Verification of a technique of non-contact blade vibration stress measurement by discrete-phase method

Аuthors

Simonov E. L.

Company «Aviadvigatel», 93, Komsomolsky avenue, Perm, GSP, 614990, Russia

e-mail: simonov_evgenii@mail.ru

Abstract

This paper deals with technique verification of turbomachine blade tip-timing (BTT) measurement by discrete-phase method. The article touches upon the issue of dynamic loading as one of the most dangerous forces exerted on gas turbine engine (GTE) wheel blades.
Alongside with a conventional method of GTE rotor blade vibration stress measurement using slip-ring devices the technique of non-contact blade vibration measurement (discrete-phase method) is intensively developing. Discrete-phase method (DPM) enables researchers to receive full strain distribution pattern simultaneously on all wheel blades, to measure blade vibrations of a two-shaft GTE. DPM is applied by experts in gas-turbine machines in Russia («CIAM», «LMZ»), Germany («MTU»), Great Britain («Rolls-Royce»), France («Fogalenanotech», «Snecma»), USA («General Electric») and others.
The purpose of this paper is to implement experimental computation optimization of BTT with CAE systems technology application.
The above method includes measurement and comparison of the actual time of rotor passing by sensor with expected time (time of an absolutely rigid blade passing). In certain time frames at known frequency of rotor rotation, the deviation of circumferential blade cross-section and its turn by centrifugal and gas forces relative to equilibrium position is determined. If the blade vibrates with frequency not aliquot to rotation frequency, time frames at each turn will be different. Registering them by DPM devices it is possible to determine blade vibration amplitude.
The test bench for BTT technique optimization has been designed and manufactured in OJSC «Aviadvigatel» (Perm).
Fifth-stage rotor blade vibration stress measurement of high pressure compressor of GTE PS-90A is carried out by BTT at the same time with strain-gauge measurement of rotor compressor on the test bench of OJSC «Aviadvigtel».
To define blade vibration parameters by DPM devices the turbomachine preparation is performed particularly in the circumferential direction some impulse (capacitive) sensors having no mechanical contact with turbomachine rotating parts are located on the compressor stator. Capacitive sensors type HPC-30-62-200-04 of SPE «Mera» (Mytischi) production are placed on the test bench. For identification of a blade and initial rotor position parallel to test bench rotation axis the induction sensor DCHV-2500 used to define phase and produsing one impulse per a turn is installed.
Methodology. The computational analysis of rotor blades stress-strain behavior (SSB).
To convert circumferential cross-section movement into blade vibration stress the data of blade movement are required. For this purpose we will perform dynamic stress distribution calculation for every vibration mode investigated.
For mathematical model verification of DPM the relation of blade vibration stress in the strain-gauge installation sites to circumferential blade cross-section displacement has been defined.
Circumferential cross-section movement in the capacitive sensors installation sites above the blade and the movement relationship with vibration sites have been calculated by finite-elements method.
In order to build a mathematical model of BTT measurement by DP technique, the blade vibration stress relation to circumferential cross-section movement has been defined by an analytical analysis as well as experimentally.
Natural frequencies obtained by an experiment appeared to be lower than estimated ones, which can be explained by more rigid boundary conditions, taken into consideration (fixing all contact surfaces units in all directions).

Conclusions

The following convergence of test results with estimation has been received: natural frequency lapse does not exceed 1,6 %, vibrations amplitude lapse 7,3%.
The comparative results analysis of rotor blades vibration measurement received by strain-gauge and BTT measurement is carried out. The received lapse ranged from 8,7 % to 19,0 %, which is acceptable at the present stage of researche. At the moment in OJSC «Aviadvigatel» conducted to decrease the lapse to the levels which do not exceed 10 %.
Calculation of rotor blades vibration stress is executed at vibrations on the lowest vibration modes (for example on the 5-th stage of GTE high pressure compressor PS-90A). SSB calculation and modal characteristics of high pressure compressor rotor blades is carried out in finite elements analysis package ANSYS. The mathematical model connecting the dynamic SSB with blade circumferential cross-section movement is built.
Further research trend involves implantation algorithm formation of natural vibration modes definition and operational vibrations stress in tempo of tests. Furthermore geometry dispersion of the profile part and the lock consideration as well as natural blade frequency is planned to include in the computation.
The received results at the moment are used by investigation of the perspective fifth generation engine on the benches of OJSC «Aviadvigatel».

Keywords:

discretely-phase method, resonance, strain gages, blades, vibration, gas-turbine engine, own frequencies and vibration modes

References

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