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The Society shall not be responsible for statements or opinionsadvanced in papers or in discussion at meetings of the Societyor of its Divisions or Sections, or printed in its publications.Discussion is printed only if the paper is published in an ASMEjournal or Proceedings.Released for general publication upon presentation.Full credit should be given to ASME, the Professional Division,and the author Is).

Instrumentation for Flow Measurements

in Tur6omachine Rotors

W. F. O'BRIEN

Mem. ASME

H. L. MOSES

Assoc. Mem. ASMEAssociate Professors,

Mechanical Engineering,Virginia Polytechnic Instituteand State University,Blacksburg, Va.

Flow measurements taken on the rotors of turbomachines are of great value forimproved understanding and advancement of design techniques. On-rotor ex-periments have been limited in the past because of instrumentation problems,especially with the data transmission system. Recent advances in miniatureelectronic systems and transducer technology have produced a renewed interestin this area. These considerations are discussed, and research on a telemetry-type data transmission system is described with experimental verification usinga strain gage on an axial flow fan blade.

Contributed by the Gas Turbine Division of the American Society of Mechanical En-gineers for presentation at the Gas Turbine and Fluids Engineering Conference & Prod-ucts Show, San Francisco, Calif., March 26-30, 1972. Manuscript received at ASMEHeadquarters, December 14, 1971.

Copies will be available until January 1, 1973.

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y. 10017

Copyright © 1972 by ASME

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1 Numbers in parentheses designate References

at end of paper.

Tip

Fig. 1 Compressor rotor

Instrumentation For Flow Measurements

in Turbomachine Rotors

W. F. O'BRIEN H. L. MOSES

INTRODUCTION

Experimental investigations of flow inturbomachines must include consideration of spe-cial instrumentation problems. This is partic-ularly true for on-rotor measurements, wherestationary instrumentation problems are combinedwith those due to the rotation. The principal ad-ditional problems are large centrifugal forces,vibration, and data transmission to a stationarysystem. Special consideration must also begiven to the support and location of instrumenta-tion and probes.

Measurements of interest include fluid pres-sure, temperature and velocity, along with bladestrain and temperature. Data including bothmean and fluctuating values are highly desirable.Such experiments are of great value for improvedunderstanding of rotor flow and advancement ofdesign techniques. The desire for higher per-formance machines and an increased interest inverification of improved computational methodshave caused a renewed interest in on-rotormeasurements.

In the past, most experimental work onturbomachine blading has been limited to station-ary, two-dimensional cascades, and many featuresof the rotating machine design were more artthan science. Measurements on actual turboma-chines were confined mostly to overall perform-ance, with some detailed measurements on station-ary components. It was realized, however, thatrotation could have an appreciable affect onperformance. Theoretical approaches to problems,such as losses and stall, have been developedfor some cases but have not been verified bydirect measurement. Experimental correlations,such as those for secondary flow losses, have beenbased on overall performance and have been question-able.

Although of recognized value, experimentaldata on the flow in rotating passages have beenlimited, primarily because of instrumentationproblems. However, recent advances in trans-ducers and electronic transmission devices permit

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improved techniques for the collection of accu-rate, wide frequency band data. This paper dis-cusses some of the problems associated with on-rotor measurements and describes a data trans-mission system for such measurements.

PREVIOUS WORK

Flow measurements on the stationary com-ponents of turbomachines have been favored forsimplicity and have been adequate in many in-stances. The need for flow data from rotor pas-sages, however, has led to several reported in-vestigations. There has also been recent reportedinterest in improved methods for rotor-to-statordata transmission.

In a 1949 paper, Weske (1) 1 reported staticpressure measurements on the rotating blades ofan axial blower at three radius locations on theblade. A scanning device was employed to physical-ly transmit one pressure measurement at a time tothe stationary components. With the extremelylow-frequency response instrumentation, it wasfound that radial displacement of the boundarylayer on the rotating blades delayed the stall ofthe root sections and induced early stall at the


11...StationaryRotating

Fig. 2 Instrument system for transmission ofrotor data

Analyzer

VCO

DemodulatorPhase Lock

LoopTransducer

SignalConditioning

Oscilloscope

TapeRecorder

Antenna

tip section of the compressor blades. At aboutthe same time, Runckel and Davey (2) measuredstatic surface pressure distributions on NACA16-series blades. They used a measurementtechnique which permitted the simultaneous trans-mission of 24 pressures through a mercury sealingdevice. Westphal and Godwin (3) used the samepressure transmission technique to obtain staticsurface pressure distributions on NACA 65-seriesblades in 1957. They were particularly concernedwith the comparison of 65-series blade pressuredistributions, as obtained from rotor and cascadetests. Good agreement at design conditions be-tween rotor and cascade measurements was found,but cascade measurements did not agree withactual rotor blade measurements at off-designangles of attack. Secondary flows induced byrotation were blamed for the differences. Staticpressure, total pressure, and flow directionmeasurements were also made on the stator vanesof the compressor, and tuft visualization tech-niques were used for flow direction mapping onthe rotor blades. Measurements were made up tothe stall point of the compressor, at variousspeeds, angles of attack, and mass flows. In1952, Michel et al. (4) reported static and totalpressure measurements on large rotating centrif-ugal compressors, transmitting pressure signalsacross the rotating interface using sealed bear-ings.

Slip-ring techniques adapted for use in gasturbine development were discussed by Potter (5).Recent interest in improved instrumentation

techniques for the transmission of data fromhigh-speed rotating shafts has concentrated on"no-sliding-contact" techniques, eliminating suchdevices as slip rings, sealed bearings, or variouspressure-sealing devices. Besides elimination ofthe wear and signal noise problems associated withsliding contact, such techniques exploit the high-frequency, multiple data-handling capabilitiesof recently available microelectronic devicesand transducers. Lesco et al. (6) recently re-ported the development of a digital-type, "no-sliding-contact" data transmission system and acheck of its accuracy for transmission of signalsfrom a small turbine engine. Reed and Rizzi (7)discussed design concepts for telemetry-typerotary data transmitters for use on high-speedshafts. Hoeppner (8) commented on potential ap-plications for such devices, especially turbineengines. Podgornyi et al. (9) have investigatedstresses on turbine disks using strain gages, andEmets (10) has transmitted pressure measurementsfrom rotating turbine blades. Adler (11) hasreported on the development of miniature strainand temperature transmitters, capable of opera-tion under continuous radial accelerations of30,000 g and temperatures of 150 C. Other non-contacting techniques have been employed, as theoptical technique for vibration measurement re-ported by Bromley and Monahan (12). Turbine bladevibration monitoring was reported by Arikawa andMatsuoka (13). The vibration data were trans-mitted without contact, using a telemetry system.A special purpose telemetry system which employed

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Flow ControlVanes

8"

Stationary Antenna Support

Rotating Telemetry Package

Stationary Antenna

Rotating Antenna

Rotating Compressor Housing

Rotors

AWW1

••k

Support Bearings

DynamometerDrive

StdtorsFig. 4 Assembly of experimental apparatus

21 transmitters to provide 16 strain measurement

channels and 30 temperature measurement channels

was described by Colangelo and Schlereth (14).

The system was of an ultra-miniature design es-

pecially suited for a jet engine environment.

Problems of measurament of fluctuating pressures

from DC to over 100 kHz in jet engine testingwere discussed by Fischer (15).

The aforementioned literature shows that

the problems of measurement of flow parameters

on turbomachine rotors have received attention,

as have the methods for obtaining such measure-

ments. The early investigations reported were

instrumental in verifying the on-design pressure

distributions in axial flow compressors. The

measurement technique was mechanical in nature.

More recently, the availability of small elec-

trical transducers and improved slip-ring tech-

niques prompted additional work, primarily in

the area of measurement of mechanical strain.

The most recent work has concentrated on the

study of methods for data transmission, especial-ly radio telemetry techniques.

ROTOR FLOW AND RELATED MEASUREMENTS

The actual flov7 in the rotating blade pas-

sages of a turbomachine is extremely complicated.

It is highly turbulent, three-dimensional, un-

steady, and often separated. At best, there is

only a qualitative understanding of some of the

phenomena involved. Much of this understandingis based on knowledge of the flow in stationarycascades, with a physical interpretation of theeffects of rotation. Better understanding and

analyses of such limiting factors as losses, un-

steady flow, and stall are needed.Although the problem is more general, the

flow in the rotor blade passages of an axial-flow compressor (Fig. 1) is of particular in-

terest. The flow is already highly turbulent,

three-dimensional, and unsteady when it enters

the passage. Shear flows created by viscous ef-fects upstream, as well as boundary layers de-

veloped in the passage, cause secondary flows.

The secondary flow, in turn, has an appreciable

effect on the boundary-layer development, par-

ticularly on the end walls and corners, and

thus on losses and stall. The flow is furthercomplicated by tip clearance between the moving

blade and stationary casing. Near stall there

is a strong interaction between the boundary

layer and free stream, which eliminates independ-

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MIME111111111111111111

111111111111111 11111111111111•1111111111111111Fig. 5 Rotor blade strain data signal

ent consideration of either. Unsteady flow iscaused by inlet distortion and stall as well asby flow interaction with stator blades.

A quantitative knowledge of such flows musttimately be based on experiment, and because of

the additional complications of high rotationalspeeds, careful selection of the required measure-ments is important. Such experiments are limitedby available, or at least feasible, instrumenta-tion. Consideration must be given to the relevanceof the experimental data in extending the physicalunderstanding of compressor flows, developingempirical correlations, and verifying theoreticalanalyses.

With respect to the difficulties involvedand the relevance of the data, blade surfacepressure is perhaps the most important flow param-eter to be considered. Pressure data, both meanand fluctuating, are useful in studying the un-steady effects of inlet distortion and stall,noise, and the effect of boundary layers andsecondary flow on the work done by the rotor. Forexample, such data would permit the verificationof theoretical methods for predicting unsteadylift forces, as well as the change in lift dueto secondary flow.

Other measurements of interest include sur-face shear stress and velocity distributions inthe blade passage. Shear stress, or skin fric-tion, can be determined with a heat-transfer de-vice or a differential pressure across a boundary-layer fence or Preston probe. This data, whichshould be directional, would be useful in study-ing the boundary-layer development and separa-

tion. Complete velocity distributions in therotating blade passage would, of course, be use-ful, but this hardly seems feasible at the presenttime.

INSTRUMENTATION ON TURBOMACHINE ROTORS

The measurement problem on turbomachineryis similar to that encountered on any rotatingdevice. The problem is that, in general, thedesired data is in a coordinate system which isrotating relative to the observer. There are many .

possible methods which can be considered forobtaining the desired data. These range fromdirect instrumentation connections with flexiblewires or tubes, through various sealing or elec-trical connection techniques involving slidingcontact, to the most recently studied techniquesinvolving the use of electromagnetic waves,principally radio telemetry. Other electromag-netic wave techniques, such as holography, haverecently been employed for measurements on turbo-machinery.

There are many special problems to be con-sidered when instrumenting turbomachinery, mostof which exist without regard to the type of in-strumentation chosen. Generally, rotors areturning at high speed and produce very high ac-celeration fields. It is presently not uncommonto encounter a requirement for mounting a rotating

instrumentation system in a 30,000-g field. Itis likely that other environmental considerationswill include a very wide temperature range foroperation and a severe vibration field. The databandwidth required depends upon the purpose ofthe individual test, but typically wide banddata is required. For example, if the generationof noise by a compressor blade is to be studied,pressure measurements on the rotating bladesmust include the 20 to 20 KHz pressure variations.Flow studies can require considerably wider band-widths, if such things as the effect of statorwakes on the flow over the rotating blades andturbulence are of interest. A calculation ofthe blade passage frequency in a modern high-speed machine easily shows the need for very wideband data.

If the on-rotor components of a rotary in-strumentation system are electrical, considera-tion must be given to the supply of this power.It can either be supplied by batteries on therotor or by some means of transmission from thestator to the rotor while the system is operating.These methods can include slip rings or can bethemselves of an electromagnetic nature.

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Fig. 3 Compressor and rotating portion of instru-ment system

It is to be expected that the usual flowdata, such as pressures, temperatures, velocities,etc., and, in addition, mechanical strain informa-tion, will be desired. This data must be trans-duced, and it is apparent that the transducerswill be subject to the same environments as therotating instrumentation system. In particular,the problem of maintaining measurement fidelityunder the high acceleration fields experiencedis a difficult one. All transducers involve mov-ing elements of non-zero weight, and their outputmust be, to some extent, affected by acceleration.The selection of very stiff, lightweight trans-ducers is indicated, but it has been suggestedthat a local measurement of the accelerationfield and the subsequent subtraction of the ac-celeration-induced transducer response would beanother solution.

The stator components of an instrumentationsystem for the acquisition of rotating data areconventional but may be complex. Depending onthe mode of data transmission which is employed,a telemetry receiver or other special signalconditioning devices may be required. The trans-mitted data may typically involve a bmad-bandmix of Fourier components having both phase andamplitude relationships. Multi-channel recordingand spectrum analysis of the data combined withspecial display techniques will likely be required.

Devising means for the calibration ofrotary data systems is another formidable prob-lem. Consideration of the typical multi-elementinstrumentation system will reveal that calibra-tion is normally accomplished by simulating an in-

put to the transducer. The response of thetire system to the simulated input is then notea.It is obvious that it is more difficult to providea simulated input to a transducer located on arotating body. Ideally, calibration should beaccomplished under rotating conditions to accountfor effects not present when the experiment isstationary.

A DATA TRANSMISSION SYSTEM

Consideration of the rotor flow measurementproblems associated with high-speed turbomachineryled to the conclusion that telemetry methods werewell suited for the application. Experimentalinvestigations have shown the feasibility oftelemetry-type data transmission systems forturbomachinery. One system of this type hasbeen constructed employing integrated circuitcomponents. First test results with a straingage transducer indicate that reliable flow param-eter data can be obtained with the system.

A variety of telemetry techniques weresuitable for this application. After consiaeraoiestudy, the direct FM technique was chosen as themost suitable for turbomachinery investigations.Among the more important reasons for this selectionwere the wide data bandwidths which can be pro-vided with direct FM telemetry, and the simplicityof such systems. In conventional multi-channeltelemetry, direct FM is not often chosen becauseof the requirement for a transmitter for each datachannel. For the ultra-short transmission dis-tances involved in turbomachinery telemetry, thisproblem need not be of primary concern.

A single channel, miniature data telemetrysystem was constructed and tested on an axial-flow compressor. Fig. 2 shows a block diagram ofthe instrument system. The data transmitter wascomposed of a signal conditioning and a voltagecontrolled oscillator (VCO) section. These func-tions were accomplished with two integrated cir-cuit devices and the necessary external components.The signal conditioning circuitry was designed toexcite and amplify data from strain gage typetransducers. Power supply for the rotating datatransmitter was provided by 9-v batteries. Theinstallation of the transmitter within a specialadapter for the axial-flow compressor is shown inFig. 3. Data were transmitted employing a capaci-tive antenna by frequency modulation of a 1-MHzcarrier. The rotating portion of the antenna maybe seen as a disk in Fig. 3. A matching telemetryreceiving antenna was mounted in the dischargeduct of the axial-flow compressor. Fig. 4 showsa section of the entire test apparatus, includingthe compressor and drive system arrangement.

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The high-frequency signal from the receivingantenna was demodulated by a receiver of specialdesign. Constructed with integrated circuits,the receiver employed the phase-locked loopprinciple of FM demodulation. The demodulateddata were viewed on an oscilloscope. Fig. 5

shows a typical presentation of data transmittedfrom strain gage transducers installed on one ofthe compressor blades. The natural frequency ofthe blade is readily apparent on the oscilloscopephotograph. Also, the various harmonics presentin the signal can be seen superposed on the funda-mental vibration frequency. The transmissionsystem produced a data bandwidth of DC to 20 KHz,and Fig. 5 includes data within this range.

As noted in the discussion of rotor flow,axial flow compressor blade surface pressuremeasurements are considered important. The datatransmission system was designed for operationwith ultra-miniature (0.125-in.-dia) strain gagepressure transducers. Future designs will pro-vide for the simultaneous transmission of pres-sure information from six transducers. Bothmean and fluctuating pressure measurements willbe made, and it is expected that valuable newinformation will be obtained. Of special in-terest are pressure data relating to stall, noise,inlet distortion, boundary layer and secondaryflow effects on compressor blades.

ACKNOWLEDGMENTS

The assistance of Hudson R. Carter, Under-graduate Research Assistant in the MechanicalEngineering Department at Virginia PolytechnicInstitute and State University, is gratefullyacknowledged. Mr. Carter assisted in the design,construction, and testing of telemetry data trans-mission system.

REFERENCES

1 Weske, J. R., "An Investigation of theAerodynamic Characteristics of a Rotating AxialFlow Blade Grid," NACA TN 1128, Feb. 1947.

2 Runckel, J. F., and Davey, R. S., "Pres-sure-Distribution Measurements on the RotatingBlades of a Single-Stage Axial-Flow Compressor,"NACA TN 1189, Feb. 1947.

3 Westphal, W. R., and Godwin, W. R.,"Comparison of NACA 65-Series Compressor-BladePressure Distributions and Performance in aRotor and Cascade," NACA TN 3806, March 1957.

4 Michel, D. J., Mizisim, J., and Prian,

V., "Effect of Changing Passage Configuration onInternal Flow Characteristics of a 48 Inch Cen-

trifugal Compressor; I-Change in Blade Shape,"NACA TN 2706, Lewis Flight Propulsion Laboratory,May 1952.

5 Potter, E. A., "Slip Ring Techniques forAircraft Gas Turbine Engine Development," StrainGauge Readings, Vol. 6, No. 2, June-July 1963,

pp. 11- 32.6 Lesco, D. J., Sturman, J. C., and

Nieberding, W. C., "Rotating Shaft-Mounted Micro-

electronic Data System," Lewis Research Center,NASA TN D-5678, Feb. 1970.

7 Reed, F. F., and Rizzi, W. J., "Kine-matic Monitoring Systems for Rotating EquipmentInstrumentation," United Aircraft Corporation,Trevose, Pennsylvania, In Proceedings, 14th In-ternational Aerospace Instrumentation Symposium,Boston, Mass., June 3-5, 1968.

8 Hoeppner, C. H., "Miniature Telemetry,"Industrial Electronics Corp., Satellite Beach,Fla., in Proceedings, 14th International Aero-space Instrumentation Symposium, Boston, Mass.,June 3-5, 1968.

9 Podgornyi, A. N., et al., "Investigationof the Stressed State of Turbocompressor Disks,"in Dynamics and Strength of Machines, Russian,1968.

10 Emets, P. 0., "Comparison of the Re-sults of Pressure Measurements at Rotating Tur-bine Blades with Computer Calculations of theFlow Past the Blade," Teploenergetiks, Vol. 15,Russian, Feb. 1968.

11 Adler, A. J., "Wireless Temperature andStrain Measurement," Aerotherm Corp., MountainView, California, in Instrument Society ofAmerica, 24th Annual Conference, Houston, Texas,Oct. 27-30, 1969, Proceedings.

12 Bromley, L., and Monahan, M. A., "Vi-bration Measurement by Holographic and Conven-tional Interferometry," U. S. Naval ElectronicsLaboratory Center, San Diego, Calif., in Instru-ment Society of America, 24th Annual Conference,Houston, Texas, Oct. 27-30, 1969, Proceedings.

13 Arikawa, S., and Matsuoka, H., "RotatingVibration Test of Turbine Blade by TelemeteringSystem," Kobe Technical Institute, Kobe, Japan,in Mitsubishi Heavy Industries Technical Review(English Edition), Vol. 5, Jan. 1968.

14 Colangelo, D., and Schlereth, F.,"Special Purpose Telemeter for a Jet Engine En-vironment," Telemetry Journal, Vol. 6, No. 2,Feb.-March 1971, pp. 15-18.

15 Fischer, J. E., "Fluctuating PressureMeasurement from DC to Over 100 KHz in JeL Engine

Testing," Kulite Semiconductor Products Applica-tion Note KPS-ANI2.

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