THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS Three Perk Avenue, New York, N.Y

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS Three Perk Avenue, New York, N.Y GT-415 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or ot Its Divisions or Sections, or printed In Its publications. Ditcueslon is printed only If the paper is published In an ASME Journal. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center (CCC) provided 53/article is paid to CCC, Rosewood Dr., Danvers, MA 193. Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright 1899 by ASME All Rights Reserved PitMed In U.SA. INSTABILITY PATHOLOGY OF A HIGH SPEED CENTRIFUGAL COMPRESSOR William C. Oakes, Patrick B. Lawless and Sanford Fleeter School of Mechanical Engineering Purdue University West Lafayette, Indiana ABSTRACT An experimental investigation is performed to characterize a high-speed centrifugal compressor as it approaches instability and during subsequent surge cycles. To achieve this, data are analyzed from the inlet, the diffuser and the exit of the compressor. Analysis of the data indicates the presence of two rotating stall modes prior to and during surge cycles. A nine cell mode pattern is shown to erupt prior to the initial surge cycle. The second rotating stall mode is a single cell mode that coincides with the initiation of the surge cycles. Both stall modes are shown to be located at or near the diffuser. NOMENCLATURE Discrete Fourier Transform f. Frequency ke Spatial mode number Sampling index Arbitrary integer Number of transducers in spatial array Number of samples n3 Spatial mode number Spacing between integration points Quasi-steady pressure Dynamic pressure transducer response Subscripts Inlet total Impeller exit static Discharge Plenum Dynamic pressure transducer number Sampling index INTRODUCTION Certain problems that plagued the early development of gas turbine engines continue to be of concern in even the most advanced engine designs. Compressor stall and surge is one such problem, with a goal of current research efforts being to extend the stable operating range. This requires knowledge of the instability itself, with an understanding of the phenomena preceding the stall or surge critical to the eventual control of these instabilities. Thus, the acquisition and analysis of fundamental data in compressors describing both the instability and the instability initiation process is required. To date, much of the effort directed at understanding the instability initiation process has been focused on axial compressors. In most axial compression systems the onset of rotating stall plays and important role in the initiation. of a compressor surge event. Thus, there has been an intense focus on rotating stall behavior in these systems. Detailed studies of stall inception have been performed by Gamier et al., [1], Day [] and Trifonidis et al., [3], McDougall et al., [4] and Camp and Day [5]. The picture which has evolved as these studies progressed portrays two different stall inception pathologies in axial compressors. In the first, rotating stall develop either long length scale (modal) disturbances which are characteristic of fluid system resonances. In the second, a short length scale (pip) disturbance forms when the diffusion capability of individual airfoils is exceeded. The significance in the distinction between these two paths is in the effectiveness of methods used to detect, and possibly suppress, the stall condition. Unfortunately, there has been much less work investigating instability initiation in high-speed centrifugal compressors typical of those employed in aeropropulsion applications. The generally accepted trend is for the impeller to exhibit rotating stall on lower speed lines and, on higher speed lines, for surge to occur. On high speed lines, the diffuser is the component most often associated with the performance fall off triggering this surge event. However, little detailed data exists as to the nature of the disturbances which occur in the compressor prior to the first surge cycle. Of specific interest is the flow breakdown in the diffuser region, and the mechanism that is responsible for the establishment of any periodicity, i.e. rotating stall, in the diffuser flow. If they exist, such disturbances hold potential to Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Indianapolis, Indiana June 7-June 1, 1999

2 serve as potential surge warning indicators as well as to provide insight into possible strategies for surge control. Work on centrifugal compressor stability includes that of Hunziker and Gyarmathy who investigated the dependency of the stability of the subcomponents to the overall stability [6]. Ffowcs Williams and Graham [7] demonstrated active control for developed surge in a high-speed centrifugal compressor. Ffowcs Williams, Harper and Aftwright [8] found nonaxisymmetric disturbances at the initiation of surge but did not investigate the nature of the disturbance occurring prior to surge. Oakes et al. [9] identified a spatially coherent disturbance occurring prior to surge when a centrifugal compressor is slowly throttled into surge. This paper is concerned with the characterization of the disturbances occurring at or before surge initiation in highspeed centrifugal compressors. Specifically, the goal is to determine if any periodic events are evident during the approach to surge and if any of these event are compatible with the existence of rotating stall in the impeller or some form of propagating disturbance in the diffuser. This is accomplished through experiments directed at characterizing the surge initiation process of a high-speed centrifugal compressor typical of those employed in aeropropulsion applications. To this end, high response dynamic pressure transducers are installed in the Purdue High Speed Centrifugal Research Compressor inlet, diffuser and exit plenum. Data from these transducers are acquired as the compressor is slowly throttled into surge along a constant speed line. Both time-frequency spectral and transient performance measurements are employed to characterize the instability signature. FACILITY The Purdue High Speed Centrifugal Research Compressor Facility is shown in Figure 1. The drive line consists of an Allison 5-C3G turboshaft engine (C3) producing 55 shaft horsepower (415 kw). The test article is driven through a slave gearbox. Flow enters the test compressor inlet after passing through a series of screens and honeycombs to reduce the compressor inlet total pressure, Figure. This both decreases the power required from the driving engine and also reduces the mechanical strain during the surge and stall experiments. After passing the screens, it enters a plenum where additional screens and honeycomb form a modified Sprenkle to reduce flow disturbances. At the end of the plenum, a contraction brings the air to the impeller inlet. Inlet surveys have shown that the impeller inlet flow is uniform. After the air passes through the impeller and the vaned diffuser, it discharges into a collection plenum and then exhausts to the discharge pipe. The compressor is throttled by a butterfly valve driven by a gear motor in the discharge pipe. Downstream of the valve and flow straighteners is a vortex flow meter to measure the compressor flow rate. The research compressor was designed by Rolls Royce Allison and consists of a titanium impeller and a radial vaned diffuser. The centrifugal impeller has a nominal operating speed of 48,45 rpm, a maximum pressure ratio of 5.4, and a maximum mass flow rate of 5.5 Ibm/sec (.5 kg/sec). Thirtysix inducer bleed slots are spaced circumferentially around the compressor shroud.5 in. (1.3 cm) from the impeller leading edge. The performance of the research compressor is presented in Figure 3. Instrumentation Steady state instrumentation is utilized to determine the performance characteristics of the compressor, including the definition of the surge line. The inlet total pressure and temperature are measured in front of the inlet contraction. A series of three, four-element total pressure rakes are used to verify that the single probe accurately represents the inlet conditions. Four three-element total pressure probes measure the exit pressure, with static pressure taps mounted in the endwall of the diffuser corresponding to each probe location. The stage pressure ratio is calculated by mass averaging the exit rake data. All steady pressure probes are connected to three 5 psid Scanivalve Pressure Scanners. An HP3497A data acquisition unit controlled by an Apple Macintosh FX computer is used to sample the output of the transducers. The exit temperature is measured by three, type K thermocouples distributed circumferentially at the diffuser exit plane. The HP3497A unit is used to sample the thermocouple signals. An Endress and Hauser DSC Vortex Flowmeter is used to measure the volumetric flowrate. Static pressure and temperature measurements downstream of the flowineter are used to calculate the mass flow rate of the compressor. To allow characterization of transient performance, a range of dynamic transducers are employed, as shown in Figure 3. Exit static pressure is measured with a single Kulite XTE-I9 dynamic pressure transducer mounted in the discharge plenum wall opposite of the exhaust pipe connection. An Omega PX1 dynamic pressure transducer is mounted in the endwall of the inlet plenum to measure the transient inlet total pressure. The instantaneous mass flow is measured using a TSI Inc. hot-film sensor, model 11-, mounted in the impeller inlet section and calibrated against the vortex flowmeter. To provide for characterization of spatially coherent events such as rotating stall, an array of eight equally spaced PCB13A miniature microphones is mounted in the endwall of the impeller inlet, 3.5 in. (8.6 cm) upstream of the compressor face. As only low-frequency disturbances were of interest in this study, blade and splitter pass frequencies are

3 eliminated from the microphone signals via a third order Butterworth filter with a cut off frequency of,4 Hz. The microphones are dynamically calibrated to account for nonlinearities of these filters. As the compressor inlet is subatmospheric, the steady pressure across the microphones is equalized to prevent saturation. This is achieved by mounting the devices in a chamber vented to the compressor inlet plenum. Additional pressure transducers are installed between the impeller inlet and the array of PCB dynamic pressure transducers located.58 inlet diameters upstream of the impeller. The location and transducer designation is listed in Table 1. Table 1. Inlet Transducer Locations Inlet Transducer Designation Distance Upstream of the Impeller (Inlet ' Diameters) A second array of eleven high temperature Kulite LE-15-5A dynamic pressure transducers is mounted in the outer endwall of the diffuser section. Each transducer is mounted in the center of the diffuser passage and 5%downstream of the leading edge of the diffuser vanes. These transducers are evenly spaced around the circumference in every other vane passage. The same analog filters described above are employed to eliminate blade pass frequency. DATA ANALYSIS The data analysis is performed in four parts. The first is the joint time-frequency analysis of the signal from a single pressure transducer to identify frequency bands where instability is encountered. A spatial Fourier analysis of the simultaneously sampled microphone arrays is then made to identify spatially coherent events during the instability initiation process. The pressure characteristics of the individual components are then examined to determine the contribution of each component to the overall system performance characteristic. selected with the width of the window sufficient to capture several periods of the lowest frequency event of interest. A discrete Fourier transform is then performed on this data, and the process is repeated by advancing the window in time until the entire data time record has been analyzed. The resulting data are then represented by a three dimensional surface with axes of time, frequency and magnitude. This data is then used to identify frequency bands where potential spatially coherent disturbances may exist. Spatial Mode Resolution Fourier analysis of signals from an array of circumferentially distributed simultaneously sampled transducers has proven effective in identifying spatially coherent phenomena such as rotating stall [3,11,1]. In such a technique, a Fourier transform is performed in the spatial domain on a simultaneous signal from each transducer in an array with the resultant harmonics corresponding to the mode (n) of the spatial pattern, i.e. the number of wavelengths per machine circumference. In practice, the signals must typically be pre- filtered with a bandpass filter around the frequency of interest to achieve an acceptable signal-to-noise ratio. Because of the limited number of circumferentially distributed tranducers that can be practically employed in such a technique, there exists a possibility of signal aliasing. Using filtering techniques as discussed above can reduce the effects of aliasing. Also, aliased modes can be identified using two arrays with different numbers of transducers, as each signal will be aliased into a different mode. Temporal-Spatial Transform The spatial transform of simultaneously sampled signals described above is useful in identifying spatially coherent events. However, the lack of temporal information precludes the identification of the direction of the wave. If the phenomenon of interest can be expected to exist for a sufficient length of time during the transient event, a temporal-spatial transform can be employed. Such a technique has been successfully used in turbomachinery acoustic analyses where steady state disturbances are examined [13]. First, the technique employs a temporal transfer on a short sample of recorded data for each transducer in an array. For the temporal Fourier transform, the pressure from the j th transducer pi(t) is discretely sampled N s, times at a rate of 1/At. The temporal discrete Fourier transform is given by Joint Time Freoueney Analysis A joint-time frequency analysis is performed on the recorded response of a single dynamic pressure transducer as the compressor is throttled into instability to identify discrete frequency events. First, a fraction of the data, or window, is N a, -1 imml N g, Fj(fm)= m P jje n At 1= (1) 3

4 where Fi(f.) is the discrete Fourier transform of the i th transducer found at frequency f, = AT, 1 is the sampling index, and the arbitrary integer m=,1,,..., Npt. A second transform of the complex output of the temporal Fourier transform can determine the direction of propagation of the modes. The spatial Fourier transform uniquely determines modes of speed greater and less than zero, representing propagation with and against impeller rotation respectively. The transform is given by N-1 F(fnz,k)=71\ - Ifj(f m )e 1= i76c-"n () where F(fin,Ice) is the dual temporal-spatial transform of the rotating stall pressure perturbation. N is the number of transducers in the circumferential array and the mode number Ice may have integer values from / to N A, /. Combining Equations I and yields the joint temporalspatial Fourier transform *tit il Fjje L at e (3) The rotating stall pressure signature is thereby determined as a function of frequency and spatial mode. For the inlet array of eight microphones employed in these experiments, therefore. the,1,,3 and 4 modes can be identified as well as the -1, -, and -3. Because the 4th mode is the Nyquist mode, the direction can not be determined. Similarly, for the diffuser array of II transducers, modes,1,,3,4 and 5 as well as I, -, -3, -4 and 5 can be identified. Analysis with Non-Uniform Sensor Spacing When conducting tests of high-speed turbomachinery, it is common to lose transducers during the experiment due to the severe environment. Indeed, in these experiments, two transducers in the diffuser section became inoperable during the experiments. However, spatial mode analysis can be performed with transducers missing with only minor reductions in mode resolutions. The discrete Fourier transform of the form N 1 1 / N Fjuce). pie i= (4) is a numerical integration of the Fourier integral. More formally, the trapezoidal approximation to the Fourier integral can be written as 1 N r. injk, I N L nipie 1= where ; accounts for the spacing between integration points. For the spatial mode magnitude analysis, these correspond to the transducer spacing shown in Figure 4 (a). For the case of an evenly spaced transducer array, n may be normalized and becomes unity for all j's. However, if transducers are removed from the array, for example if the 9 th and 11 th of an II transducer array are removed, the values of ; become nj= 1.5,1,1,1,1,1,1,1.5, (6) respectively for the remaining transducers. The resulting transform will result in a change in magnitude of the fundamental mode, with energy redistributed into the higher the modes at the fundamental frequency of the disturbance. Numerical experiments performed for the transducer counts employed in this study demonstrated the ability to resolve a dominant fundamental mode. It should be noted that because the transform remains periodic with e iitjk, IN (7) the Nyquist criteria remains unchanged and the behavior of aliased modes is the same with a complete or partial transducer array. EXPERIMENTAL TECHNIQUE The stalling behavior of the compressor is characterized with the inlet dynamic pressure transducer array, the diffuser dynamic pressure transducer array and the plenum dynamic pressure transducer. After the compressor achieves a steady operating point with the throttle valve open, the valve is closed at a constant rate. This moves the compressor towards higher pressure ratios until instability is encountered. The tests were performed with impeller speed held constant at the 9% speedline. Data acquisition is initiated at the beginning of the throttle movement and continues until the trigger level of a selected inlet dynamic pressure transducer is reached indicating the compressor has surged. The dynamic transducer signals are sampled at a rate of 8, Hz using three National Instruments EISA-A analog-to-digital (5) 4

5 conversion boards installed in a Micron P133 computer, which allows twelve channels of data to be recorded simultaneously. A total of two seconds of data spanning the post and pre-trigger regions is saved for analysis. The quasi-steady performance of the compressor is characterized by measuring the impeller inlet total (P a), impeller exit static (P I ), and plenum (P ) pressures as well as the mass flow. The mass flow is measured using a hot film sensor mounted in the impeller inlet section and calibrated against the vortex flowmeter. These data are recorded using the same data acquisition system as above but with a sampling frequency of 4, Hz. The performance data are recorded for separate throttling events from the spatial mode identification. However, inlet and diffuser transducer signals were recorded to verify similar behavior during the different runs. An averaging filter with a ms window is then applied to these data. RESULTS Joint Time-Frequency Analysis The results of a joint time-frequency analysis of the plenum dynamic pressure signal during throttle-down is shown in Figure 4. For these data, the window for the Fourier transform spanned 18 impeller revolutions. A discrete frequency event is observed near 1,45 Hz (Point A) and continues until a much stronger, broadband event is encountered at Point B which corresponds to the start of a surge cycle. Additional analysis of the data near the event at Point A determine8 the frequency to be 1, Hz. At the inlet, an analysis of a microphone signal also exhibits activity at 1,48 Hz as shown in Figure 5 at Point A. The start of the first surge cycle is clearly shown at Point B, as well as a higher frequency disturbance at 5 Hz indicated by Point C. Similar data in the diffuser passage are presented in Figure 6, The disturbance observed in the plenum is present in the diffuser, Point A, along with a lower frequency disturbance centered at 5 Hz, Point B. This lower frequency disturbance appears for only about 5 impeller revolutions near the beginning of the first surge cycle shown by Point C. Thus, at all three locations a persistent event was indicated at 148 Hz, with additional discrete frequency events located in the inlet at 5 Hz and in the diffuser passage at 5 Hz. Disturbance Location To determine the location of the disturbances within the research compressor, a comparison is made of the magnitude of each of the frequency spectra at the plenum, diffuser, and the three axially spaced inlet locations. The results for the disturbance at 148 Hz are presented in Figure 7, with the maximum signal strength located in the diffuser. The signal of the next largest magnitude occurs in the endwall of the plenum. The severely attenuated signal at the impeller inlet is indicative of a disturbance located in the difftiser: The signal continues to attenuate upstream of the impeller at a near linear rate. A similar comparison of the joint time-frequency analyses for the 5 Hz disturbance shows the diffuser to again be the location of maximum amplitude, Figure 8. The decay rate is much more severe upstream of the impeller prior to the first surge cycle than for the disturbance at 148 Hz. The signal of lowest magnitude is at the plenum endwall. Analysis of the disturbance at 5 Hz in the inlet was not detected further downstream in either the diffuser or plenum. Spatial Mode Identification: 5 Hz Disturbance To analyze the spatial coherence of the disturbances noted above, spatial Fourier analyses are performed on the signals from the inlet and diffuser transducer arrays. The signals are first numerically filtered around the frequencies identified in the above described joint time-frequency analyses. The signals from the inlet dynamic pressure transducer array are numerically filtered in a 8 Hz band around the 5 Hz disturbance frequency and the resulting spatial mode magnitudes presented in Figure 9. At times greater than t = 3 revolutions, the first mode magnitude rises briefly. The mode increases again at t = 38 revolutions and again decreases. These locations correspond to the low pressure regions of two mild surge-like oscillation cycles prior to the first large surge cycle. At t = 43 revolutions, which corresponds to the beginning of the first deep surge cycle, the mode increases a third time. At the initial appearance of the first mode, the higher order modes are not elevated. At t = 38 at the second mild surge cycle, the second mode is elevated as well as the third and fourth modes but to a lesser extent. The higher modes are elevated during the first deep surge cycle as well. Further information is obtained from the diffuser transducer array. Since the total number of transducers in both arrays precluded simultaneous data acquisition, a separate data set is examined for the diffuser array. The results of the spatial Fourier analysis of the diffuser array on the disturbance centered at 5 Hz are presented in Figure 1. The response is similar to that of the inlet array. Note, however, that in this particular case a single mild surge cycle occurs prior to the initial deep surge cycle rather than the two mild surge cycle shown by the previous data set. The first mode magnitude rises at t = 35 revolutions and remains elevated until t = 375. The higher modes have a similar response. The higher mode activity is attributable to only having 9 of the 11 transducers 5

6 operational, with transducers #9 and #11 failing early in the experimental program. As discussed earlier, such a transducer array can characterize the spatial modes but there is an artificial increase in the magnitude from spatial harmonics. The first mode clearly rises to the highest magnitude indicating a first spatial mode. A rise in the first mode magnitude is also observed at the beginning of the first deep surge cycle, although at a lower magnitude than during the mild surge. Figure 11 illustrates the mode magnitude behavior during the initial surge cycles. There is an increase in magnitude at t = 385 revolutions followed by a reduction and then another increase in magnitude that corresponds to the minimum pressure location in the surge cycle. A similar behavior is also seen in the second deep surge cycle with an increase at t = 55 revolutions Spatial Mode Identification: 148 Hz Disturbance The signals from the inlet dynamic pressure transducer array are filtered about 148 Hz which corresponds to the second discrete frequency disturbance identified by the joint time-frequency analyses. The results from a spatial Fourier analysis are shown in Figure 1. At t = 5 revolutions, the first mode magnitude increases slightly and remains at an elevated level until t = 9 revolutions when the first deep surge cycle begins. The higher order modes show increased magnitudes in this region, but to a much lower level. The exception to this is during the single mild surge cycle at t = 85 revolutions where the third and fourth modes show an increase in magnitude. The diffuser dynamic pressure transducer signals are also filtered about 1,48 Hz and the Fourier modes determined from these signals are shown in Figure 13. The second mode magnitude shows a significant increase in amplitude at t = 375 revolutions and remains elevated until the first deep surge cycle at t = 95 revolutions. While there is an increase in amplitude of the higher order modes, the amplitude is much smaller than the second mode. While the spatial domain analysis of the two arrays are performed on separate experiment runs, the frequency response determined by the joint time-frequency analyses exhibits similar behavior in both. Thus, the difference in spatial mode magnitudes between the diffuser and inlet data indicates that a higher order mode is being aliased into the first and second modes for the two spatial transducer arrays. This is also suspected when one considers the impeller rotational frequency is 76 Hz. The frequency of the disturbance (148 Hz) would be a quite unusual for a singlecell stall condition, as it would be rotating at greater than twice the impeller speed. Thus, the information obtained from the spatial Fourier transform must actually be a spatially aliased disturbance of higher cell count. For the inlet array of eight dynamic pressure transducers, waveforms of modes 7, 9, 15, 17, etc. will fold into a first mode. Similarly, for the diffuser transducer array, waveforms of modes 9,13,, etc will fold into the second mode. Thus, the 9 mode is the only common mode that appears as a 1 mode for the array of eight transducers and a for the array of 11 transducers. Mode Direction A temporal-spatial transform is now employed to determine the direction of the modes. Figure 14 shows the results of this analysis using the 8 transducer inlet array. Clearly, the +I mode is the dominant mode. Aliased modes can be (the number of dynamic pressure transducers in the array). The possible modes are then, -15, -7, +1, +9, +17, etc. As the 9 mode has been identified earlier, it must therefore be propagating in the positive direction, which is defined as the direction of impeller rotation. A smiliar analysis of the 1 mode confirms that it also propagates in the direction of impeller revolution. Spatial Mode Comparison Figure 15 shows the spatial mode magnitudes for the 1 mode at 5 Hz and the 9 mode at 1,48 Hz along with the plenum pressure. The 9 mode magnitude increases at 75 revolutions and remains elevated until the first surge cycle. The one mode increases at t = 8 impeller revolutions corresponding to the first mild surge cycle. Figure 16 shows the window of time around the initial surge cycle. It can be seen that the appearance of the 1 mode has minimal effect on the 9 mode, with both existing simultaneously. A similar phenomena occurs at t = 85 revolutions which corresponds to the first deep surge cycle. As the surge cycle progresses, the 1 mode magnitude continues to increase until the pressure rise begins to recover. During the recovery portion of the surge cycle, there is minimal amplitude of either the 1 or 9 mode. Both modes reappear at t = 95 revolutions as the next surge cycle begins. Pressure Characteristic and Spatial Mode Comparison The stable compressor characteristic is compared with the magnitude of the disturbances at the frequencies identified by the spatial mode analysis. Figure 17 shows the impeller and overall compressor pressure rise for the compressor along with the +1 and +9 mode magnitudes identified by the Fourier analyses around 5 and 1,48 Hz, respectively. The 9 mode increases as the compressor pressure rise increases. This mode continues to increase until the mild surge is initiated at a flow coefficient of.555. The initiation of the mild surge corresponds to an increase in the 1 mode magnitude and the decay of the 9 mode. The data are averaged over a window of. sec. Because of the non-linear fluctuations of flow coefficient with respect to time, the data are not uniformly 6

7 distributed prior to and during surge. The time scale is shown at a flow coefficient.576 which is representative prior to mild surge initiation at.555. During the mild surge event, the data are more widely spaced, as indicated along the overall pressure characteristic. The impeller performance exhibits similar behavior, with the pressure rise continuing into the first mild surge. The impeller performance is not significantly affected by the +9 mode stall appearance. The slope of the characteristic also remains negative indicating the impeller to be a stabilizing component in the stage. The diffuser, however, shows a distinct reduction in pressure rise at a flow coefficient of.588, which corresponds to the increase in magnitude of the 9 mode, Figure 18. The +9 cell stall pattern appears to be functioning as a blockage, reducing diffuser performance and thereby decreasing the stability of the stage. The diffuser pressure rise remains essentially constant until.566 where the pressure rise begins to decrease. The magnitude of the 9 mode shows a further increase until the mild surge is initiated at.555. At this point, the 1 mode shows a marked increase in magnitude and the diffuser performance is diminished further. Similar to Figure 17, the data are not evenly distributed, with the data in the surge cycle being more widely spaced. SUMMARY & CONCLUSIONS Instability initiation was investigated in the Purdue High Speed Centrifugal Compressor using two circumferentially distributed arrays of dynamic pressure transducers. One array of 8 transducers was located approximately one-half an inlet diameter upstream of the impeller with the other array of 11 transducers located in the endwall of the vaned diffuser. Spatial Fourier analyses of the signals from these arrays were used to identify spatially coherent disturbances during constant speed loading of the research compressor. The results showed the presence of two distinct rotating stall patterns prior to and during surge. A 9-cell stall stall appears as a discrete frequency disturbance at 148 Hz several hundred impeller revolutions prior to the initial surge cycle. The maximum signal strength is in the diffuser section and is attenuated at the inlet and in the exit plenum indicating that the source of the signal is at or near the diffuser. The attenuation of the signal is not as severe in the exit plenum as it is at the impeller inlet. Examination of the quasi-steady pressure characteristics shows that appearance of the disturbance corresponds to a loss of diffuser performance indicating it is functioning as a blockage in the diffuser. The performance of the impeller, however, continues to rise. A discrete frequency disturbance at 5 Hz occuring at the onset of mild and deep surge was identified to be a single cell stall. The maximum signal strength is measured in the diffuser section indicating that region as the source of the signal. The mode is severely attenuated at the inlet and at the exit plenum. The appearance of the +1 mode corresponds to the zero slope point along the overall pressure characteristic. A further reduction in the diffuser performance corresponds to the appearance of this mode. The two rotating stall modes exist both independently and in combination with each other. Rotating stall appears at the initiation of each surge cycle similar to the precursors observed in axial compressors. During the pressure recovery portion of the surge cycle, the compressor is free of the rotating stall modes. The existence of the 9 mode rotating stall for several hundred revolutions prior to surge initiation may provide an activation parameter for future high speed control schemes. ACKNOWLEDGEMENTS This research sponsored in part by the Army Research Office (ARO). This support is most gratefully acknowledged. The support of Rolls Royce-Allison is also gratefully acknowledged. REFERENCES 1. Gamier, V.H., Epstein, A.H. and Greitzer, E.M., 199, "Rotating Waves as a Stall Inception Indication in Axial Compressors," Journal of Turbomachinety, Vol. 113, No., April, pp Day, I.J., 1991, "Stall Inception in Axial Flow Compressors," Journal of Turbomachinety, Vol. 115, No. 1, January, pp McDougall, N.M., Cumpsty, N.A. and Hynes, T.P., 199, "Stall Inception in Axial Compressors," Journal of Turbomachinery, Vol. 11, No. 1, January, pp Tryfonidis, M., Etchevers,., Paduano, J.D., Epstein, A.H. and Hendricks, G.J., 1995, "Pre-Stall Behavior of Several High-Speed Compressors," ASME Journal of Turbomachinety, Vol. 117, No. 1, pp Camp, T.R. and Day, I. J., 1998 "A Study of Spike and Modal Stall Phenomena in a Low-Speed Axial Compressor", Journal of Turbomachinely-Transactions of the ASME. vol. 1 no. 3, July 1998, pp Hunziker, R. and Gyannathy, G., 1993, "The Operational Stability of a Centrifugal Compressor and Its Dependence on the Characteristics of the Subcomponents", ASME Paper 93-GT Ffowcs Williams, J.E. and Graham, W. R., 199 "An Engine Demonstration of Active Surge Control," ASME Paper 9-GT Ffowcs Williams, J.E., Harper, M.F.L and Allwright, Di., 1993 "Active Stabilization of Compressor Instability and 7

8 Surge in a Working Engine," Journal of Turbomachinery- Transactions of the ASME. vol. 115 no. 1, Jan 1993, pp Oakes, W., Lawless, P. B., Fleeter, S., 1996, "High Speed Centrifugal Compressor Surge Initiation Characterization," A/AA Paper Frigne, P. and Van Den Braembussche, R., "Distinction Between Different Types of Impeller and Diffuser Rotating Stall in a Centrifugal Compressor With Vaneless Diffuser," ASME Journal of Engineering for Gas Turbines and Power, Vol. 16, April 1984, pp Lawless, P.B. and Fleeter, S., 1993, "Rotating Stall Acoustic Signature in a Low Speed Centrifugal Compressor: Part 1 - Vaneless Diffuser," Journal of Turbomachinery -Transactions of the ASME. vol. 117 no. 1, Jan 1995, pp Lawless, P.B. and Fleeter, S., 1993, "Rotating Stall Acoustic Signature in a Low Speed Centrifugal Compressor: Part - Vaned Diffuser," ASME Paper 93 - GT Sawyer, S., Feiereisen, J.M., and Fleeter, S., 1996, "The Influence of Rotor Detuning On The Acoustic Response Of An Annular Cascade," AIAA Paper Plenum Discharge Diffuser Pressure Transducer Array Inlet save ransauce Impeller Exit and Diffuser Inlet Static Pressure Taps Discharge Plenum Pressure Transducer ot Fi be Inlet Pressure Transducer Vent Line totransducer Plenum Figure. High speed research compressor inlet and instrumentation Allison 5-C3 Turbashaft Engine Research Compressor Discharge Plenum High-Speed Shaft Slave Gearbox Diffuser Impairer 4.5 Research Cornpressor Shaft Pressure Ratio 3.5 Low-Speed Shaft kg/sec Figure 1. Purdue High Speed Centrifugal Compressor Research Facility Figure 3. Research compressor performance map 8

9 COD 3WD kpa D as 4.3. COO kr' s t I A V I MD ":". SOD 13 5 SW IDES line cirri:wrier Revelations) SW IDES 11M Opener Revolutions) Figure 4. Joint time-frequency analysis of plenum transducer signal Figure 6. Joint time-frequency for diffuser endwall pressure transducer 4WD MOD MOD!zoo g ZOO rc 15 IMO 5:117 SW IDES, N.-NW TITO (Irfpcfilar Revolutions) Mag n itude (kpa) 1. Diffuser Plenum Inlet 1 Inlet Inlet 3.8 r.6 r A I 1. k I %. c, Figure 5. Joint time-frequency analysis for a single inlet pressure transducer Figure 7. Fourier transform magnitudes for the disturbance at 148 Hz at the plenum, diffuser and along the impeller inlet

10 Diffuser Inlet 1 Inlet Inlet 3 Plenum Spatial Mode Magnitude (kpa) 4-5th,Mode ) th mode - 4-3rd Mode 4. SiSI \ 4-1st Mode o Figure 8. Fourier transform magnitudes for the disturbance at 5 Hz at the plenum, diffuser and along the impeller inlet Figure 1. Spatial mode magnitudes of the diffuser transducer array filtered about 5 Hz and plenum pressure Mode Mag n itude (kpa) (edn ) amssaid innueld Spatial Mode Magnitude (kpa) _ Plenum Pressure 15 1st Mod (ed >1) amssaid tunuold Figure 9. Spatial mode magnitudes of the inlet array filtered around 5 Hz and plenum pressure Figure 11. First spatial mode magnitude of the diffuser transducer array filtered about 5 Hz and plenum pressure 1

11 O itr lie 17 9 Exit Plenum Pressure (KPa) Mode Magnitude (KPa).5 I " I liii Impeller Revolutions Figure 1. Spatial mode magnitudes of inlet transducer array filtered about 148 Hz and plenum pressure Figure 14. First mode magnitudes from temporal-spatial analysis of a single diffuser transducer 17 Spatial Mode Magnitude ( K Pa) Impeller Revolutions 9 ca. Spatial Mode Magnitude ( KPa) Plenum Pressure cf E a) EC Figure 13. Spatial mode magnitudes for the diffuser transducer array filtered about 148 Hz and plenum pressure Figure 15. Spatial mode magnitudes from the diffuser array and plenum pressure 11

12 Spatial Mode Magnitude (KPa) iniin iimil Plenum Pressure Plenum Pressu re (KPa) Mode Magnitude (kpa) Mild Surge S\ 1 Mode Magnitude Diffuser Pressure Rise OT=. Sec 9 Mode Magnitude (1 E Flow Coefficient, 4) Pressure Coefficient, w Figure 16. Spatial mode magnitudes from the diffuser array and plenum pressure Figure 18. Diffuser pressure coefficient with spatial mode magnitudes from diffuser array Made Mag nitude (kpa) O T=. Se * Impeller - - \ - - _.A..--- Pressure A \ 9 Mode - : Rise 1.5 Magnk:idke 7-1 Mode \, - Magnitude 1 7 \.5 Overall EOT.. Sec A---Pressur Rise E- \ Mild Surge Pressure Coefficient, kir Flow Coefficient, 93 Figure 17. Overall compressor and impeller pressure coefficients with spatial mode magnitudes from diffuser array 1