Free Turbine Rotor Integrity Evaluation Procedure in Case of Engine Magnetic Suspension Failure AL131ST

Aeronautical and Space-Rocket Engineering


Аuthors

Treshnevskaya A. S.*, Makarychev A. S., Kuz’min M. V.**, Kirsanov A. R.

Lyulka Experimental Design Bureau, branch of the United Engine Corporation – Ufa Engine Industrial Association, 13, Kasatkina str., Moscow, 129301, Russia

*e-mail: Treshnevskaya_as@okb.umpo.ru
**e-mail: maxim.kuzmin@okb.umpo.ru

Abstract

The purpose of the work consisted in developing a technique for calculating the emergency failure of the AL31ST engine free turbine magnetic suspension of the as close as possible to the real operating conditions. Reliable computing of the emergency failure and, as a result, the rotor falling on the standby bearings represents a problem, since the said computing should account for all dynamic effects and shock loads that occur while the rotor, spinned-up to the operating speed, contact with the rings of the emergency bearings. As long as the ceramic material of the emergency bearings rolling bodies is the silicon nitride, it is necessary to develop an adequate model of micro-cracks spread in the non-homogeneous structures.
The article provides computation of the AL31ST engine free turbine magnetic suspension emergency failure. In the beginning, a numerical model of the shaft train with emergency bearings with account for all weight and inertia characteristics of the structural elements, with the layer of the auxiliary elements under supports of the free turbine rotor and applied boundary conditioins as close as possible to the real operating conditions, was developed. A Johnson–Holmquist model of plastic destruction was applied for the rolling bodies simulation. The shaft line was spinned up to the operating speed, then the auxiliary elements were removed from the computation, and due to gravity the rotor fell on the back-up bearings. Elastoplastic computing was performed with the LS-DYNA/explicit code.
Maximum stresses on rings of the standby bearings were of 73,0 kgf/mm2. The rated Mises stresses in structural elements in contact with standby bearings were of 55,0 kgf/mm2. All obtained stresses do not exceed the strength limit. Thus, in case of a magnetic suspension failure, the standby bearings should ensure the AL31ST engine free turbine rotor integrity.
Natural frequencies and waveforms analysis of the AL31ST engine shaft train was performed as well. The linear computing was performed with the LS-DYNA/implicit code. The results of modal analysis revealed that natural speeds did not occur in the operating range from 62 to 97 Hz, which corresponded to the 70–110 % of the operating speed at the first mode, which was 5300 rpm.
Summing up, it may be said that the standby rolling bearings are strongly recommended to be employed in the AL31ST engine free turbine structure with a view to reliability enhancing. In case of the magnetic suspension failure, the standby bearings will ensure the integrity of the AL31ST engine free turbine rotor. They take over if the magnetic field fails, or rotor deviation from its axial position, preventing its contact with the body and mechanical disruption. As long as no natural frequencies and forms originate in the operation range, one may suppose that the rotor bending oscillations would not become an extra impact while its contact with the backup bearings.

Keywords:

free turbine, magnetic suspension emergency failure, safety bearings, ceramic rolling bodies, pliancy of supports, rotor spinning, rotor natural frequency

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