Calculation and experimental evaluation of damper efficiency for decreasing vibratory stresses in turbine rotor blades

Аuthors

Shorr B. F.^*, Melnikova G. V., Serebryakov N. N.^**, Shadrin D. V.^***, Bortnikov A. D.^****

Central Institute of Aviation Motors named after P.I. Baranov, CIAM, 2, Aviamotornaya str., Moscow, 111116, Russia

*e-mail: shorr@ciam.ru
**e-mail: serebryakov@ciam.ru
***e-mail: shadrin@ciam.ru
****e-mail: adb@ciam.ru

Abstract

The subjects for study are dampers of various masses installed under the platforms of turbine blades.

The research issue is prevention of turbine blades failures caused by higher level of variable stresses.

The goal of the work consists in experimental and computational definition of effectiveness of shock-absorbing insertions' masses (3.3 g, 4.7 g and 5.8 g) for variable stresses reduction in the full-size turbine wheel.

The methodology of the work includes two trends: computation and experimental. The computation trend is based on modeling the damper using MSC.Nastran contact elements and estimating the reduction of vibratory stresses, by integrating the equation of motion in the time-domain employing the standard non-linear integration procedure by the Newmark method. The effect of the insertion on vibration frequencies of the blade was also studied. The experimental trend is based on a comparative analysis of the amplitudes of vibratory stresses in the blades both with installed damper and without them. Tests are performed on the CIAM bench test (manufacturer is Test Devices company). The turbine wheel is assembled for testing in a special way: one sector of the wheel is damper free, and the rest three sectors were equipped with dampers of various masses. The blades were prepared with strain gages, and in each sector the blades with maximum response to external excitation from the air supplied to the test chamber were selected. Tests were carried out for an unheated wheel.

The calculations revealed that the most effective reduction of vibratory stresses in the blade occurs when the holddown pressure of the damper to the bottom surface of the blade platform are 200–800 N. Such forces for damper mass of 3.3 g were caused by centrifugal forces at rotational speeds of the wheel in the range 35–70 % of the maximum rotational speed; this range is 29–58% for the damper of 4.7 g, and for the damper of 5.8 g, it is 26–51%. The affect of dampers weighing 4.7 g and 5.8 g is ineffective, starting, respectively, from 90% and 82% of the maximum rotational speed. According to calculations, the damper with mass of 3.3 g allows reduce the vibratory stresses by 22% at a resonant mode at the 87% of maximal rotational speed.

The tests revealed that, in comparison with damper and without damper, the blade frequency with shock absorber of 3.3 g increased by also 16%, and the oscillations' amplitude decreased by 25%. This correlates satisfactorily with the computation data.

Conclusions were drawn that the calculated and experimental results in these studies showed, in general, a satisfactory agreement with respect to both the reduction of vibratory stresses and the change in the resonant frequency when a damper was installed. Some discrepancy between the calculated and experimental data on the effect on the vibratory stresses of “heavy” inserts of 4.7 g and 5.8 g may occur due to the assumptions in calculations, as well as to the errors in the experiment and processing of the test results. To evaluate the effect of the amplitudes of vibratory stresses in the blade without damper, stiffness and mass of the damper, as well as friction coefficients on the effectiveness of the damper to reduce the vibratory stresses in the blade, additional experimental and calculated studies are required.

Keywords:

turbine wheel, blade, damper mass, damper stiffness, damper effectiveness, friction force, vibratory stress decline in a rotor blade

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