Evaluating effects of friction contacts between shrouds in integrally shrouded blade rows

Simone Bistolfi, Nicolò Bachschmid, Emanuel Pesatori
Franco Tosi Meccanica

The beneficial effects of the contact between shrouds are described extensively in recent literature: natural blade frequencies are increased and additional damping is available. Different models are proposed for analyzing its linear and non-linear behavior, selection of optimum contact forces are proposed for reducing vibration amplitudes to a minimum. Results from different non linear analyses that use different models all based generally on a reduced modal model of the blade row and on the harmonic balance approach for modeling the non linear contact forces, are sometimes contradictory: some claim e.g. that increasing excitation amplitude leads to a reduction of the dynamic magnification factor (due to friction damping increase) some other claim the opposite.
In the present paper the procedure for analyzing the effects of friction contacts in blade rows using ANSYS is described: static non linear analysis is required for finding contact conditions, linear equivalent contact models must be selected for performing linear cyclic symmetry analysis for finding natural blade row frequencies, which depend on the model used. Finally non linear dynamic frequency response is calculated for different frequencies around the first linear natural frequency, for finding the maximum response (resonance). These calculation, being the use of cyclic symmetry impossible, should be performed taking account of the complete row with friction contacts between shrouds; this is computationally not affordable. But being the excitation sinusoidal (with a certain engine order) and the deflection also sinusoidal with a corresponding nodal diameter number, a suitable group of blades can be selected with suitable boundary conditions, for covering a half sinusoid between two nodes. Changes in vibration behavior due to changes in excitation amplitude are also evaluated. Friction forces can be emphasized using of a “contact shim” which can be inserted in a cavity between adjacent shrouds, for the blades that would separate due to centrifugal elongation.