experimental study active control surge centrifugal ... · 384 n. chaoqunet al. other hand, in...

International Journal of Rotating Machinery2000, Vol. 6, No. 5, pp. 383-392Reprints available directly from the publisherPhotocopying permitted by license only

(C) 2000 OPA (Overseas Publishers Association) N.V.Published by license under

the Gordon and Breach SciencePublishers imprint.

Printed in Malaysia.

Experimental Study on Active Control of Surgein a Centrifugal Compression System*

NIE CHAOQUNt, CHEN JINGYI and CHEN NAIXING

Institute of Engineering Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing 100080, P.R. China

(Received 24 April 1998; In finaljbrm 10 March 1999)

An experimental study has been carried out on the active control of surge in a centrifugalcompression system. With a computerized on-line control scheme, the surge phenomenon issuppressed and the stable operating range of the system is extended. In order to design theactive control scheme and choose the desired parameters of the control system inputs, specialemphases have been placed on the development of surge inception and the nonlinearinteraction between the system and the actuator. By use of the method designed in the presentwork, the results of active control onsurge have been demonstrated for the different Bparameters, different prescribed criteria and different control frequencies.

Keywords: Compressor, Aerodynamics, Experimental study, Surge, Flow instability,Active control

INTRODUCTION

Similar to the situation ofaeronautical propulsion, itis well known that the surge phenomenon may seri-ously hinder industrial fans from operating steadilyand improving in behavior. In order to ensure thesafe operation of compression systems, for a longtime industrial fans are kept away from operatingunder high loading. Therefore, the possibility ofhigh-pressure ratio or high efficiency is sacrificed.

Since Epstein et al. (1989) first put forwarda preliminary active control plan to suppress insta-bility in turbomachinery, it appears possible that

active control can be used to maintain the operationofa compression system at high parameters withouthaving to change the original set-up ofthe compres-sion system. Subsequently, Ffowcs Williams andHuang (1989) obtained the results of active controlof surge in the centrifugal compressor by adoptingthe principle of anti-sound in acoustics. At the sametime, Gysling et al. (1991) made use of theaeromechanical feedback principle of the specialspring-mass-damper’s perturbation response to thecompression systems, and Pinsley et al. (1991) tookadvantage of the method of close-coupled throttleobtaining more satisfied control results. On the

This paper was originally presented at ISROMAC-7.Corresponding author. Tel." +86-10-62560740. Fax: +86-10-62575913.

383

384 N. CHAOQUN et al.

other hand, in recent years the successful experi-ments on surge also stimulate the rapid devel-opment of the active control and understandingabout the inception mechanism of rotating stall(Garnier et al., 1991; Day, 1993a; Hoying, 1993;Tryfonidis et al., 1995). A number of results onactive stabilization of rotating stall have beenobtained. It is obvious that the theoretical andexperimental analyses on active control of flowinstabilities are important areas of investigation inturbomachinery.

|n design of active control programs, there aretwo distinct methods. The first method consists oftransducers, DAQ devices, a computer and actua-tors. The main feature of this method is electro-mechanical feedback (Pinsley et al., 1991; Day,1993b; Paduano et al., 1993; Haynes et al., 1994). Inthe second method, the principle of aeromechanicalfeedback is applied to reach the objective of theactive control to flow instabilities. The controlmethod is strongly related with unsteady dynamicresponses in the system (Gysling et al., 1991;Gysling and Greitzer, 1995). It seems that the firstmethod is practical for surge while the second issuitable for rotating stall.The objective of this project is to suppress surge

with the technique of active control, in order toexpand the steady-state safe operating range. Also,the nature of the inception process of surge and thenonlinear interaction between the system and theactuator are investigated in detail.

the selected scales and shapes of the rig accordingly.The simulated results show that when B parameteris greater than 0.25, the surge occurs in the centri-fugal compression system (Fig. 1). For acquiringcontrol results at the different B parameters, threecases for B parameters (B=0.25, B=0.28, B--0.34) are designed. The experimental rig is com-posed of centrifugal fan, inlet duct, plenum andthrottle (Fig. 2). The operating point of the systemcan be compelled into surge by decreasing the crosssection of the throttle.

1.1o

1.00-

0.10-

0.50..-0.10

A B=0.30

FIGURE Simulated results. (The surge point of the sys-tem is simulated for designing the rig.)

EXPERIMENTAL FACILITIES ANDCONTROL SYSTEMS

Experimental Rig

As the experimental research is closely related to theactual engineering application, the special centri-fugal fan is chosen to be a testing rig. The surgephenomenon easily occurs near the surge boundaryline. According to the Moore-Greitzer model(Moore and Greitzer, 1986a,b), and the criterionfor the B parameter (Greitzer, 1976a,b; 1981), thedynamic behavior of the fan can be simulated for

FIGURE 2 The structural outline of active control. (Theexperimental facility includes the compression system, themeasuring and controlling system, and actuator.) 1. Motor,2. Centrifugal Fan, 3. Hot Wire No. 1, 4. Sensor No. 1, 5.Rectifier Grids, 6. Inlet of Fan, 7. Flow Adjustable Nets, 8.Hot Wire No. 2, 9. Sensor No. 2, 10. Sensor No. 3, 11.Sensor No. 4, 12. Amplifier, 13. Anemometer, 14. A/DBoard, 15. D/A Board, 16. Oscilloscope, 17. Servo Motor,18. Actuator, 19. Computer, 20. Servo Power, 21. Plenum,22. Throttle.

ACTIVE CONTROL OF SURGE 385

Measurement Systems

Four pressure transducers, a constant temperatureanemometer, the high-performance data acquisi-tion board, and the computer are set up for a real-time measuring and supervising system. In order todistinguish the behavior of dynamic response ininlet, plenum, radial and circumferential position ofthe centrifugal fan, the dynamic accuracies of thefour transducers are checked carefully by usinglaser-tube instruments. It is found that the damp-ing coefficient is equal to 0.0786, the oscillatingfrequency is equal to 2150 Hz, and the angle of lagphase retardation is equal to 0.015. It is shown thatthe four transducers agree with the demands ofsupervision to the dynamic signal. Because offragileness of the hot wire, it will not be used asthe probe providing an input signal while exertingthe plan of the active control.

Control System

1.60

1.20

0.40 Signalin Plenum

0.00 2.00 4.00 6.00 8.00 10.00

FIGURE 3 Compulsive surge (B=0.28). (The transient pro-cess of surge with closing throttle continuously.)

0.0-

Signal in Plenum

.00 4.00 .00 &O0 10.00

FIGURE 4 Spontaneous surge (B=0.28). (The transientprocess of surge with closing throttle intermittently.)

The instruments and equipment in the actualcontrol scheme include actuator, servo motor,actuating power supply, D/A board, the dynamicpressure transducers, the amplifiers and controlsoftware (Fig. 2). The electromechanical feedbacksystem is employed to obtain a series of differentcontrol outcomes and to investigate the nonlinearproperties between the system and the actuator.

DYNAMIC BEHAVIOR AND INCEPTIONDEVELOPMENT OF SURGE

Dynamic Behavior

In order to suppress surge, the dynamic character-istics are investigated for different B parameters.Figure 3 represents the compulsive surge process,and Figure 4 shows the spontaneous surge process. Itis shown that the developmental time ofthe former isshorter than that of the latter from flow stability toflow instability, and the strength ofthe oscillation ofthe former process is stronger than that of the latterfrom the inception to full developed surge. The

strength ofsurge in the compression system increaseswhile B parameters become large (Fig. 5).

Inception Process of Surge

To implement the plan of active control on surge,the fundamental feature of the surge inception isinvestigated qualitatively. As soon as the small fluidperturbations, which are related to surge, occur, theamplitude of the perturbation will increase gradu-ally, the stronger fluid oscillation will be incitedrapidly, and surge will occur finally. Therefore, thequestion is whether it is possible to put the controlmeasure into effect well before this classical surgeappears in the system. The better on-line method isbased on diagnosing the inception. Figure 6 showsthe frequency spectrum of the inception develop-ment in the plenum for the compulsive surge. Thewhole time-resolved dynamic process is divided intofour pieces (1, 2, 3 and 4). Each piece includes 1024discrete points. Then successive frequency analysisis used to examine the behavior of the four pressuredata sets individually. From the development of the


1.60

1.40.

1.20.

1.00.

0.80.

0.60.

0.40.

B=0.251.6o

1.401.20,

o.0o:.0.60-

0.40"

0.20

A, B=0.281.60

B=0.34

-O.lO o.oo O.lO 0.20 0.30 0.40 0.50b

o.0.0.60-

0.40"

0.20"-0.10 0.00 0.10

FIGURE 5 Surge limited cycles at three different B parameters. (The dynamic characteristic of su.facility, the strength of surge oscillation increases while B parameters become larger.)

P1.20

"2

during the same moment. When the control schemeis implemented, the signal at either the plenum orthe outlet of the centrifugal fan is chosen to be theinput signal for the closed control loop.

-50 0 50 100 150 200 250

FIGURE 6 Surge inception. (The identification of surgeinception is based on the divided frequency spectrum.)

low frequency signal in Fig. 6, the 2.71 Hz signalincreases successively and is detectable at data setnumber 2, about 4 s before surge. Therefore, thislow frequency signal could be considered as awarning before surge. The measurement of theinception process based on the divided frequencyspectrum analysis consists of the first step to designthe active control plan and transitional time fromthe flow stability to the flow instability provides theenough time needed for the data processing andanalyzing.

In addition, the characteristics of the surgeinception development are also investigated underthe three different B parameters and the differentpositions of pressure measurement in.detail. It isshown that the surge inception signal is weak atboth the inlet and the circumference of thecentrifugal fan, but it is strong and clear at boththe outlet of the centrifugal fan and the plenum

NONLINEAR INTERACTION WITHINACTUATOR AND SYSTEM

Many different forms of actuators are proposed bySimon et al. (1993). In recent years, the effect ofstabilization on flow instability has been achievedwith the actuators (Ffowcs Williams and Huang,1989; Gysling et al., 1991; Day, 1993b; Paduanoet al., 1993; Gysling and Greitzer, 1995). The usedactuators have one common feature. They canproduce the small controlled friendly disturbancescreated by the control model. The disturbancesprovide damping action with amplitude develop-ment of surge and rotating stall.

In our experiment, the self-designed actuator ofrotary valve type is driven by a servo motor with itsrotational speed manipulated by the computer inthe control loop. The actuator is to create the smallfriendly disturbances in the compression system.Figure 7 expresses the time-resolved dynamic signalof the pressure at the outlet of the centrifugal fanwhen the system is still away from the surge. Thesesmall disturbances are created by the actuatorunder the different speeds of the servo motor andsupposed to suppress the initial perturbation ofthe surge. It is obvious from Fig. 7 that the wave


from of the disturbances is not the ideal first har-monic wave form but including many complicatedharmonics. Therefore, in order to select the neededcontroller’s frequency, the nonlinear interactionbetween the actuator and the system are investi-gated. Here, the tendency of the first three har-monics is analyzed. The frequency as well as theamplitude claracteristics of the nonlinear influenceare shown in the Fig. 8 where the lower index "a"refers to the disturbance in the system created by the

FIGURE 7 Small friendly disturbances. (The actuator pro-vides the system with friendly disturbances at operating ranges.)

actuator and "c" refers to the controlling frequencyof the servo motor. From Fig. 8, both the frequencyand amplitude values of the first harmonic wave ofthe disturbances increase sequentially with para-bolic form when the controller’s frequency isincreased. The amplitude values of the firstharmonic wave is close to be identical to each otherwhen the controller’s frequency is less than 50 Hzand the minimum value occurs when the control-ler’s frequency is equal to about 20Hz. For thesecond harmonic wave, the variation tendency isnearly in agreement with that of the first harmonic.But when the controller’s frequency is greater than70 Hz, the amplitude value of the second harmonicchanges to a decreasing tendency. For the thirdharmonic, the changing tendency is similar to theharmonic sine wave and the wave disappears inthe compression system when the controller’sfrequency is greater than 50 Hz.On the whole, these nonlinear characteristics

between the actuator and the compression systemmust be taken into account in order to design theactual plan of the active control on surge. In our

0 Expr/mtpoiE

2.0-

0.0" I’"

3,0.

2,0

0

CJr Fitting

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’’’1’’’’’1 ’’’!’’18 30 60

4O

30.

20.

I0,

1’2 o Extractp

’’’1’’’1’’’i’’’’1’’0 20 80 lOO 120

d e

FIGURE 8 The characteristics of frequency and amplitude between the actuator and the system. (The controlled transforma-tion of small friendly disturbances provided by actuator induces the nonlinear alternation of the system in frequency domain.)


experiment, only the characteristic of the firstharmonic wave is utilized to suppress the surge,but the nonlinear behaviors of the second and thethird harmonics are also useful in understandingthe nonlinear effect of the active control.

EFFECTS OF ACTIVE CONTROL

The Designed Approach

The objective to suppress the surge phenomenon isto restrict the amplitude of the fluid oscillation. Inorder to realize this objective, the effects ofthe smallfriendly disturbances are to weaken the amplitudeof pressure and flow quantity oscillation from theinception. As a result, the operating ranges of thesystem can be expanded by exerting the smallfriendly disturbances from the actuator.According to the analyzed results of the incep-

tion, the real-time signals from both the plenum andthe outlet ofthe fan are chosen to be the input signalof the closed-loop control plan. At the same time,the adjusted pulse signal provided by the computeris elected to be the output. Within the technique ofdynamic identification, the control model can beidentified. Figure 2 stands for the structural outlineof active control to surge. By decreasing the flowcross section in the throttle sequentially, thedynamic behavior of the system begins to workas an unsteady boundary. Then, acquiring thedynamic pressure signal continuously, obtainingthe discrete variable data through the data ampli-fiers and A/D board, and conducting the real-timeanalyses and processing those data by computer. Ifthe inception is detected, the computer immediatelysends out a functional pulse signal, to rotate theservo motor at a specific function, finally to causethe actuator operating. The objective was achievedsuccessfully.

The Control Effects at the DifferentControl Frequencies

The control frequency is governed by the pulsefrequency which the computer sends out to the

actuator. In order to seek the optimum controlfrequency for active control to surge, the differentcontrol effects to restrain the surge were investi-gated. Figure 9 stands for the different experimentalresults, and Figure 10 shows the dynamic behavior.It is shown that when the control frequency is lessthan 40 Hz the fluid oscillating amplitude decreasesobviously. When the controlled frequency is in therange between 80 and 100 Hz, it is impossible thatthe surge phenomenon is controlled actively.From the results of the power spectrum analyses

(Fig. 11), it is obvious that when the main controllerfrequency is less than 50 Hz the frequency value ofthe first harmonic wave is nearly equal to that of theactuator, this means that after the active controlscheme on surge is carried out, the frequencycomponent related to 3.26 Hz (for B----0.34) disap-pears, i.e., the surge phenomenon is restrainedsuccessfully. But there are other low frequencyvalue in the system which is already not that ofthe surge. This frequency value is close to thefrequency ofthe first harmonic wave ofthe actuator(compare Figs. 8(a) and 11). When the controlfrequency is larger than 50 Hz, the low frequencyvalue related to surge is stronger than the firstharmonic wave of the actuator and its amplitudeincreases with the increase of the control frequency.

FIGURE 9 Controlled results of different frequencies attime domain. (The comparison between the different frequen-cies supplied by computer during the occurrence of surge.)


1.4

..2"

1.0-

0.8.

0.6.

SO (Hz )

1.4

1.1-

l.O"

1.2-1

1.0-]o...]

0.2 1 m,-o: ’oW "gi "oW

d e f

FIGURE 10 The dynamic characteristics of the different controller’s frequency. (The group of figures shows the controlledeffects in the view of both non-dimensional flow and pressure perturbation.)

lo An,(da) 2OH::8.0.6.0.

4.0.

0.0.

-2.0 ’’’’1

a o-] o. 63H

et (HOi,,,,, i,,,,, i,,,,,i,,,,,i,,,,, i,’i,,tO 40 40 100 160 220 280 340b

10

8.0.

6.0.

4.0.

2.0:0.0

-2.0.,

Am(dB) 1.27Hz

Ye" "ib’i Tg"k’"k ",d e fFIGURE ll The power spectrum about the active control results to surge. (The group of figures demonstrates the coupledinteraction between the main controller’s frequency and the surge inherent frequency.)


It is likely that this surge frequency is also enhancedby the higher harmonics of the actuator. On thewhole, the optimum control frequency must bechosen between 20 and 40 Hz. Therefore, after theactive control scheme on surge is carried out,the frequency of the small fluid disturbance in thesystem is already not that of the surge. Thefrequency value is the frequency of the firstharmonic wave of the actuator. The surge phenom-enon is restrained successfully. While with the mainfrequency of the controller larger than 50 Hz, thelow frequency factors of the disturbances related tosurge is stronger than.the first harmonic wave of theactuator. The higher the main frequency value ofcontroller is chosen, the larger the amplitude of thedisturbance is caused. The optimum controlledfrequency must be chosen between 20 and 40 Hz.

Jr Vo

o

O.O Z.O ZO 3.0 4.0 5.0

FIGURE 12 The controlled results in time domain. (Theresults of the active control to surge are contrasted with theprescribed amplitude criteria of pressure perturbation.)

The Control Results of theDifferent Prescribed Criteria

The prescribed criteria are set up in the range from20 to 160 dB. As soon as the amplitude of the firstharmonic wave of the inception exceeds thepreliminary criteria then does the actuator supplythe small friendly disturbances. Figure 12 shows agroup ofthe controlled results. When the prescribedcriteria are set up at the lower values (from 20 to50 dB), the results of the active control are obvious,and the amplitude of the left perturbations isacceptable, when the prescribed criteria are set over120dB, it takes long time to reach steadiness. Forengineering applications, when a deep surge occursin the system, the higher prescribed criteria resultsare not satisfying for implementing the activecontrol plan. Therefore, the prescribed criterionshould be set up to be less than 50dB. When thesignal of the inception is identified, the plan of theactive control is exerted immediately by operatingthe actuator.

Entire Control Effects

Figure 13 represents the results, it is shown thatthe amplitude of fluid oscillation is decreased

dramatically after exerting the active control plan,at the same time, the oscillating loop of the surgedraws back to the quasi-stable operating pointrapidly.

Extending Effects from Surge Boundary

With the techniques of the active control, theoperating ranges of the centrifugal fan. can beenlarged for passing through the surge boundary.In fact, the actual operating point draws back tothe quasi-stable operating point after the surge issuppressed. Figure 14 shows the experimentalresults where the operating point passes throughthe surge boundary line. It is shown that the non-dimensional flow quantity average value of theonset point ofsurge is about 0.196. The limited non-dimensional flow quantity average value of quasi-steady point that is carried out with the technique ofactive control scheme is equal to 0.148. Therefore,the steady operating range is expanded by approxi-mately 24%. If the area of the throttle is furtherdecreased, the surge will be impelled rapidly(Fig. 15). It is shown that the actuator has noability to control the new surge phenomena. Thenew unsteady point is considered to be new surgeboundary point.


1.48

1.16

0.52

B=0.34

oscillation is decreased after

1.2

I.I

o.9

O.8

O.7

o.6

[] ltaiPoit

Pom

0.0 O.I 0.2 O.3 0.4 0.5 0.6

FIGURE 14 Extending surge boundary point. (The steadyoperating range is expanded by about 24% with the techniqueof active control.)

1.2

O.8

0.4

& B=0.28

2.0 4.0 6.0 & O IO. O

FIGURE 15 New surge point. (While the area of the throttleis further decreased the surge phenomena will be impelledrapidly.)

CONCLUSIONS

1. The dynamic behavior of surge in the actualcentrifugal compression system is analyzed indetail. The experimental results show that thelength of the time which the compressionsystem enters surge compulsively is shorterthan that of the time for spontaneous surge.The strength of the former is larger than thatof the latter. The strength of the surge is posi-tively proportional to the B parameters, thefrequency of surge is negatively proportional tothe B parameter.

2. The power spectrum analysis is applied toinvestigate the frequency behavior of the incep-tion of surge. The experimental results revealthat the inception of surge can be diagnosed

at both the outlet of the centrifugal fan andthe plenum. With real-time analysis method, theinception of the surge is captured rapidly and theactive control scheme is established.

3. The essential nonlinear characteristics withwhichthe actuator draws the small friendly disturb-ances into the centrifugal compression systemare investigated experimentally. These charac-teristics are important in designing the activecontrol scheme and in understanding the controlresults.

4. The active control of surge is successfully real-ized in a range of control frequencies and pre-scribed amplitude criteria of the surge inceptionsignal. The stable range where the actual centri-fugal compression system operates is extendedby 24% with the technique of the active control.


Acknowledgments

This research project is funded by the Climb ProjectB and the National Science Foundation of China.The two effective supports are greatly appreciated.The authors are also indebted to Mr. Qiao Xiaohuifor his assistance on experiment and data acquisi-tion and processing.

NOMENCLATURE

B Greitzer parameterm Mass flowt(s) Time (second)AP The pressure difference

Subscripts

p Plenum

Superscripts

non-dimensional variable

References

Day, I.J. (1993a) Stall inception in axial flow compressors,ASME Journal of Turbomachinery, 115, 1-9.

Day, I.J. (1993b) Active suppression of rotating stall and surge inaxial compressors, ASME Journal of Turbomachinery, 115,40-47.

Epstein, A.H., Ffowcs Williams, J.E. and Greitzer, E.M. (1989)Active suppression of aerodynamic instabilities in turbo-machines, Journal ofPropulsion and Power, 5, 204-211.

Ffowcs Williams, J.E. and Huang, X.Y. (1989) Active stabiliza-tion of compressor surge, Journal of Fluid Mechanics, 204,245-262.

Garnier, V.H., Epstein, A.H. and Greitzer, E.M. (1991) Rotatingwaves as a stall inception indication in axial compressors,ASME Journal of Turbomachinery, 113, 290-301.

Greitzer, E.M. (1976a) Surge and rotating stall in axial flowcompressors, Part 1: Theoretical compression system model,ASME Journal ofEngineeringfor Power, 98, 190-198.

Greitzer, E.M. (1976b) Surge and rotating stall in axial flowcompressors, Part 2: Experimental results and comparisonwith theory, ASME Journal of Engineering for Power, 98,199-217.

Greitzer, E.M. (1981) The stability of dumping system The1980 Freeman scholar lecture, ASME Journal of FluidsEngineering, 103, 193-242.

Gysling, D.L., Dugundji, J., Greitzer, E.M. and Epstein, A.H.(1991) Dynamic control of centrifugal compressor surge usingtailored structures, ASME Journal of Turbomachinery, 113,710-722.

Gysling, D.L. and Greitzer, E.M. (1995) Dynamic control ofrotating stall in axial flow compressors using aeromecha-nical feedback, ASME Journal of Turbomachinery, 117,307-319.

Haynes, J.M., Hendricks, G.J. and Epstein, A.H. (1994) Activestabilization ofrotating stall in a three-stage axial compressor,ASME Journal of Turbomachinery, 116, 226-239.

Hoying, D.A. (1993) Stall inception in a multistage high speedaxial compressor, AIAA Paper, No. 93-2386.

Moore, F.K. and Greitzer, E.M. (1986a) A theory of post-stalltransients in axial compression systems, Part 1: Developmentof equations, ASME Journal ofEngineeringfor Gas Turbinesand Power, 108, 68-76.

Moore, F.K. and Greitzer, E.M. (1986b) A theory of post-stalltransients in axial compression systems, Part 2: Application,ASME Journal of Engineering for Gas Turbines and Power,108, 231-239.

Paduano, J.P., Epstein, A.H., Valavani, L., Longely, J.P.,Greitzer, E.M. and Guenette, G.R. (1993) Active control ofrotating stall in a low speed compressor, ASME Journal ofTurbomachinery, 115, 48-56.

Pinsley, J.E., Guenette, G.R., Epstein, A.H. and Greitzer, E.M.(1991) Active stabilization of centrifugal compressor surge,ASME Journal of Turbomachinery, 113, 723-732.

Simon, J.S., Valavani, L., Epstein, A.H. and Greitzer, E.M.(1993) Evaluations of approaches to active compressorsurge stabilization, ASME Journal of Turbomachinery, 115,57-67.

Tryfonidis, H., Etchevers, O., Paduano, J.D., Epstein, A.H.and Hendricks, G.J. (1995) Pre-stall behavior of severalhigh-speed compressors, ASME Journal of Turbomachinery,117, 62-80.

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