Turbine Blades and Exhaust Gas flow monitoring

NATO/PFP UNCLASSIFIED

Turbine Blades and Exhaust Gas Flow Monitoring Using Microwave Probe Małgorzata PERZ, Radosław PRZYSOWA, Edward DZIĘCIOŁ, Ryszard SZCZEPANIK Air Force Institute of Technology (Instytut Techniczny Wojsk Lotniczych) ul. Ksiecia Boleslawa 6 01-494 Warszawa 46, skr. poczt. 96 POLAND tel. +48 22 68 52 111 / fax. +48 22 836 44 71 [email protected]

ABSTRACT This paper proposes usage of microwave radiation for health monitoring of turbine blades during the jet engine operation. It shows that it is possible to measure blade motion components and also to identify its vibration parameters and detect fatigue cracks. Practical issues of the technique implementation are discussed. Examples of the probe signal received during ground tests of the SO-3 turbojet engine are presented and analyzed.

INTRODUCTION Polish aero-engines (usually of Russian origin) and most of their elements are maintained basing on flying hours limit. This method is not cost effective but secure at normal operation conditions. Health control of turbine blades is performed after their manufacture and during engine overhaul using flaw detection methods and destructive fatigue testing of samples. Time of blades operation is fixed far before their cycle limit (fatigue durability). Nevertheless we faces fatigue problems, mainly because of engines ageing related to economical issues and also due to faults in production, overhaul or maintenance. According to long-lasting experience of our material laboratory [1] most of turbine blades damages are primary cased by non-HCF factors. In the practice blades of our engines rarely run out amount of their fatigue durability before reaching the flying hours limit, unless there are other reasons such as maintenance-related flaws (FOD, local overheat, pitting) or production-related flaws (forging laps, faulty coating etc). This is why fatigue is the reason for many failures of turbine blades.

Perz, M.; Przysowa, R.; Dzięcioł, E.; Szczepanik, R. (2005) Turbine Blades and Exhaust Gas Flow Monitoring Using Microwave Probe. In Evaluation, Control and Prevention of High Cycle Fatigue in Gas Turbine Engines for Land, Sea and Air Vehicles (pp. 24-1 – 24-8). Meeting Proceedings RTO-MP-AVT-121, Paper 24. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp.

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Turbine Blades and Exhaust Gas Flow Monitoring Using Microwave Probe

Fig. 1. Fatigue breakaway of a chrome-coated turbine blade.

Fig. 2. Transmission Electron Microscope photograph of fracture area (4500x).

Fig. 3. SO-3 turbine blade – visible cracks (500x).

It is difficult to early detect fatigue cracks in blades during engine service using available NDT methods, what makes HCF-related problems more serious. Success of SNDŁ-1b/SPŁ-2b tip-timing system [2] and several cases of thermal and HCF failures of turbine blades of the RD-33, AŁ-21 and SO-3 engines encouraged the authors to look for method of vibration measurement and health monitoring of turbine blades [3]. Operating gas turbine is an extremely adverse environment for any kind of measurements. High pressure and temperature reaching even 1200 – 1500 o C and presence of partially ionized molecules of exhaust gas make capacitance, eddy-current or inductive probes practically useless for continuous measurement of turbine blades motion. Therefore, a prototype of a microwave motion probe (of the MUH type) was designed in the Air Force Institute of Technology as a specialized diagnostic tool, profitable to operating in high pressures and temperatures. The device forms “pulses” – images of compressor and turbine blades, for the whole operating range of an engine [5]. The signal obtained for subsequent rotations is repeatable and characteristic for a given blade row. Amplitude and shape of each pulse are connected tightly with geometry of corresponding blade, its vibration amplitude and mode.

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BASIS OF THE PROBE The principle of the device is based on the homodyne detection concept, what ensures large sensitivity and is efficient especially for a weak and noisy signal. The signal from the generator is equally divided into two paths – one is the reference signal, the other one goes to the antenna (Fig. 4). The antenna signal is modulated by the moving object (a blade tip) and reflected back to the antenna. In the homodyne detector both signals (the reference and the reflected one) are summed.

Fig. 4. Structure of the microwave motion probe.

The experiments shows that only the antenna itself should be resistant to high temperatures – the whole generator/detector system works in temperature below 100oC, so it is possible to use standard semiconductor elements and there is no need for any special cooling. Whole probe should be resistant to vibrations.

SIGNAL RECEIVED FROM TURBINE BLADES At the probe output alternating voltage is generated. An actual value thereof determines the share of microwaves emitted by the antenna, coming back after reflection from tips of rotating turbine blades. Since the emission, reflection and reception of waves take place at a distance similar to the wavelength, electrical images of blades do not directly correspond to their geometry (Fig. 5). Laws of optical geometry are not applicable for this range.

U time Fig. 5. Exemplary signal received during tests by microwave motion probe from turbine blades.

Besides, the necessary condition for obtaining pulses which correctly represent the blade image is to match frequency of the emitted microwaves with geometry of objects under examination. It means that the frequency (power supply voltage) adjustment is necessary while installing the probe. RTO-MP-AVT-121

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Turbine Blades and Exhaust Gas Flow Monitoring Using Microwave Probe Transmission and reception conditions during measurements caused the signal to get inverted polarization (Fig. 5). Local minimum values represent blade tips, but the background is represented by local maximum values with high-frequency disturbances. In the case of the turbine it is more difficult to find and distinguish what represents blade and background responses in the signal. There are two reasons for that: a number of blades higher than that of the 1st stage of the compressor, and high concentration thereof. Disturbances at local maxima probably represent short-lasting fluctuations of the working medium, which changes conditions of the microwaves propagation between the blades. The received signal amplitude and shape are formed not only by the blades themselves, but also by any blade row displacement towards the antenna aperture, caused by the turbine disk movements. In a similar way, the shaft run-out and rotor vibration of different origin result in the change of orientation of the blades and the antenna, which is not connected with the disk rotational motion. It adequately modifies shape of pulses generated by subsequent blade passes. Vibro-acoustic phenomena introduce into the resulting signal some spectrum components effected by particular sources of vibration (Fig. 8). Several parameters form mathematical model of a blade's electrical image: phase, amplitude and width of a pulse. They are connected tightly with blade geometry, vibration amplitude and mode. Other movements of the rotor, apart from behavior of blades row, also have some influence on the shape of the pulses. This makes measurements of blade vibration parameters more difficult but gives useful information about rotor dynamics. For the whole operating range of an engine, the MUH device forms electric pulses, which are images of compressor and turbine blades. The signal obtained for subsequent rotations is repeatable and also characteristic for a given blade row. Tips clearances map and the table of distances between blades sequence are unique for the machine. They not only allow to identify an engine unambiguously basing on the measurements, but also distinguish individual blades in the row.

SIGNAL REFLECTED FROM EXHAUST GAS Ideal conditions for the microwave probe to generate pulses representing blade passes under the antenna occur during the start-up, when fuel has not been injected yet. At that time there are no factors that attenuate transmission of microwaves, i.e. high temperature and fluctuations of the exhaust-gas pressure; also vibrations of the shaft and disks, which interfere with the signal, are relatively small. However, all these factors are present during the idle run, but to such extent that the blade image in the total probe signal maintains high signal-to-noise ratio. This operational range is also most suitable for analysis of gas dynamics. The device generates pulses not only due to the blades presence, but also during the antenna’s passing through the space between the blades. It could be assumed that it is possible to measure local gradients of temperature and pressure of exhaust gas. They should influence microwave propagation by changing electromagnetic properties of the medium. In the space between subsequent blades there are high gradients of temperature, due to the cooling effect of the blades. This effect causes changes in gas ionization, which means different reflection coefficient for microwaves. Also, changes in gas flow together with short-time pressure pulsation of working factor may cause local differences in propagation conditions. All those phenomena cause modulation of electromagnetic parameters of the medium. Reflection coefficient can also change due to the presence of solid portions – a product of the fuel burn, as well as parts of the engine construction. All these phenomena may cause signal “waving” at local maxima for the time when the antenna is in front of the space between the blades.

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Turbine Blades and Exhaust Gas Flow Monitoring Using Microwave Probe The special experiment was done to analyze and identify structure of the signal background. The test was focused on factors not connected with the blades or the rotor, which make the received signal modulated in time intervals when antenna is between the blades. Two homodyne signals where registered simultaneously, corresponding to the situation when (Fig. 6, 7): a) the probe was mounted directly over the engine blades, b) the probe was mounted about 30 cm further, into the jet casing.

Fig. 6. The hot part of the SO-3 turbojet engine with microwave probes mounted into the turbine casing (a) and the jet casing (b).

a

b

Fig. 7. Signal of the microwave probe received for one rotation from the turbine stage (a) and from the jet (b).

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Relative amplitude [dB]

Even though the amplification ratio of the signal (b) was higher, its level is significantly lower in comparison to the signal (a), because there are no objects to fully close the antenna circuit. In spectrum of signal (b) at frequencies of 8010 Hz and 48700 Hz, wide characteristic ranges are clearly visible (Fig. 8). They are not random components and most probably are caused by gas dynamics. Right now, it is difficult to unambiguously identify what kind of physical phenomena might modulate the reflection coefficient of the exhaust gas. 150 140 1 130 114 120 110 100 90 80 70 60 50 40 100

2

228

3

342

6

5 4 679 3 196

4

1 370

9473 7

8

18 945 9 28 418

Relative amplitude [dB]

a

150 140 1 130 120 114 110 100 90 80 70 60 50 40 100

1 000

2 228

3 342

10 000

4 1 370

6 5

8010

4 679

8

48700 9

18 946 28 419

3 196

1 000

7

9 473

100 000

Frequency [Hz]

10 000

100 000

Fig. 8. Spectrum of signal received from the turbine stage (a), and from the jet (b). Influence of gas dynamics is visible for frequencies f = 8010 Hz and f = 48700 Hz. FFT window size: 65k, type: Hanning. Origin of marked spectrum lines: 1 - rotor speed: fΩ; 2 - 2nd harmonic of rotor speed;

6 - turbine inlet vanes: 41 × fΩ; 7 - turbine blades: 83 × fΩ;

3 - 3rd harmonic of rotor speed; 4 - nozzles: 12 × fΩ; 5 - blades of 1st compressor stage: 28 × fΩ;

8 - turbine blades, 2nd harmonic: 166 × fΩ; 9 - turbine blades, 3rd harmonic: 249 × fΩ

Those characteristic frequencies for gas can also be seen in the signal (a). If we assume signal (b) to be the background of this signal, we can improve its signal-to-noise ratio by subtracting it from signal (a). Between section (a) and (b) of the engine gas path the temperature and pressure of medium are not able to equalize. That is why the spectrum components caused by engine blades are also visible in signal (b). These results have proved that in the signal received from the MUH device there is information about local disturbances and non-homogeneities of the gas flow. To use this probe to measure parameters of exhaust gas flow further effort and precise calibration procedure is needed.

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CONCLUSIONS Electrical signal received from the MUH carries both the information about components of blades movement and gas dynamics. It needs processing to obtain diagnostic information, which will be a matter of future work. The final aim is to develop a complex monitoring system for both turbine and axial compressor of the aero engine. It would improve safety of aircraft, especially the single-engine ones. Specialized microwave probes mounted on the casing could be used during the flight for monitoring engine operation – especially to assess health of blades, but also some parameters of exhaust gas. The system would alarm the pilot about reaching unsafe level of blade vibration and advise him some special procedure. Blade vibration data could be used after processing based on phase-discrete method to assess blade health and detect fatigue crack. Right now, the prototype probe can only be used in a laboratory, but in future its improved version could be mounted on the engine casing.

REFERENCES: [1]

Dudziński A.: Analiza mechanizmu niszczenia łopatek turbin silników lotniczych na podstawie wieloletnich badań materiałowych (in preparation), Air Force Institute of Technology, Warsaw 2005.

[2]

Analysis of turbine blades damages basing on material testing data.

[3]

Witoś M., Szczepanik R.: Turbine Engine Health/Maintenance Status Monitoring with Use of PhaseDiscrete Method of Blade Vibration Monitoring, NATO RTA, AVT-121, Grenada 2005.

[4]

Dzięcioł E., Jaźwiński J., Szczepankowski A.: Microwave detector as used to monitor health/maintenance status of aircraft engine’s turbine blades in hot gas. Proceedings of Conference on Turbine of Large Output, Gdańsk 2003.

[5]

Advanced measurement techniques for aero engines and stationary gas turbines. Lecture series monograph, von Karman Institute, Brussels 2004.

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SYMPOSIA DISCUSSION – PAPER NO: 24 Author’s name: R. Przysowa Discussor’s name: A. Abate Question: What is the resolution capability of the microwave probe, i.e. what tip displacement can be reliably seen? Answer: Depends on the waveform frequency, but should be good.

Author’s name: M. Perz Discussor’s name: A. Corro Question: Can the sensor be used with shrouded blades? Answer: Yes, but the signal has more noise.

Author’s name: M. Perz Discussor’s name: A. Flotow Question: 1) Do you use a magic T? 2) Is the resonance cavity in the probe ¼ λ or ½λ? 3) Must the probe be 2cm wide? Answer: 1) We will in the improved version in the future. 2) It is ½λ . 3) We will try to shrink it.

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