive perturbation are equal and stable. All other characteristics of the precursive signals indicated in Fig. 8, such as the mode, the frequency, the rotating nature and the cell number, are the same as for precursors in Fig. 6. Pressure I, Hz m=l amdiwde Pun-- I, Hz m=-3 80 GO - fr Hz m= Pre. A Hz 1000 ^` RS Pre. B Frequency Fig. 6 Pressure spectra in the frequency domain for the RS and several its precursors nred = rpm o E o 0 E a o ^ oa C impeller revolutions before develop straightly into the full stall. In comparison with this, the intermittent precursors described in the first two examples manifest a non-straight way to develop to the full stall and could therefore be considered as pre-precursors in the stall inception process. o W E Pre. A RS I Pre. B RS 2 Pre. C RS 3 9 L. o L.0 _ E a > In xls=1,0 Q I xis =0,95 d 1 x/5-0.8 a d xls=0,7 n o XIS =0.55 3X/S=O.L U g XIS=0,15 XIS=o,o5 0,55 Fig. 9 Pressure and blade strain signals in the time domain showing continuous stall precursors and the pressure increase before RS nred = rpm x!s= 1.0 a x1s=0.9! xls = 0.8 x!s x15=0s0 x x15=01 c xfs=o.ot Fig. 7 Pressure and blade strain signals in the time domain showing intermittent stall precursors with equal time interval between each onset nred = rpm Pressur e amplitude o ^e^ so II fe23.93 Hz m.-g mbar 3f wll tee. p 30 \ fp. Lt.02 Hz m=-l fa=3g,16 Hz m=-g fe =30,18 Hz-G 0 Pre. A L Hz 1000 Pre. B Pre. C Pre. 0 Frequency Fig. 8 Pressure spectra in the frequency domain for the RS and several its precursors nred = rpm In still another example, shown in Fig. 9 and also at n = rpm, the compressor has been further throttled in comparison to the operating point of Fig. 7. The condition seems to be more favourable for the stall onset that the precursors (A, B and C) are no longer intermittent but merged together to become a continuous one right in front of the full stall (RS 1, 2 and 3). As shown in Fig. 10, these precursors still possess the same character as in the above two examples (with one exception for precursor A -- see explanation in next paragraph), but exist continuously for about 66 RS Pressure amplitude Rim 80 },,82.03 Hz m=-3 mdor RS 3 fe =34.18 Hz =-G Pre.0 60 fr' Hz m=-g RS 2 fe =34.18 Hz m=-l Pre. B GO - 1e =23.93 Hz m=-g RS1 fe =34.18 Hz =-L 0 - Pre. A Hz 1000 Frequency Fig. 10 Pressure spectra in the frequency domain for the RS and its precursors nred = rpm On the whole, all the precursors described above, whether intermittent or continuous, independent of how long it exists, have no sign of amplitude increase within the duration of their existence. Therefore this type is called stable-amplitude precursor. Progressive-amplitude precursor. This type of stall precursor is detected again at the speed line of rpm, but the starting operating point is with less throttling than that in Fig. 7 and 9 as indicated by the values of mass flow in the compressor map (Fig. 4). Repeated tests have been conducted to confirm its existence. As shown in Fig. 11, the wave of pressure oscillation has a clear modal shape, with much longer time period than that in Fig. 7 and 9. In this figure, judging by the blade strain growth, the part of precursive signal can be distinguished and a progressiveamplitude character is clearly displayed. This precursive signal proceeds for as long as 270 impeller revolutions and gradually evolves into the full amplitude stall. In Fig. 12, the result of frequency analysis for the precursor part and the full stall part confirms the modal character of these signals and gives the same value of frequency for both signals (fp = 6.84 Hz). However the result of the phase analysis and the blade excitation analysis has failed to give any number of stall cells, indicating the non-rotating nature of this stall. This type of stall, called standing stall, has been 4

5 also encountered in the previous investigations, e.g. in Haupt et al, 1986 and Erre. 010(1 hs F 0 E i ^ xls = 1.0 xis= Q = xls=0.e s/^jv^i.. x,5= 0.7 x/5=o J xi5=0.3 - ^ x1s=0.15 x/s= S Fig. 11 Pressure and blade strain signals in the time domain showing progressive type of stall precursor nred = rpm Pressu amplitt pmt 60,r Precursive pressure increase. Fig. 13 is a low-filtered reproduction of Fig. 7 and 9, and with enlarged pressure scale. From there, a trend of pressure increase before the full stall can be observed in the parts of Pre. A, B and C from Fig. 9. The pressure increase during the occurrence of rotating stall (mostly intermittent type and broadband frequency characteristics) has been investigated in the past ( Haupt et al, 1987a and Seidel et al, 1991) and interpreted as a symptom of reverse flow effect. Here this increase is found to be emerged also in the stall inception process, therefore it can also serve as an indication of the onset of the stall. o ` E o E oa E,,/solo a xls=0.95 o xls= 0.8 x/s=07 xls=0.55 x/s=0 c xrs=0.15 x/s= s Fig. 13 Low filtered pressure signals of Fig. 7 and 9 showing the precursive pressure increase before RS nred = rpm mbar Hz 200 Frequency Fig. 12 Pressure spectra in the frequency domain for the stall and its precursor nred = rpm It should be noted that one of the three precursors in Fig. 9, namely the precursor A, also belongs to the progressive-amplitude type. But, in comparison to Fig. 11, the evolvement to the full stall is not smooth and the stall pattern is of rotating type. Stall Table 1 Different types of stall precursors More discussions. In this paragraph, we will compare the results of stall inception process, presented for a centrifugal compressor in this paper, with that published in the literature for axial compressors. Before doing this, test results of all three types of stall precursors described above are summarized in Table 1. Generally, the behavior of progressive-amplitude precursor is very similar to the modal waves detected in the inception process of rotating stall in axial compressors by Gamier et al, 1990 and Day, But in centrifugal case, the progressive evolvement from precursor to the full stall has been observed for a non-rotating standing stall only, while for the case of rotating stall this evolvement is not smooth. The stable-amplitude precursor, which happens mostly, is somewhat peculiar for the centrifugal machine, especially the intermittent type -- the behavior as a pre-precursor. In axial machines, however, the precursors frequently detected by jype f (Hz) T m a(%) ote Pre. A stable-amplitude, intermittent RS o Pre. B stable-amplitude, intermittent RS 5 Pre. C stable-amplitude, intermittent RS Stall RS Stall RS Pre. A stable-amplitude, intermittent RS Pre. B stable-amplitude, intermittent RS 7 Pre. C stable-amplitude, intermittent RS Pre. D stable-amplitude, intermittent RS Stall RS o Pre. A progressive-amplitude, continuous RS Stall RS '" Pre. B stable-amplitude, continuous RS Stall RS Pre. C stable-amplitude, continuous RS Stall RS 11 Pre progressive-amplitude, standin stall Stall standing stall

6 Day, 1991 and1993 are the localised disturbance with cells of shortlength scale, and the duration of its existence, right in front of the full stall, is only about 4-6 rotor revolutions. In fact, if we look carefully at the very beginning of full stall waves in either Fig. 5 or 7 in present paper, the one or two small amplitude waves, judging by their duration of existence, are just the same as the localised disturbance in axial compressor case, only the length scale of cells is longer. In general, from the results reported in present paper, stall precursors with much longer duration can exist in centrifugal case. It seems that the flow condition in centrifugal compressors is less favourable for the onset of stall, and if the stall happens, the precursive cells are of longer-length scale. It takes some time for the precursors in centrifugal compressor to strengthen themselves, often intermittently, to finally establish the full stall. The physical Static wall pressure Pa.. Bar 1.50 reason is perhaps linked to the different mechanism of tip clearance flow for the centrifugal machine because of the stronger Coriolis field, and the less effect of rotor-stator mis-coupling because of the long flow path in the impeller. Some explanation for the longer existence of precursive stall in centrifugal case might be deduced from the recent result by Chen et al, 1993 concerning the transient internal pressure patterns during rotating stall in a centrifugal compressor impeller. There the interactive effect between the symptoms of reverse flow and the inlet separated flow is demonstrated and interpreted as the cause for the stall formation. The stall formation time is influenced by the time needed for the reverse flow to go through the blade channel, which is in turn dependent on the distance travelled by this flow. Therefore, the long flow path in the centrifugal impeller blade channel should be Static wall pressure P, t,, Bar S Time ms Time ins Presst amplil P Presst amplil P.t.t K ' S Frequency Id lz Fig. 14 Blade passage frequency during RS at x/s = 0.15 nred = rpm Static well pressure Bar S Frequency khz Fig. 16 Blade passage frequency during stall inception at x/s = 0.15 nred = rpm Static well pressure Bar ^Y^"^ `..-r I^11^ ^M Time ms Time ms Pressi amplil PaM 75.0 Pressi amplil Pa Frequency hllz Frequency kllz Fig. 15 Blade passage frequency during RS at x/s = 0.55 nred = rpm Fig. 17 Blade passage frequency during stall inception at x/s = 0.55 nred = rpm 6

7 one of the reasons to lead to the longer time duration of the stall inception process. Another point for the comparison is that in present paper, pressure signals in the inception process, except the range of precursors, are not quiet but pulsating. In fact, this has also been pointed out by Day et al, 1993 when the unstable behavior is compared for low and high speed axial compressors. From a closer look at Day's result for the high speed engine compressor, it seems that, similarly to the result for centrifugal machine, some pre-precursors in the range of pulsating signals can also be found, and their behavior is the same as for the localised disturbances described in front of the full stall for axial compressors. Blade passage frequency. As mentioned above, for all types of precursors, there is no clearly observed increase of blade strain signal during the stall inception process. In order to clarify this, we recall that the strain signal of blade vibration represents the result of a periodical excitation of the rotating stall wave on the blade, and the rotating stall wave, in its one period, is consisted of basically two different patterns of blade passage signals -- the normal pattern with evident blade loading, and the stalled pattern with the blade loading collapsed (Chen et al, 1993). It is the periodical alteration of these two patterns that forms the transient flow process of rotating stall and the corresponding blade excitation. Obviously, the more different are blade loadings between these two patterns, the more will be the fluctuation of the pressure wave formed by the rotating stall. Therefore it is desired to look further into these blade passage signals during the stall inception process to see how they influence the precursor characteristics. We are not going to analyse the details of the whole internal pressure patterns, but limit to the blade passage signals for only two representative locations, the one at the impeller inlet, x/s = 0.15 (Fig. 14 and 16), and the other at impeller middle with splitters, x/s = 0.55 (Fig. 15 and 17). We will compare the blade passage signals for the precursor and its corresponding full stall, so the signals in figures are taken from the first precursor (Pre. A) and the full stall (RS 1) of Fig. 5. In each of these figures, we will compare the signals for two parts within one period of stall pressure oscillation, the part S corresponds the blade passage signal of the stalled pattern, while the blade passage signal in part corresponds normal pressure loading. The discrete blade passing frequencies are always well depicted in these figures because the transducers are located in C W E o E a o,c E the casing wall within the region of the impeller, but the pressure amplitudes in the frequency domain are quite different. For the full stall, a pronounced difference between pressure amplitudes of parts and S can be observed for both locations (Fig. 14 and 15) and thus can be used for the stall detection. But for the precursor signals, pressure amplitudes of these two parts are magnitudes of the same order (Fig. 16 and 17). This result shows that in the inception process, since the precursor has not been developed to the shape of full stall, it is hardly to have any significant blade signals for warning the onset of the stall. In addition, because of the intermittent appearance and short duration of existence of stall precursors, it is also difficult to pick up a warning by the on-line frequency analysis of the precursive signals, except when the inception process is of progressive character for a rather long duration (Fig. 11) or has a prolonged precursive pressure increase as shown in Fig. 13. Inception process for surge. The inception process for surge has also been examined during the test for the speed range of rpm. Only the result for n = rpm is shown in Fig. 18, where three surge cycles are displayed with two being the deep surges. As investigated previously by Haupt et al, 1987b and Jin et al, 1992, a deep surge cycle always begins and ends with one cell rotating stall (m = +1), and in most cases, another rotating stall with different cell numbers can be found to proceed in front of the surge cycle. This has also been observed in present research. In Fig. 18, the preceding rotating stall has much less amplitude than the one cell stall at the beginning of surge, and therefore, from the viewpoint of the inception process, can also be considered as a precursor for surge, and it behaves also similarly as described above for rotating stall precursors -- a continuous type, with stable- or progressive-amplitudes, and with precursive pressure increase. Another observation is that, for lower speed lines of rpm, there are several parts of intermittent rotating stall proceeded before surge, and these parts form a wave of very low frequency (about 1.25 Hz), which is on the same order as the frequency of the subsequent surge cycles (about 0.83 Hz). This wave persists for quite a long time (only four periods are included in Fig. 18) and therefore can be considered as another precursor for surge. It should be noted that, unlike the rotating stall precursors, in the case of surge, the amplitudes of blade strain signals in the precursive parts show a dramatic intermittent increase as indicated surge ^ surge f surge try d o x/s=1,0 X/5=0,9 X/sr0,8 x/s=0,7 n x/s=0.55 a x/s=q4 xis=0.3 x/s=0,15 x/s-0,05 0.5s_ Fig. 18 0,8 s 4 imw^pa^ fin ram tt1. 1-"u^.'AMtA'W{i hro".-+ Y:^1'i;^«.^.y 0.8 s f 0.8 s^ 1,2 s 1,2 s ^ Pressure and blade strain signals in the time domain during surge showing RS in front of the surge cycle and low frequency precursive wave n red = rpm 7

8 It should be noted that, unlike the rotating stall precursors, in the case of surge, the amplitudes of blade strain signals in the precursive parts show a dramatic intermittent increase as indicated in Fig. 18. Detailed investigation of the stall characteristics for these precursive signals is not described in present paper but the phenomenon is the same as in Seidel et al, 1991 concerning the stall with large cell numbers and broadband pressure fluctuation. COCLUSIOS Experimental investigation on the stall inception process in a centrifugal compressor has revealed three types of precursive pressure signals: the stable-amplitude precursor, the progressiveamplitude precursor and the precursive pressure increase. Their behavior can be summarized as follows: 1. For the stable-amplitude precursor, the most frequent type of stall precursors, the frequency of pressure signal is generally different from that of the full stall. But in the case of progressiveamplitude type, the frequency for the precursor and the full stall is the same. 2. Except for one case of standing stall, where the evolvement from the progressive-amplitude precursive disturbance to the fully stall is smooth, sudden transition for the amplitude of pressure and blade strain signals from precursor to the full stall is always observed independently of the precursor types. 3. Except for one case of standing stall, the precursive signals of all types propagate as rotating stall, and the stall cells for all precursors are of longer-length scale, the same order as for the full stall cells. 4. The duration of existence for the stable-amplitude precursors is ranged for impeller revolutions, while the progressive-amplitude precursor can last as long as 270 impeller revolutions. 5. The stable-amplitude precursor could emerge in an intermittent way for several times before the full stall, thus acting as a pre-precursor. At the speed line of rpm, depending on the deepness of the compressor throttling, different types of stall precursors can be observed -- progressive-amplitude precursor, or stable-amplitude intermittent and then continuous precursor accompanied with the precursive pressure increase. 6. It is difficult to have any significant warning on the stall onset from the frequency analysis for stable-amplitude precursive signals with sudden stall onset, nor from the information of precursive signals such as blade passage and blade strain. The more favourable case for the stall warning is for the progressiveamplitude precursor with smooth evolvement to the full stall and the case of precursive pressure increase. In addition, the preceding rotating stall before surge cycle can be considered also as a precursor for surge. The same is for the low frequency wave observed at lower speed range and consisted of intermittent rotating stall signals. The frequency of this wave is on the same order as of the appearance of surge cycles. ACKOWLEDGEMET The joint research in this paper is funded by the German Research Association (DFG) and the ational Science Foundation of China (SFC). These supports are gratefully acknowledged. The authors would like to thank Dr. Haupt, U., Dr. Jin, D. and Mr. Seidel, U. for their cooperation and valuable comments. The contribution of Mr. Tanneberg, P. in running the tests, Mr. Wichmann in the measurement and Mr. Ohm, A. in data processing is also gratefully acknowledged. REFERECES Chen, J., Hasemann, H., Seidel, U., Jin, D., Huang, X. and Rautenberg, M., 1993, "The Interpretation of Internal Pressure Patterns of Rotating Stall in Centrifugal Compressor Impellers", ASME 93-GT-192 Day, I.J., 1991, "Stall Inception in Axial Flow Compressors", ASME 91-GT-86 Day, I.J., 1993, "The Unsteady Behavior of Low and High Speed Compressors", ASME 93-GT-26 Epstein, A.H., Ffowcs Williams, J.E. and Greitzer, E.M., 1986, "Active Suppression of Aerodynamic Instabilities in Turbomachines", AIAA , also J. Propulsion and Power, v. 5, o. 2, pp Gamier, V.H., Epstein, A.H. and Greitzer, E.M., 1990, "Rotating Waves as a Stall Inception Indication in Axial Compressors", ASME 90-GT-156 Haupt, U., Abdel-Hamid, A.., Kaemmer,. and Rautenberg, M., 1986, "Excitation of Blade Vibration by Flow Instability in Centrifugal Compressors", ASME 86-GT-283 Haupt, U., Rautenberg, M. and Abdel-Hamid; A.., 1987a, "Blade Excitation by Broadband Fluctuations in a Centrifugal Compressor", ASME 87-GT-17, also ASME J. of Turbomachinery, v.110, o.1, pp , 1988 Haupt, U., Jin, D., Seidel, U. and Rautenberg, M., 1987b, "On the Mechanism of Blade Excitation due to Surge on Centrifugal Compressors", JSME 87-Tokyo-IGTC, v. 2, Tokyo, Japan Haupt, U., Seidel, U., Abdel-Hamid, A.. and Rautenberg, M., 1988, "Unsteady Flow in a Centrifugal Compressor with Different Types of Vaned Diffusers", ASME 88-GT-22, also ASME J. of Turbomachinery, v. 110, o. 3, pp , 1988 Hoenen, H. and Gallus, H.E., 1993, "Monitoring of Aerodynamic Load and Detection of Stall in Multistage Axial Compressors", ASME 93-GT-20 Inoue, M., Motoo, K., Takahito, I. and Youichi, A., 1990, "Detection of a Rotating Stall Precursor in Isolated Axial Flow Compressor Rotors", ASME 90-GT-157 Jackson, A.D., 1986, "Stall Cell Development in an Axial Compressor", ASME 86-GT-249, also ASME J. of Turbomachinery, v. 109, Oct., pp Jin, D., Haupt, U., Hasemann, H. and Rautenberg, M., 1992, "Excitation of Blade Vibration due to Surge of Centrifugal Compressors", ASME 92-GT-149 McDougall,.M., Cumpsty,.A. and Hynes, T.P., 1989, "Stall Inception in Axial Compressors", ASME 89-GT-63, also ASME J. of Turbomachinery, v. 112, Jan., pp Seidel, U., Chen, J., Haupt, U., Hasemann, H., Jin, D. and Rautenberg, M., 1991, "Rotating Stall Flow and Dangerous Blade Excitation of Centrifugal Compressor Impeller", Part 1: Phenomenon of Large-number Stall Cells, ASME 91 -GT